Final Report Summary - NANODIRECT (Toolbox for Directed and Controlled Self-Assembly of nano-Colloids)
Executive Summary:
Nanodirect was a small collaborative project between universities and research centers focused on the development of a toolbox for directed self-assembly of nanoparticles. Self-assembly is a popular concept in the field of nanotechnology, especially for nanoparticles, which are one of the most prominent and promising candidates for technological applications. Self-assembly has been identified as a potentially important process where the building blocks spontaneously organize into ordered or structures driven by the desire to minimize energy. Nanodirect started from the premise that in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control may be required. An so although this may seem like a contradiction in terminis, directed self-assembly may be a viable route for self-assembly.
Nanodirect aimed at presenting the European Reseach Area with a toolbox for directed assembly, to investigate and to compare the potential of novel strategies for material design. Four different work packages each focused on a different tool for directed self-assembly. The first element of the toolbox was the use of chemistry as a directing agent, by making nanoparticles with specific shapes or providing specific sites by which they interact. We also developed a number of so called model systems for research, i.e. these are systems in which the different parameters that characterize the nanoparticles (size, shape, aspect ratio) can be varied in a controlled manner. These model systems where shared between the different partners of the consortium. These model nanoparticle systems were also such that were made to be responsive to external stimuli, such as the directing fields.
The other workpackages were aimed at investigating to what extent flow fields, electric fields and the presence of interfaces and walls could be used to direct self-assembly. Hence complementary to the use of chemistry as a directing agent, the use of different fields and macroscopic templating agents has been explored. Flow fields combined with the use of a complex suspending matrix, such as a polymer solution, lead to beautiful 1D and 2D particle trains or crystalline sheets, which have been explored experimentally and using mesoscale simulations. Electric fields can be used to generate an even richer are of structures and phases, yet it is diffult to scale this method down to small particle sizes. Very promising results have been obtained when assembling nonspherical particles at liquid liquid interfaces by the effects of surface tension
Project Context and Objectives:
Nanodirect aimed at developing a toolbox containing elements that should enable one to design and fabricate nano-based composite materials by directed self-assembly.
Synthetic chemistry is a first element of the toolbox and hence range of nanoparticle building blocks with controlled sizes, shapes and aspect ratios have been synthesized and characterized. Specifically, within Nanodirect the effects of nanoparticle shape, the directionally of interactions and the use of composite particles have being used as strategies for directed self-assembly. Gold and silver nanoparticles with a wide range of controlled shapes and sizes have been synthesized with an emphasis on the efficiency and scale-up. These metallic nanoparticles are of interest for their plasmonic properties, and concerning the latter it is crucial how these can be tailored. In order to assemble the particles in hierarchical structures, they can be either deposited on a microscopic template (e.g. CNT) or made to interact directionally by e.g. hydrogen bonds or charge attractions, which leads to clusters and a subsequent control over the optical response. The templating method clearly offers most control and leads to regular structures with very specific optical responses. The use of anisomeric nanoparticles as elements to create percolating networks at low concentrations has been explored. To this end, model systems with tunable interactions were designed and prepared, as a basis for usage in the rest of the project. Rod-like viruses were chemically modified to be either sterically stabilized or thermoresponsive. Novel types of sticky ellipsoidal latex particles were synthesized and their ability to form networks at low volume fractions was investigated as well. The nanoparticles based-gels are have been shown to build-up yield stresses at very low solid fractions. Finally, synthesis of composite colloid nanoparticles with tailored properties was also explored and shown to open a canopy of possibilities.
Within Nanodirect we also critically evaluated the experimental methods to characterize the hydrodynamic properties such as the particle size, shape and polydispersity of metallic nanoparticles. The classical electron microscopy measurements only yield geometric aspect ratios and the statistics are usually limited. Plasmon-enhanced light scattering methods turn out to be rather insensitive. A method employing time resolved flow induced dichroism turned out to be well applicable to metallic nanoparticles and yields robust results. The effects of the hydrodynamic properties on the diffusivity of nanoparticles with a large aspect ratio have been investigated, both theoretically and experimentally.
Complementary to the use of chemistry as a directing agent, the use of different fields and macroscopic templating agents has been explored. The effects of flow fields for directed self-assembly in dense nanoparticle suspensions, the effects of flow on suspensions of non-spherical particle suspensions or the potential for exploiting the anisotropic viscoelastic stress induced by a shear flow to direct assembly of particles have been studied. This is carried out experimentally as well as using coarse-grained numerical simulations. Some striking observations include the directed self-assembly of particles into spectacular strings and as yet unreported 2D crystalline sheets inviscoelastic media. These methods may be easily applicable during rather standard (polymer) processing operations. Both the experiments and the mesoscale simulation methods have pointed to the important role of shear thinning of the matrix fluid and a rather subtle dependence on the elastic properties and overall level of viscosity of the fluid, yet a clear and final insight is still lacking. It has also been investigated how the DSA by viscoelastic stresses can be used in applications, taking the case study of a suspension of protein particles assembled into fibers without denaturation of a protein.
The effects of the interactions with solid walls and the use of fluid-fluid interfaces to direct self-assembly during deposition or in convective self-assembly is the final element of the toolbox. For the interactions with solid walls, novel experimental setups, such as total internal reflection fluorescence microscopy, evanescent wave dynamic light scattering and the flexure microgap rheometer have been developed or adapted to nanoparticle systems. Using the evanescent wave light scattering, rotational and translation motion of particles (hard spheres and fd-virus) close to a solid wall have been studied. The interactions have been shown to be weak. Theoretical work has focused on the interactions of both spherical and rod-like particles with a wall in order to rationalize the experimental findings. So far, simulations were also possible for dilute systems, and experiments on concentrated suspensions. The TIRFM technique has been used to provide detailed results on the interplay between walls, anisotropic nanoparticles and the depletion forces, which may contribute to our understanding of how to deposit particles onto solid walls. This is being further investigated.
For the liquid-liquid interfaces surface rheological techniques have been further advanced and the focus has been on studies of the effect on how to exploit the effects of particle shape in dictating dynamics, structure and rheological properties of particles at interfaces. Both rod- like and disk-like particles have been studied and both show very promising results for making high interface materials (such as foams, films, emulsions) with tailored properties. Especially mechanical properties of interfaces can be controlled to a large extent by exploiting particle shape effects. Moreover the geometry of multiphase mixtures can be used as a scaffold for nanoparticle assembly, for both spherical and non-spherical shapes. This is being investigated further experimentally for graphene oxide sheets. Methods to develop hierarchically structured interfaces and the use of directionally interacting particles at interfaces are currently being explored.
The expected impact of Nanodirect is that it provides a knowledge-based approach to directed self-assembly by coupling nanoparticle tunability and external stimuli. More specifically, the development of novel experimental tools, model systems and simulation methods results in a rational approach to design nanoparticle-based materials with controlled structures. Collaboration with an industrial advisory board, exchanges of young researchers with industry and the detachment of scientist form industry to academic labs explores the potential applications for a broad range of industries.
Project Results:
1. Chemistry as toolbox
During the four years of NanoDirect, the work within WP2 has focused on the characterization and up-scaling of existing colloidal nanoparticle systems, the development of novel nanoparticles with directional interactions and the study of phase behavior and self-assembly directed by various parameters.
Up-scaling has been demonstrated for four different nanosized systems showing directional interactions. In summary: the formation of Au nanorods, based on surfactant-directed seeded growth and which is usually limited to a few mg, has been optimized for gram-scale production with a tight control of dimensions and aspect ratio; disk-like particles of Au, Ag and Cu were also prepared by colloid chemistry methods, within a wide range of dimensions and achieving in all cases sub-gram scale production, which allowed us to carry out various studies within other work packages (in the case of Au plates, the use of well-defined smaller Ag plates was essential to form particles with larger aspect ratios); fd-virus was grown and purified following standard biochemical protocols, which can be multiplied to obtain gram-scale production, followed by surface modification; ellipsoidal polystyrene or PMMA particles were prepared by controlled stretching of monodisperse spherical particles in polyvinyl alcohol (PVA) thin films, upon mild heating, achieving large scale production (through parallel processing) with aspect ratios ranging from 1.25 to 10 and less than 1% of polydispersity. A similar procedure with biaxial stretching can be used for large-scale production of nanodisks.
Directional interactions
Surface modification has allowed the design of different strategies for chemistry-directed interactions. Examples are the following:
Gold nanorods with hydrogen-bonding directed interactions. Gold nanorods were selectively modified on their tips with pH sensitive bifunctional molecules, such as cysteine (CYS) or mercaptophenol (MPh), which bind to gold surfaces and are likely to form intermolecular hydrogen bonding. End-to-end directed interactions reach a maximum at a pH equal to the pKa of the linking molecule, in agreement with a hydrogen-bonding theory. Reversible assembly could be obtained as a function of pH.
Gold nanorods in standing superlattices. Surface modification with cationic gemini surfactants and interfacial aggregation lead to spontaneous formation of self-assembled supercrystals of standing nanorods, with directional optical properties.
Conclusion: The interplay between tailoring shape and using macro- or microscopic templating is the most promising route for directed self-assembly, but there is an entire universe of methods which can be used as can be seen in figure 2.
Phase behavior studies
Selected systems were used for phase behavior studies. For example, fd-viruses were studied upon surface modification with pNIPAM, so that the gelation behavior could be studied and tuned by combining the ionic strength, particle concentration, polymer structure, temperature and electric field (studies in WP4). Silica rods were used to study the phase behavior of shorter rods, with the advantage that the phase behavior could be studied both in real and reciprocal space. Additionally, well-defined platelets served to find different types of nematic phases, including a biaxial nematic, which had been predicted but not found before. On the other hand, telechelic polymers were used to study mixtures of large star (soft) and small hard spheres, which showed the existence of multiple glassy states.
Characterization tools for nanoparticles
As particle shape plays an important role in controlling the optical, magnetic, and mechanical properties of nanoparticle suspensions as well as nanocomposites. However, characterizing the size, shape, and the associated polydispersity of nanoparticles is not straightforward. Electron microscopy provides an accurate measurement of the geometric properties, but sample preparation can be laborious, and to obtain statistically relevant data many particles need to be analyzed separately. Moreover, when the particles are suspended in a fluid, it is important to measure their hydrodynamic properties, as they determine aspects such as diffusion and the rheological behavior of suspensions. The classical approach to measuring the hydrodynamic properties for such nanoparticles is using dynamic light scattering (DLS). This method required both the polarized and depolarized light to be analysed. Moreover, the observable property, i.e. diffusivity, has a logarithmic dependence on the aspect ratio. The response of a very dilute suspension of particles to flow, and measuring the response to a transient flow field using linear dichroism has been shown to be a more reliable method to obtain hydrodynamic aspect ratio and polydispersity for nonspherical gold nanoparticles for two reasons. First, the use of the evolution of the orientation angle makes effects of polydispersity less important. Second, the use of an external flow field gives a mathematically more robust relation between particle motion and aspect ratio, especially for particles with relatively small aspect ratios.
Conclusion: Novel characterization methods have been explored, focusing on anisotropic nanoparticle systems and their composites. Brilluoin scattering remains a highly specialized technique. DLS and rheo-optical methods are readily available and offer new possibilities for nanoparticles compared to electron microscopy especially regarding dynamical properties.
Selected References resulting from the work of WP2:
Sanchez-Iglesias, M. Grzelczak, B. Rodriguez-Gonzalez, P. Guardia-Giras, I. Pastoriza-Santos, J. Perez-Juste, M. Prato, L.M. Liz-Marzan, "Synthesis of Multifunctional Composite Microgels via In-situ Ni Growth on PNIPAM-Coated Au Nanoparticles", ACS Nano 2009, 3, 3184-3190
Zhang, Z.K. N. Krishna, M.P. Lettinga, J. Vermant, and E. Grelet, "Reversible Gelation of Rod-Like Viruses Grafted with Thermoresponsive Polymers", Langmuir 2009, 25, 2437-2442
M. Grzelczak, J. Vermant, E.M. Furst, L.M. Liz-Marzan, Directed Self-Assembly of Nanoparticles, ACS Nano 2010, 4, 3591-3605.
Z. Zhang, J. Buitenhuis, A. Cukkemane, M. Brocker, M. Bott, J.K.G. Dhont, Charge reversal of the rodlike colloidal fd virus through surface chemical modification, Langmuir 2010, 26, 10593-10599.
A.Sanchez-Iglesias M. Grzelczak, J. Perez-Juste, L.M. Liz-Marzan, Binary Self-Assembly of Gold Nanowires with Nanospheres and Nanorods, Angew. Chem. Int. Ed. 2010, 49, 9985-9989
Zhenkun Zhang, Patrick Pfleiderer, Andrew B. Schofield, Christian Clasen, and Jan Vermant, Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles J. Am. Chem. Soc. 2011, 133, 392-39
Voudouris, P, Choi, J, Gomopoulos, N, Sainidou, R, Dong, HC, Matyjaszewski, K, Bockstaller, MR, Fytas, G. "Anisotropic Elasticity of Quasi-One-Component Polymer Nanocomposites", ACS NANO 5, 5746-5754 (2011).
J.M. Romo-Herrera, R.A. Alvarez-Puebla, L.M. Liz-Marzan, Controlled Assembly of Plasmonic Colloidal Nanoparticle Clusters, Nanoscale 2011, 3, 1304-1315.
N.K. Reddy, J. Perez-Juste, I. Pastoriza-Santos, P.R. Lang, J.K.G. Dhont, L.M. Liz-Marzan, J. Vermant, Flow Dichroism as a Reliable Method to Measure the Hydrodynamic Aspect Ratio of Gold Nanoparticles, ACS Nano 2011, 5, 4935-4944
E. Antoniou, P. Voudouris, A. Larsen, B. Loppinet, D. Vlassopoulos, I. Pastoriza-Santos, L. M. Liz-Marzan, Dispersions of Polymer-Grafted Silver Nanoprisms in the Bulk and Near Solid Surfaces", JOURNAL OF PHYSICAL CHEMISTRY C 116, 3888-3896 (2012).
Reddy, N., Zhang, Z., Lettinga, P., Dhont, J., Vermant, J. Probing structure in colloidal gels of thermo-reversible rod-like virus particles: Rheology and scattering. Journal of Rheology, 56 (5), 1153-1174 (2012).
2. Directed self assembly by flow.
During the four years of NanoDirect, the work within WP3 has focused on the using flow for directed self-assembly. This report will first discuss the different tools developed; both experimental platforms and mesoscale simulation techniques, and subsequently the obtained results will be discussed.
Equipment and simulation tools developed.
We also built a flexture-based microgap rheometer which enabled studying fluids in either bulk flows or in confinement. For bulk flows, the setup has the advantage that it can be combined with X-ray scattering from the vorticity plane, i.e. the plane in which deformation and orientation of the micro and nanostructures can be observed.
Flow of complex systems is closely linked to the rheology of nanoparticle based suspensions and nanocomposites. Within Nanodirect two aspects related to the rheology were studied. First rheologically complex fluids were used as media in which substantial normal stresses develop. The idea is that flow induced anisotropy can be used to direct structuring was investigated. Secondly, the use of rheology for densen suspensions of nanocomposites required us to adapt tools, both in terms of measurment geometry and methodologies. For example, A cone-partitioned-plate fixture has been developed and validated in order to be able to reliably measure nonlinear shear rheology in highly elastic nanocomposite systems.
To study dense nanosuspensions; and in particular to understand how these systems rearrange and structure over long times (related to shelf life), the use of large amplitude oscillatory shear (LAOS) has received quite some attention, Rheology is blind, but offers the advantage that it does not suffer from the requirements for optical contrast (either too little or too much). Monitoring the higher harmonics in the stress signal by Fourier-Transform (FT) rheology may provide useful insight on the progressive transition from linear to nonlinear viscoelastic response. However, the physical interpretation of FT-rheology data is still not obvious. For example the process of yielding in a colloidal glass formed by star-like nanoparticles was studied and the underlying mechanisms are cage breaking and reformation as well as stress storing and relaxation within the period of oscillatory shear which are affected by an interplay between shear and Brownian motion and thus relate with Peclet number variation with strain and frequency An entirely new way of analyzing linear and nonlinear oscillatory material has also been developed. A new quantitative sequence of physical processes (SPP) method views generic oscillatory responses within the Frenet-Serret frame as sequences of planar 2D curves embedded in the 3D space defined by the strain, strain rate, and stress axes. This offers a possibility to understand in more detail the physical processes taking places when the deformation amplitude is increased and the microstructure is altered by flow.
Finally, to study the hydrodynamic forces in complex fluids, which is important to understand the DSA by viscoelastic forces, a standard AFM was modified in order to take a closer look at the effect of fluid rheology on lubrication forces.
MESOSCALE SIMULATION TECHNIQUES: The properties of soft matter- nanoparticle systems are mainly determined by their mesoscopic structure. Due to the large separation of length and time scales between the atomic scale of the solvent and the mesoscale of the solute, the simulation of the dynamical behavior of such composite soft matter isparticularly challenging. Traditional molecular dynamics simulations are often inappropriate because of the unaffordable simulation time, especially when hydrodynamics needs to be taken into account. For a colloidal suspension in a viscoelastic solvent, e.g. proteins in a cell, or spherical or rod-like colloids dispersed in polymer solutions the situation is even more complex. The solvent itself is now a complex fluid too. To be able to address and unravel dynamical and rheological properties of such complex systems requires a simplified mesoscopic model of a viscoelastic solvent, preferentially in combination with a mesoscopic hydrodynamic simulation technique such as the multiparticle collision dynamics (MPC) approach or the and Responsive particle dynamics.
The techniques differ in the level of coarse graining and in the size of the systems that can be simulated. The MPC technique obtains a higher efficiency in reproducing the viscoelasticity of the solvents using molecular models, or at least being able to incorporate more of physical insight into the real system. The silulations are computatonially extremely efficient, and here is even an on-line real time demo of the code which can be tried out at:
http://www.fz-juelich.de/ics/ics-2/EN/Forschung/HydrodynamicsSimulation/_node.html(opens in new window)
Conclusion: Several techniques, mainly related to the rheological properties, were further developed and adapted to nanoparticle systems, but are more generally applicable. The use of LAOS has proven to be of use mainly for dense colloidal systems, where simple physical processes underly the response dynamics. Specific tools (to address issues in the experiments on nanocomposite systems were developed. A major effort has been the development of mesoscale simulation methods, required to bridge the wide range of length and time scales in soft matter composites containing nanoparticles. Two simulation approaches were explored, one deterministic and particle collision based (MPCD), the other based on Brownian dynamics and coarse grained. MPCD is a tool oriented at a fundamental understanding of the phenomena, whereas Rapid can be sued to explore a broader range of systems.
Directed self-assembly of rod like particles
Suspensions of rodlike nanoparticles and their solidlike response at extremely low volume fractions can be exploited technologically. Understanding the link between the physicochemical parameters such as size, aspect ratio, volume fraction, and interparticle forces with the resulting microstructure and the subsequent rheological response remains challenging. Within Nanodirect, suspensions of monodisperse rodlike virus particles, whose surface is modified by grafting with a thermoreversible polymer poly(N-isopropylacrylamide) in WP.2 are used as a model system. The repulsive and attractive contributions to the total interaction potential can be changed independently by varying the ionic strength and the temperature. The anisotropic nature of the electrostatic potential may result in different self-assembled microstructures. T. Rheological measurements of the near critical gel properties as a function of concentration and ionic strength proved to be more sensitive compared to scattering in resolving the structural differences, as characterized by a gel strength S, and a structural parameter n. A percolating structure is formed at very low volume fractions (we believe to be the world record holder for gel formed on earth, zero gravity experiments having gone lower), which show a weak dependence on the ionic strength with the anisotropy of the repulsive interactions playing the main role in creating more open structures. Surprisingly, the intrinsic stiffness of the rodlike particles does not affect the moduli of the gel states very strongly.
For stable rodlike suspensions, the use of orientable rod like particles as viscosity modifiers may well be more interesting as a tool to modify the dynamics of the suspensions. This was investigated by again using fd-virus nanoparticles in extensional flow. Another way to strengthen the efficiency of flow is to increase the concentration of particles. Real space confocal studies have shown for the first time the effects of flow on the organization of rodlike particles subjected to flow. To date, these results have not yet been confirmed for nanoparticles, but the governing parameter has been shown to be the rotational Peclet number, and everything can be expected to scale with the rotational diffusivity. This explains how shape anisotropy and size will intervene, and this method will be applicable for nanoparticles.
Directed self-assembly by viscoelastic stresses
Experiments on directed self-assembly in viscoelastic liquids were carried out to investigate the usefulness. As reported sporadically in the literature, due to the changes in hydrodynamic interactions, string-like structures can be observed in sheared viscoelastic fluids. To clarify the influence of the fluid rheology and to understand the mechanism of chaining formation, optical microscopy observations as well as small angle light scattering (SALS) experiments were performed using parallel plate geometry. Suspensions of monodisperse polystyrene spheres were dispersed in shear thinning viscoelastic liquids. Aligned strings of particles were observed when shearing faster then a critical shear rate, which was found to be independent of particle size. However, the role of particle migration was found to be of prime importance and migration effects were quantified.
To understand the fundamental underlying the chaining, three types of experiments have been performed. On the one hand, studies on single particle dynamics have been used both experimentally and using the MPCD simulations, as well as standard continuum mechanical calculations. Depletion interactions were induced to measure the interaction strengths and AFM measurements to quantify the hydrodynamic interactions. All these results combined suggested that the degree of shear thinning is the controlling parameter, although this awaits experimental verification. The mesoscale simulation methods were also used to explore certain aspect further in detail. For example, the effect of using bidispersed sized particles and effects of polydispersity were investigated. These showed effects of segregation according to size, potentially this could be exploited in size sorting method.
Directed assembly in emulsion droplets
By producing a monodisperse emulsion by means of a viscoelastic monodisperse emulsion, nanoparticle clusters with controlled morphology could be produced. For nanoparticle assembly using emulsion droplets the following approach was taken. A nanoparticle-loaded oil is emulsified in water using a surfactant. The emulsification process breaks up initially large droplets into smaller ones, each carrying their nanoparticle payload. In a final step the hexane oil is slowly removed by evaporation, forcing the particles to assemble into clusters, the size of which can be controlled via droplet size and nanoparticle loading. Clusters were made containing as many as hundreds of nanoparticles, or as few as two. Control over cluster size will be afforded by a judicious choice of the viscoelastic properties that control droplet breakup. In a later stage this approach will be combined with a controlled shear to try to obtain exquisite control over the cluster uniformity.
Conclusion: Flow strength constitutes an important and processing related parameter to direct the structuring of nanoparticle suspensions. For rodlike particles, orientation can be induced by flow and the interplay between directional interactions and flow structuring can be used to tailor the microstructure and the resulting properties at will. These systems have potential as rheology modifiers, either to impart solid like rheological properties at low volume fractions or to affect the extensional flow properties. For nanoparticles these aspects could then be achieved while maintaining optical transparence.
Selected References resulting from the work of WP3:
Zhang, Z.K. N. Krishna, M.P. Lettinga, J. Vermant, and E. Grelet, "Reversible Gelation of Rod-Like Viruses Grafted with Thermoresponsive Polymers", Langmuir, 25(4), 2437-2442 (2009)
R.Pasquino F.Snijkers N.Grizzuti and J.Vermant Directed self-assembly of spheres into a two-dimensional colloidal crystal by viscoelastic stresses, Langmuir, 26 (5), pp 3016-3019 (2010)
S. Rogers, P. T. Callaghan, G. Petekidis and D. Vlassopoulos,Time-dependent rheology of colloidal star glasses, J. Rheology, 54, 133-158, (2010)
Effect of Viscoelasticity on the Rotation of a Sphere in Shear Flow F. Snijkers, G. D Avino, M. Hulsen F. Greco, Pier Luca Maffettone,J. Vermant, J. Non. Newt. Fluid. Mech., 166, 363-372 (2011).
Anke Kuijk, Alfons van Blaaderen, and Arnout Imhof Synthesis of Monodisperse, Rodlike Silica Colloids with Tunable Aspect Ratio, J. Am. Chem. Soc., 133 (8), pp 2346-2349 (2011)
S. A. Rogers, B. M. Erwin, D. Vlassopoulos, and M. Cloitre, -Oscillatory Yielding of a Colloidal Star Glass, J. Rheol., 55, 733-752 (2011).
S.C. Ji, R. Jiang, R.G. Winkler, and G. Gompper, Mesoscale hydrodynamic modeling of a colloid in shear-thinning viscoelastic fluids under shear flow.J. Chem. Phys. 135. 134116 (2011).
I. S. Santos de Oliveira, A. van den Noort, J. T. Padding, W. K. den Otter and W. J. Briels. Alignment of particles in sheared viscoelastic fluids. J. Chem. Phys. 135, 104902 (2011)
Frank Snijkers and Dimitris Vlassopoulos , Cone-partitioned-plate geometry for the ARES rheometer with temperature control J. Rheol. 55, 1167 (2011).
G. D'Avino F. Snijkers R. Pasquino M.A. Hulsen F. Greco P.L. Maffettone J. Vermant Migration of a sphere suspended in viscoelastic liquids in Couette flow: Experiments and simulations Rheoogica Acta, 51: 215-234 (2012)
Simon A. Rogers, A sequence of physical processes determined and quantified in LAOS: An instantaneous local 2D/3D approach, J. Rheol. 56, 1129 (2012)
Naveen Krishna Reddy, Zhenkun Zhang, Jan Vermant, M. Paul Lettinga and Jan K. G. Dhont, Probing Structure in Colloidal Gels of Thermo-Reversible Rod-Like Virus Particles: Rheology and Scattering Journal of Rheology, 56, 1153 (2012)
3. Directed self assembly by electric fields.
During the four years of NanoDirect, the work within WP4 has focused on the using mainky AC electric fields for directed self-assembly.
We studied the phase behavior and pattern formation of colloidal particles in AC electric fields as this does not lead to electrophoresis of the particles, which would merely deposit them on one of the electrodes (electrodeposition). Furthermore, we now have two parameters, field strength and field frequency, to vary. Another advantage of using an AC electric field is that it acts by dielectric polarization, which will work on any kind particle, or by electric double layer polarization, which will act on most particles. Various new methodologies were developed to probe the assembly of colloids in electric fields: We used various light scattering techniques, confocal microscopy, and computer simulation. New instruments were developed, in which electric fields can be applied and particle assembly can be monitored. Directed assembly of particles makes use of the following three principles:
1.Orientation of (nonspherical) particles by alignment of their induced dipole moment with the external electric field.
2.Alignment of particles by the mutual dipole-dipole interaction of their induced dipole moments.
3.Migration of particles by the action of a field gradient on the induced dipole moment (dielectrophoresis).
Experimental methods to study the assembly in electric fields by light scattering and optical (confocal) microscopy have been adapted and developed. The samples cells consisted of ITO coated glass slides which are connected to external function generators; Structure formation taking place inside the cell in response to the electric field can be observed by placing the cell on a microscope, or interrogating it using scattering techniques such as dynamic light scattering (DLS) or birefringence
Simulation tools: Theory and simulations tools were developed to describe the structures observed using electric field directed self-assembly, as well as to predict that should be obtainable in the large parameter space of the experiments. Simulations were done using the Monte Carlo method in the NVT ensemble, at low packing fractions. To handle the long-range dipolar interactions, we use Ewald summations with conducting boundary conditions. To improve equilibration and sampling speed in the systems with strings, cluster moves were introduced to move particles in a cylindrical volume collectively
Results of DSA by electric fields.
Experiments were done to study the assembly of particles of various shapes (rods, plates, and in some cases spheres) directed by external fields. For Rods, the fd-virus nanoparticles from WP2 were used and Various phases, patterns and dynamical states have been found At low field amplitudes there is equilibrium between non-chiral nematic and isotropic regions; the N-phase. At larger field amplitudes there is a transition to coexistence between a non-chiral nematic and chiral nematic; the N-phase. The large non-chiral domains break up to smaller, disconnected domains in the N*D-phase. The chiral texture disappears and the non-chiral nematic domains melt and form within the dynamical state Ds. In the dynamical state Df, melting and forming of nematic domains is very fast. At high frequencies, in the H-phase, the system becomes homogeneous, and the rods are aligned along the electric field. The N-phase directly transforms to the H-phase at high frequency on increasing the field amplitude and the H-phase transforms to a dynamical state at higher field amplitudes.
For shorter silica rods or dumbells in an AC field, more detailed insight into the nature of the microstructures could be obtained. Here the true power of directed self-assembly was shown. Without an electric field the particles did not assemble into any ordered phase on any appreciable time scale. To assemble the colloidal dumbbells a high frequency electric field to induce dipolar interactions between them. This readily led to ordering of the particles. Similar, albeit less striking results were obtained with the silica rods. Another experimental model system is one that consists of very thin (20 nm) gold plates. Although the plates are not very monodisperse in diameter (between 500 and 1000 nm), they readily form stacks due to the secondary minimum in the DLVO potential. Using an electric field, columns can be formed regardless of ionic strength. Here, the columns run perpendicularly to the electric field. This means that the individual plates are aligned parallel to the field with their long axis. After some time, the columns continue to assemble side by side into broad bands. This still requires only modest field strengths of around 10 V/mm. The strong plasmon absorption of the plates causes a strong polarization dependent extinction by these structures. Turning off the field causes the platelets to rapidly disperse.
Combination of electric bottle and shear flow
A system of rodlike silica particles with an aspect ration of 6 was studied in shear flow at a number of different volume fractions. The particles had a diameter of 550 nm and a length of 3300 nm, were labelled with a layer of fluorescent dye (fluorescein, FITC), and were refractive index matched in a mixture of water and DMSO (dimethylsulfoxide) in which they are charge stabilized. The electric bottle incorporated into a parallel plate shear cell offers a convenient way to increase the volume fraction gradually, such that the shear can be applied to the same system in the same experiment.
The behavior of this system was studied at increasing volume fractions where a different phase is found in the absence of shear. Upon increasing the shear rate we found the following progression of shear induced structures:
Isotropic to paranematic
Nematic to oriented nematic
smectic to oriented smectic to oriented columnar to small Sm/Col domains to paranematic
Clearly, the most elaborate succession of structures is found in the smectic. Snapshots of the structures in the plane of zero velocity illustrate this. On the whole, the order in the structure diminishes with increasing shear rate, but with interesting new structures being formed. The results demonstrate the utility of the instrument in studying structures in a dispersion of rodlike particles directed by shear flow.
The work carried out in Nanodirect Work package 4, demonstrated means to assemble of both spherical and nonspherical colloidal particles by applying external electric fields. Instruments were developed to study this assembly using confocal microscopy and light scattering techniques. The sample cells apply AC electric fields of the desired geometry to make use of polarization of the particles by the electric field, which orients them parallel to the field. The induced dipole moments subsequently lead to modified mutual interactions that may induce order among particles. In many experiments the particles used were around a micron in length and several hundred nanometer in length so that they could be studied in more detail. For these particles use can be made of the dielectric polarization of the particles in the external field, which is proportional to the particle volume. For particles below about 100x100x100 nm3 in size, however, electric double layer polarization is still strong enough to allow their assembly to be directed by external fields, as is shown by the experiments on fd virus. This is effective at field frequencies in the range of 10Hz - 10kHz.
A comparison with theory and computer simulations shows that interactions can be modeled well by the theory given, even though it is approximate for interacting particle systems. Also the nanoparticle assemblies are reproduced well in the simulations. With this understanding it becomes possible to predict the nanostructures that could be obtained. This is important because the experimental work shows that the number of different ordered structures in relatively simple shaped particles is already very large.
Selected References resulting from the work of WP4:
Kang, K., Dhont, J. K. G., Eur. Phys. J. E 30, 333 (2009).
Kang, K., Dhont J. K. G., Soft Matter 6, 273 (2010)
Demirörs, A. F., Johnson, P. M., van Kats, C. M., van Blaaderen, A., Imhof, A., Langmuir 26, 14466 (2010).
Kuijk, A., van Blaaderen, A., Imhof, A., J. Am. Chem. Soc. 133, 2346 (2011).
Kuijk, A. , Byelov, D. V., Petukhov, A., van Blaaderen, A., Imhof, A., Faraday Disc. (accepted); DOI: 10.1039/C2FD20084H
4. Directed Self-assembly by walls and interfaces
During the four years of NanoDirect, the work within WP5 has focused on the use of interfaces and wall to template or direct self-assembly.
Particles at interfaces
Equipment developed: One of the intriguing features of nanoparticle laden interfaces is the appearance of significant mechanical properties imparted to a monolayer. Typical measurement techniques for measuring the properties of complex fluid-fluid interfaces have been designed for relatively simple fluid surfaces and often focused on surfactants. Particle laden interfaces turned out to behave as insoluble and elastic interfaces. To study these, properly designed surface rheometers measure both the stress and deformation history for fluid interfaces at which the constitutive property is not known a priori.
Sensitive techniques for measuring the shear rheology of (nano)particle-laden interfaces have been developed a constitute an important part of the toolbox for studying DSA at fluid-fluid interfaces. They have been validated, theoretically underpinned and tested by different members of the consortium. There is still a need for developing adequate tools for dilatational rheology, although some progress has been made. Also future work should focus on developing adequate constitutive models to describe the performance of these interfaces in products or during processing steps. Yet the basic elements of the toolbox are available.
Directed self-assembly at fluid-fluid interfaces.
Capillary forces, arising due to the presence of a surface or interfacial tension can effectively pin particles to an interface and make their expulsion difficult. The presence of an interface adds significant complexity to the colloidal interactions. For example, compared with similar particles in a bulk liquid, the electrostatic repulsion between charged particles is greatly enhanced at a water low dielectric medium interface. At the interface a particle has an asymmetric counterion distribution that results in a dipole-dipole interaction through the low dielectric constant phase. Within nanodirect the nature of the electrostatic interaction was investigated and it was argued that the effect of the finite ion sizes of the hydrated ions on the water side need to be taken into account to arrive at quantitative predictions of the electrostatic forces.
The versatility of the technique was subsequently further explored to different types of particles. Using rough particles also enables one to generate strong interfaces, but even more spectacular results could be obtained with nanosheets. Graphite Oxide (GO) sheets were selected to be studied as surface-active particles to produce technologically relevant materials. GO nanoparticles consist of single or few layers of carbon obtained by strong oxidation of graphene. Their lateral size (approximately 10m) and thickness gives very high aspect ratios and surface to volume ratios. GO particles have excellent mechanical properties and can be reduced in situ to restore the high electrical conductivity of pure carbon sheets. Such properties make GO sheets a good candidate for the design of complex high-performing materials through interface-driven assembly. Within Nanodirect we demonstrated how these sheets assemble at the interface forming compact films of nanometer thickness. These films are highly resistant to deformation (both in compression and shear) and at the same time are extremely deformable, resembling the behavior of elastic membranes. When graphene oxide sheets are introduced in a mixture of simple liquids undergoing phase separation, particles segregate at interfaces forming a strong film that can arrest phase separation. This allows to arrest the liquid system into metastable structures, for example co-continuous emulsions stabilized by particles. This process yields 3D complex architectures of particles that can be separated from the fluids to be used for electronic applications (conductive foams, light supercapacitors, etc.)
Conclusion: Exploiting nanoparticle self-assembly at fluid-fluid interface is a very power novel method to create 1D, 2D or 3D structures. The use of lateral capillary forces to create strong assemblies has shown to be extremely useful. This method is very robust has potential to be combined with relatively classic processing methods such as coating techniques, although some work is required;.
Particles near a solid wall and directing by confinement.
Equipment developed. Two main techniques were either further developed or adapted to the use of nanoparticles for the study of nanoparticles both of them relying on the properties of the evanescent wave and how it is being scattered. We have developed the experimental tools to investigate the translational and rotational dynamics of rod like particles in the ultimate vicinity of a hard wall by evanescent wave dynamic light scattering (EWDLS) with polarization analysis. Experiments were performed on aqueous solutions of rod shaped gold particles. Anew velocimetry technique based on EWDLS to measure the near wall velocity of nano particles showed as yet unparalleled resolution.
A second technique adapted to nanoparticles was total internal reflection microscope (TIRM) was adapted and tested in order to use fluorescense instead of scattering. The advantage of using fluorescense is that much smaller colloids can be studied, that mixtures can be probed, where the component of interest is fluorescently labeled, and that self-dynamics can be studied in concentrated dispersions. We applied this method to study particle-wall interactions with anisotropic depletants In a standard TIRM experiment the scattered intensity, Is, from a colloidal sphere in the ultimate vicinity of a solid surface, which is illuminated with an evanescent wave is recorded. By analyzing the fluctuations of Is it is possible to determine the interaction potential between the sphere and the wall. Alternatively the fluorescent intensity, IF, from a fluorescently labelled sphere might be detected and analyzed accordingly. The advantages of using a fluorescent probe would be a significantly increased sensitivity and a reduced signal to noise ratio, as compared to the scattering probe. Such that the technique, which is so far limited to probes spheres with a minimum radius of about 500 nm, could also be used with much smaller spheres. However the advantages of the TIRFM concept might be counterbalanced by the fact that fluorescent dyes tend to bleach.
Conclusion: Two techniques for studying near wall-dynamics have been evaluated for use to Nanoparticles within nanodirect. Setups for EWDLS and TIRF have been adapted tested for the use with nanoparticle systems. The techniques are suitable to measure near all dynamics of both spherical and non-spherical nanoparticles. They are very sensitive but non-trivial to use. To study the interplay between walls and flows, these techniques can also be complemented with a flexure microgap rheometer, which moreover was adapted for combined use with X-rays, to study the interplay between the nanosuspension structure , flow and the effects of confining walls.
Directed self-assembly near solid walls Near Wall Dynamics of concentrated suspensions of spherical nano-particles have been studied using EWDLS and TIRF. On increasing concentration, suspensions of spherical nano particles are known to self assemble into a crystalline structure at a volume fraction of close to 0.5. For spheres, the effects of the hydrodynamic interactions with the wall reamin important even down to the nanometer scale. shaped particle assemble into a nematic phase at a volume fraction. As a first step to study the dynamic pathway of this shift, we investigated the translational and orientational near wall dynamics of dilute suspensions of gold nano rods. For this purpose the evanescent wave instrument had to be equipped with additional features to analyse the polarization dependence of the scattered light. For the microscopic understanding of flow or shear driven self assembly, it is crucial to know the velocity profile and the local shear rate in the vicinity of the interfaces confining the suspensions. We have set up a new evanescent wave velocimetry technique for the experimental determination of these parameters with an unprecedented resolution in the ten nanometer range.
To analyze the results for rods near walls, theoretical and simulation work was carried out and the effect of the walls on slowing down of the deposition rates (due to an enhanced hydrodynamic friction) were analyzed in detail, showing that even in the nanoscale regime the hydrodynamic forces are dominating. To further direct the assembly also under the influence of flow, the effective interactions between a single colloid and the solid wall in bi-disperse systems were investigated. Of special interest are mixtures where so-called depletion interactions are important, pushing the particles to the wall. Such interactions between relatively large colloids and the wall are induced by a second species of many small colloids or polymers. The potential application of such systems towards wall-induced self assembly lies in the easy tuning of the potential by means of the concentration and the size of the depletant. The resulting anisotropic, directed attraction towards the wall can induce structures at high concentrations of the large colloids. As a first step in the direction of wall-assisted self assembly, we investigated the depletion attraction of colloidal spheres as induced by disk- and rod-like colloids, also under the influence of shear flow.
Conclusion: Whereas directed self-assembly by solid walls may be of practical importance in coating and deposition processes, overall the effects are weak. The detailed studies presented here show that even in the nanoparticle regime the hydrodynamic forces dominate the near wall dynamics. Depletion forces may help to control the interaction, but flow weakens the effects, due to alignment and organization of the depletants. Confinement effects also help to promote near wall aligment or structuring, at reasonable values of the flow strength. Overall, the use of solid walls to promote self-assembly is a method with limited potential. Most possibly, walls with controlled surface energy would provide better control, but this complicates matters even more.
Selected References resulting from the work of WP5:
B.Cichocki E. Wajnryb, J.K.G. Dhont, P.R. Lang, The intensity correlation function in evanescent wave scattering, J. Chem. Phys., 132, 074704/1-12, 2010. and in Padding, J. T.; Briels, W. J. J. Chem. Phys. 132, 054511 (2010).
Steven Vandebril, Aly Franck, Gerald G. Fuller, Paula Moldenaers and Jan Vermant A double wall-ring geometry for interfacial shear rheometry, Rheologica Acta, 49:131-144 (2010).
Masschaele K, Park BJ, Furst EM, Fransaer J., Vermant J.,
Finite Ion-Size Effects Dominate the Interaction between Charged Colloidal Particles at an Oil-Water Interface â?¨ â?¨ PHYSICAL REVIEW LETTERS, 105 Article Number: 048303 (2010)
July, C.; Kleshchanok, D.; Lang, P.R. Depletion interactions caused by polydisperse platelets
Soft Matter, 7, 6444 - 6450 (2011)
Zhenkun Zhang, Patrick Pfleiderer, Andrew B. Schofield, Christian Clasen, and Jan Vermant, Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles J. Am. Chem. Soc., 133 (3), pp 392-395 (2011)
P. Pfleiderer, S.J. Baik, Z. Zhang, E. Grelet, C. Clasen, J. Vermant, A sliding-plate microgap rheometer for simultaneous use with small-angle x-ray scattering (SAXS) in the vorticity direction, Rheologica Acta, submitted.
Laurence de Viguerie, Rabea Keller, Ulrich Jonas, Rodiger Berger, Christopher G. Clark, Jr., Christopher Klein, Thomas Geue, Klaus Mullen, Hans-Jorgen Butt, Dimitris Vlassopoulos; "Effect of the Molecular Structure on the Hierarchical Self-Assembly of Semifluorinated Alkanes at the Air/Water Interface", Langmuir 27 (14), 8776-8786 (2011)
Tom Verwijlen, Paula Moldenaers, Howard A. Stone and Jan Vermant, Study of the Flow Field in the Magnetic Rod Interfacial Stress Rheomete, Langmuir, 27, 9345-9358 (2011)
Editorial: dynamics and rheology of complex fluidâ??fluid interfaces Gerald G. Fuller and Jan Vermant Soft Matter, 2011,7, 7583-7585, See entire issue - Themed issue initiative edited by nanodirect.
Luna Imperiali, Ken-Hsuan Liao, Christian Clasen, Jan Fransaer, Christopher W Macosko, and Jan Vermant, Interfacial rheology and structure of tiled graphene oxide sheets, Langmuir, Volume 28, 7947-8280 (2012)
July C, Kleshchanok D, Lang PR. Shear-affected depletion interaction. Eur Phys J E Soft Matter. 35(7):60 (2012)
Fuller, G. G., and Vermant, J. Complex Fluid-Fluid Interfaces: Rheology and Structure. Annual Review of Chemical and Biomolecular Engineering, 3(1), 519-543 (2012).
Potential Impact:
Nanodirect was in line with the objectives of the NMP theme in trying to improve the competitiveness of European industry in the area of nanomaterials. The project generated a knowledge-based approach to the rational design of new, nanoparticle based materials. The project was intended to be 'enabling science' which has potential applications in a variety of disciplines. With the proposed toolbox companies and research institutes can now go beyond the current state of the art, and this in several areas, to present the European industry with a competitive edge. Nanodirect did not promise to bring a quantum leap in technology. The research evolved from the work on colloidal materials, yet going well beyond the current state if the art. Many of the results will also be useful in more traditional areas of colloid science and technology, which are sometimes classified as 'soft nanotechnology'
As laid out in the work program, the objectives were threefold:
- Different roadmaps or strategies for directed assembly and novel materials design have been compared, with an assessment on their potential for scale-up. As discussed in the S&T sections, some methods such as the electric field assembly, while easy to use in the colloidal domain (100 nm to 1 micrometer remain cumbersome to use for nanoparticles (well below 100 nm). However, some other methods have surpassed the expectations, such as the use of liquid interfaces for driving self-assembly.
- Integrated building blocks, directing tools and experimental tools to test and software tools to for assessing directed self-assembly and enabling a rational basis for nanoparticle based materials design, development and processing. In order to make sure these can be used by researchers in the ERA, five out of six partners participate in an Fp7 infrastructure initiative called ESMI European soft matter infrastructure (http://www.esmi-fp7.net(opens in new window)) were the different elements of the toolbox (synthesis, simulation and experimental tools) for the different directing tools (chemistry, Flow, E fields and walls and interfaces) are made available under the infrastructure initiative of the EU.
- Creating durable partnerships with research groups and selected industrial partners: This was first achieved by establishing a users platform, consisting of leading EU companies, active over a broad range of areas. Secondly, there were exchanges of young and senior researchers between the companies and the research institutions. And finally, several of the company representatives active in the users platform took up a part time positions in some of the participating universities (1 day/week), typically delegated by their company, as this proved to be an efficient route for exchanging information and disseminating results. In some cases the companies requested more of a consulting activity for the future interactions with the nanodirect benificiaries.
The potential impact may occur in different sectors of industrial activity. The results within Nanodirect show the roadmaps for generating specific structures, mainly with tailored optical or mechanical properties. Currently, there are some specific applications being discussed with some members of the users platform and also some ideas are being further explore with companies outside of the users platform, after the first right for refusal was exercised. However, the impact of nanodirect can mainly be expected in the longer run, as the toolbox starts being used. Nanodirect research was focused on creating structures and the canopy of possibilities can now be used for designing materials with tailored properties.
Main dissemination activities
As nanodirect was aimed to be enabling science, disseminating research results through publications and conference contributions was one of the most important aspects. There was a special effort to have joint publications between partners to emphasize to the outside world the coordinated, collaborative effort. Also young researchers were given the ability to attend international conferences. More than 100 papers were published or are in press and several more can be expected in the next few months. More than 20 (and in the end probably more than 30 papers will be joint publications) Often these results appeared in high-impact factor journals (Nature, JACS, PRL, Ang. Chem., ACS Nano, Nanoscale, J. Materials chemistry, Langmuir).
As special highlights form nanodirect dissemination we could cite:
A joint review was written between 2 partners of Nanodirect on directed self-assembly of nanoparticles in the leading journal ACS Nano. It was one of the top 3 most read, most downloaded papers in both 2010, 2011 and is an ISI highly cited paper with already close to 200 citations.
There was a themed issue in SOFT matter on WP5 related topics : Dynamics and Rheology of Complex fluid-fluid interfaces, with 3 Nanodirect contributions ( Editorial: dynamics and rheology of complex fluid fluid interfaces , Gerald G. Fuller and Jan Vermant Soft Matter, 2011, 7, 7583-7585) and a themed issue on Bridging the Gap between Hard and Soft Colloids has been published. (Editorial bridging the gap between hard and soft colloids Dimitris Vlassopoulos and Michel Cloitre Soft Matter, 2012,8, 4010-4013)
From the workpackage 5 there will be a textbook on complex fluid-fluid interfaces: rheology and structure. This book will be published with Oxford University press, and the manuscript will be submitted in the spring of 2013.
Main public events organized by Nanodirect
1. Workshop on Synthesis and Surface Modification of Nanocolloids a Topical Meeting WP2, Baiona, Spain - 18-20 February (2009): The aim of the workshop was to create an opportunity to discuss research and future work perspectives in the field of colloidal synthesis of nanoparticles and their applications.
2. International Workshop on Rheology and Structural Design of Complex Fluids, Leuven, February 7th and 8th, 2011, The meeting took place in the Arenberg Castle on the Campus of the K.U. Leuven and attracted 60 participants, with half of the oral contributions from academia and the other half from industry.
3. A summer school, with participation of all nanodirect on "Directed assembly of Nanoparticles". The aim of the short course was to give participants an overview of the different approaches. The fundamentals and a critical analysis of the approaches were offered. Lecturers included L. Liz-Marzan (U Vigo), D. Vlassopoulos (FORTH), C. Clasen (KU Leuven),J. Vermant (KU Leuven), J.K. Dhont (FZ-Jülich), G. Gommper (FZ-Jülich), W. Briels (Twente) and A. Imhoff Utrecht). The meeting was held July 10-11th 2012 in Baiona, Spain.
Overall, there were more than an impressive 60 invited conference contributions by Nanodirect members, 50 oral contributions and more than 20 posters. 2 senior researchers and 1 junior researcher received an ERC grant and there were 2 conference awards and a publication award related to Nanodirect research. The results were actively disseminated, at the highest levels of our field and with impressive quantitative and qualitative output.
Exploitation of results
Nanodirect provides the European industry with a toolbox for structuring materials in new way. These materials are becoming the first step in increasing the value of products and their performance, rather than focusing the production steps. Multifunctional surfaces and materials with tailored properties and predictable performance based on directed self-assembly of nanoparticles can be achieved. Focus during the last 2 years was on identifying potential applications. During the annual project meetings and the workshops, the members of the users platform presented valuable input. Nanodirect as a project focused heavily on creating structures with controlled anisotropy and identifying pathways to achieve those structures. Work is still needed in translating these results on structures in specific end user properties, but this was not the focus of the participants within nanodirect. Nevertheless, the following areas of applications have been identified:
- Photonic, plasmonic and other optical applications, mainly using the metallic nanoparticles or ordered suspensions.
- Rational formulations strategies for tailoring the rheological properties using non-spherical particles, in shear and extensional flows
- Methodologies to address the long time dynamics and the shelf life of concentrated products.
- Interface templating of graphene based materials to make conductive coatings or 3D structures.
- Using interfacial rheology and controlling interaction between particles as a tool to rationally design materials such as emulsions, foams and high internal phase emulsions.
Several of the tools (though not all) developed within nanodirect have been made available to the wider public through the FP7 infrastructure project ESMI, coordinated by FZ-Julich and to which 5 of the 6 partners participate. The synthesis, experimental platform and the theory and simulation tools are made available to a wide audience, within academia and outside. In particular for the industrial interest within the framework of ESMi there have been several companies, which have used the tools developed in WP5 to look at complex fluid interfaces.
In conclusion, Nanodirect has achieved its goals as presenting enabling science and in identifying and structuring the tools and methods to direct self-assembly of nanoparticles. The speed at which the international research community has picked up research results from Nanodirect is impressive, and similar funding initiatives to Nanodirect have appeared in the US and some of the Asian countries. Nevertheless, challenges remain in translating the control that can be obtained over structures, to generating materials with specific and desired properties. The latter requires the knowledge generated within Nanodirect, which is focused at the methodological level, to be combined with expertise on the material properties and challenges in specific applications. Interested parties can always contact the coordinator or specific partners within Nanodirect to explore routes for future participation.
List of Websites:
Address of the project public website, if applicable as well as relevant contact details
http://www.nanodirect.eu(opens in new window)
Coordinator:
Prof. Dr.ir. Jan Vermant.
Department of Chemical Engineering
W. De Croylaan 46, B-3001 Leuven
KU Leuven, University of Leuven
Nanodirect was a small collaborative project between universities and research centers focused on the development of a toolbox for directed self-assembly of nanoparticles. Self-assembly is a popular concept in the field of nanotechnology, especially for nanoparticles, which are one of the most prominent and promising candidates for technological applications. Self-assembly has been identified as a potentially important process where the building blocks spontaneously organize into ordered or structures driven by the desire to minimize energy. Nanodirect started from the premise that in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control may be required. An so although this may seem like a contradiction in terminis, directed self-assembly may be a viable route for self-assembly.
Nanodirect aimed at presenting the European Reseach Area with a toolbox for directed assembly, to investigate and to compare the potential of novel strategies for material design. Four different work packages each focused on a different tool for directed self-assembly. The first element of the toolbox was the use of chemistry as a directing agent, by making nanoparticles with specific shapes or providing specific sites by which they interact. We also developed a number of so called model systems for research, i.e. these are systems in which the different parameters that characterize the nanoparticles (size, shape, aspect ratio) can be varied in a controlled manner. These model systems where shared between the different partners of the consortium. These model nanoparticle systems were also such that were made to be responsive to external stimuli, such as the directing fields.
The other workpackages were aimed at investigating to what extent flow fields, electric fields and the presence of interfaces and walls could be used to direct self-assembly. Hence complementary to the use of chemistry as a directing agent, the use of different fields and macroscopic templating agents has been explored. Flow fields combined with the use of a complex suspending matrix, such as a polymer solution, lead to beautiful 1D and 2D particle trains or crystalline sheets, which have been explored experimentally and using mesoscale simulations. Electric fields can be used to generate an even richer are of structures and phases, yet it is diffult to scale this method down to small particle sizes. Very promising results have been obtained when assembling nonspherical particles at liquid liquid interfaces by the effects of surface tension
Project Context and Objectives:
Nanodirect aimed at developing a toolbox containing elements that should enable one to design and fabricate nano-based composite materials by directed self-assembly.
Synthetic chemistry is a first element of the toolbox and hence range of nanoparticle building blocks with controlled sizes, shapes and aspect ratios have been synthesized and characterized. Specifically, within Nanodirect the effects of nanoparticle shape, the directionally of interactions and the use of composite particles have being used as strategies for directed self-assembly. Gold and silver nanoparticles with a wide range of controlled shapes and sizes have been synthesized with an emphasis on the efficiency and scale-up. These metallic nanoparticles are of interest for their plasmonic properties, and concerning the latter it is crucial how these can be tailored. In order to assemble the particles in hierarchical structures, they can be either deposited on a microscopic template (e.g. CNT) or made to interact directionally by e.g. hydrogen bonds or charge attractions, which leads to clusters and a subsequent control over the optical response. The templating method clearly offers most control and leads to regular structures with very specific optical responses. The use of anisomeric nanoparticles as elements to create percolating networks at low concentrations has been explored. To this end, model systems with tunable interactions were designed and prepared, as a basis for usage in the rest of the project. Rod-like viruses were chemically modified to be either sterically stabilized or thermoresponsive. Novel types of sticky ellipsoidal latex particles were synthesized and their ability to form networks at low volume fractions was investigated as well. The nanoparticles based-gels are have been shown to build-up yield stresses at very low solid fractions. Finally, synthesis of composite colloid nanoparticles with tailored properties was also explored and shown to open a canopy of possibilities.
Within Nanodirect we also critically evaluated the experimental methods to characterize the hydrodynamic properties such as the particle size, shape and polydispersity of metallic nanoparticles. The classical electron microscopy measurements only yield geometric aspect ratios and the statistics are usually limited. Plasmon-enhanced light scattering methods turn out to be rather insensitive. A method employing time resolved flow induced dichroism turned out to be well applicable to metallic nanoparticles and yields robust results. The effects of the hydrodynamic properties on the diffusivity of nanoparticles with a large aspect ratio have been investigated, both theoretically and experimentally.
Complementary to the use of chemistry as a directing agent, the use of different fields and macroscopic templating agents has been explored. The effects of flow fields for directed self-assembly in dense nanoparticle suspensions, the effects of flow on suspensions of non-spherical particle suspensions or the potential for exploiting the anisotropic viscoelastic stress induced by a shear flow to direct assembly of particles have been studied. This is carried out experimentally as well as using coarse-grained numerical simulations. Some striking observations include the directed self-assembly of particles into spectacular strings and as yet unreported 2D crystalline sheets inviscoelastic media. These methods may be easily applicable during rather standard (polymer) processing operations. Both the experiments and the mesoscale simulation methods have pointed to the important role of shear thinning of the matrix fluid and a rather subtle dependence on the elastic properties and overall level of viscosity of the fluid, yet a clear and final insight is still lacking. It has also been investigated how the DSA by viscoelastic stresses can be used in applications, taking the case study of a suspension of protein particles assembled into fibers without denaturation of a protein.
The effects of the interactions with solid walls and the use of fluid-fluid interfaces to direct self-assembly during deposition or in convective self-assembly is the final element of the toolbox. For the interactions with solid walls, novel experimental setups, such as total internal reflection fluorescence microscopy, evanescent wave dynamic light scattering and the flexure microgap rheometer have been developed or adapted to nanoparticle systems. Using the evanescent wave light scattering, rotational and translation motion of particles (hard spheres and fd-virus) close to a solid wall have been studied. The interactions have been shown to be weak. Theoretical work has focused on the interactions of both spherical and rod-like particles with a wall in order to rationalize the experimental findings. So far, simulations were also possible for dilute systems, and experiments on concentrated suspensions. The TIRFM technique has been used to provide detailed results on the interplay between walls, anisotropic nanoparticles and the depletion forces, which may contribute to our understanding of how to deposit particles onto solid walls. This is being further investigated.
For the liquid-liquid interfaces surface rheological techniques have been further advanced and the focus has been on studies of the effect on how to exploit the effects of particle shape in dictating dynamics, structure and rheological properties of particles at interfaces. Both rod- like and disk-like particles have been studied and both show very promising results for making high interface materials (such as foams, films, emulsions) with tailored properties. Especially mechanical properties of interfaces can be controlled to a large extent by exploiting particle shape effects. Moreover the geometry of multiphase mixtures can be used as a scaffold for nanoparticle assembly, for both spherical and non-spherical shapes. This is being investigated further experimentally for graphene oxide sheets. Methods to develop hierarchically structured interfaces and the use of directionally interacting particles at interfaces are currently being explored.
The expected impact of Nanodirect is that it provides a knowledge-based approach to directed self-assembly by coupling nanoparticle tunability and external stimuli. More specifically, the development of novel experimental tools, model systems and simulation methods results in a rational approach to design nanoparticle-based materials with controlled structures. Collaboration with an industrial advisory board, exchanges of young researchers with industry and the detachment of scientist form industry to academic labs explores the potential applications for a broad range of industries.
Project Results:
1. Chemistry as toolbox
During the four years of NanoDirect, the work within WP2 has focused on the characterization and up-scaling of existing colloidal nanoparticle systems, the development of novel nanoparticles with directional interactions and the study of phase behavior and self-assembly directed by various parameters.
Up-scaling has been demonstrated for four different nanosized systems showing directional interactions. In summary: the formation of Au nanorods, based on surfactant-directed seeded growth and which is usually limited to a few mg, has been optimized for gram-scale production with a tight control of dimensions and aspect ratio; disk-like particles of Au, Ag and Cu were also prepared by colloid chemistry methods, within a wide range of dimensions and achieving in all cases sub-gram scale production, which allowed us to carry out various studies within other work packages (in the case of Au plates, the use of well-defined smaller Ag plates was essential to form particles with larger aspect ratios); fd-virus was grown and purified following standard biochemical protocols, which can be multiplied to obtain gram-scale production, followed by surface modification; ellipsoidal polystyrene or PMMA particles were prepared by controlled stretching of monodisperse spherical particles in polyvinyl alcohol (PVA) thin films, upon mild heating, achieving large scale production (through parallel processing) with aspect ratios ranging from 1.25 to 10 and less than 1% of polydispersity. A similar procedure with biaxial stretching can be used for large-scale production of nanodisks.
Directional interactions
Surface modification has allowed the design of different strategies for chemistry-directed interactions. Examples are the following:
Gold nanorods with hydrogen-bonding directed interactions. Gold nanorods were selectively modified on their tips with pH sensitive bifunctional molecules, such as cysteine (CYS) or mercaptophenol (MPh), which bind to gold surfaces and are likely to form intermolecular hydrogen bonding. End-to-end directed interactions reach a maximum at a pH equal to the pKa of the linking molecule, in agreement with a hydrogen-bonding theory. Reversible assembly could be obtained as a function of pH.
Gold nanorods in standing superlattices. Surface modification with cationic gemini surfactants and interfacial aggregation lead to spontaneous formation of self-assembled supercrystals of standing nanorods, with directional optical properties.
Conclusion: The interplay between tailoring shape and using macro- or microscopic templating is the most promising route for directed self-assembly, but there is an entire universe of methods which can be used as can be seen in figure 2.
Phase behavior studies
Selected systems were used for phase behavior studies. For example, fd-viruses were studied upon surface modification with pNIPAM, so that the gelation behavior could be studied and tuned by combining the ionic strength, particle concentration, polymer structure, temperature and electric field (studies in WP4). Silica rods were used to study the phase behavior of shorter rods, with the advantage that the phase behavior could be studied both in real and reciprocal space. Additionally, well-defined platelets served to find different types of nematic phases, including a biaxial nematic, which had been predicted but not found before. On the other hand, telechelic polymers were used to study mixtures of large star (soft) and small hard spheres, which showed the existence of multiple glassy states.
Characterization tools for nanoparticles
As particle shape plays an important role in controlling the optical, magnetic, and mechanical properties of nanoparticle suspensions as well as nanocomposites. However, characterizing the size, shape, and the associated polydispersity of nanoparticles is not straightforward. Electron microscopy provides an accurate measurement of the geometric properties, but sample preparation can be laborious, and to obtain statistically relevant data many particles need to be analyzed separately. Moreover, when the particles are suspended in a fluid, it is important to measure their hydrodynamic properties, as they determine aspects such as diffusion and the rheological behavior of suspensions. The classical approach to measuring the hydrodynamic properties for such nanoparticles is using dynamic light scattering (DLS). This method required both the polarized and depolarized light to be analysed. Moreover, the observable property, i.e. diffusivity, has a logarithmic dependence on the aspect ratio. The response of a very dilute suspension of particles to flow, and measuring the response to a transient flow field using linear dichroism has been shown to be a more reliable method to obtain hydrodynamic aspect ratio and polydispersity for nonspherical gold nanoparticles for two reasons. First, the use of the evolution of the orientation angle makes effects of polydispersity less important. Second, the use of an external flow field gives a mathematically more robust relation between particle motion and aspect ratio, especially for particles with relatively small aspect ratios.
Conclusion: Novel characterization methods have been explored, focusing on anisotropic nanoparticle systems and their composites. Brilluoin scattering remains a highly specialized technique. DLS and rheo-optical methods are readily available and offer new possibilities for nanoparticles compared to electron microscopy especially regarding dynamical properties.
Selected References resulting from the work of WP2:
Sanchez-Iglesias, M. Grzelczak, B. Rodriguez-Gonzalez, P. Guardia-Giras, I. Pastoriza-Santos, J. Perez-Juste, M. Prato, L.M. Liz-Marzan, "Synthesis of Multifunctional Composite Microgels via In-situ Ni Growth on PNIPAM-Coated Au Nanoparticles", ACS Nano 2009, 3, 3184-3190
Zhang, Z.K. N. Krishna, M.P. Lettinga, J. Vermant, and E. Grelet, "Reversible Gelation of Rod-Like Viruses Grafted with Thermoresponsive Polymers", Langmuir 2009, 25, 2437-2442
M. Grzelczak, J. Vermant, E.M. Furst, L.M. Liz-Marzan, Directed Self-Assembly of Nanoparticles, ACS Nano 2010, 4, 3591-3605.
Z. Zhang, J. Buitenhuis, A. Cukkemane, M. Brocker, M. Bott, J.K.G. Dhont, Charge reversal of the rodlike colloidal fd virus through surface chemical modification, Langmuir 2010, 26, 10593-10599.
A.Sanchez-Iglesias M. Grzelczak, J. Perez-Juste, L.M. Liz-Marzan, Binary Self-Assembly of Gold Nanowires with Nanospheres and Nanorods, Angew. Chem. Int. Ed. 2010, 49, 9985-9989
Zhenkun Zhang, Patrick Pfleiderer, Andrew B. Schofield, Christian Clasen, and Jan Vermant, Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles J. Am. Chem. Soc. 2011, 133, 392-39
Voudouris, P, Choi, J, Gomopoulos, N, Sainidou, R, Dong, HC, Matyjaszewski, K, Bockstaller, MR, Fytas, G. "Anisotropic Elasticity of Quasi-One-Component Polymer Nanocomposites", ACS NANO 5, 5746-5754 (2011).
J.M. Romo-Herrera, R.A. Alvarez-Puebla, L.M. Liz-Marzan, Controlled Assembly of Plasmonic Colloidal Nanoparticle Clusters, Nanoscale 2011, 3, 1304-1315.
N.K. Reddy, J. Perez-Juste, I. Pastoriza-Santos, P.R. Lang, J.K.G. Dhont, L.M. Liz-Marzan, J. Vermant, Flow Dichroism as a Reliable Method to Measure the Hydrodynamic Aspect Ratio of Gold Nanoparticles, ACS Nano 2011, 5, 4935-4944
E. Antoniou, P. Voudouris, A. Larsen, B. Loppinet, D. Vlassopoulos, I. Pastoriza-Santos, L. M. Liz-Marzan, Dispersions of Polymer-Grafted Silver Nanoprisms in the Bulk and Near Solid Surfaces", JOURNAL OF PHYSICAL CHEMISTRY C 116, 3888-3896 (2012).
Reddy, N., Zhang, Z., Lettinga, P., Dhont, J., Vermant, J. Probing structure in colloidal gels of thermo-reversible rod-like virus particles: Rheology and scattering. Journal of Rheology, 56 (5), 1153-1174 (2012).
2. Directed self assembly by flow.
During the four years of NanoDirect, the work within WP3 has focused on the using flow for directed self-assembly. This report will first discuss the different tools developed; both experimental platforms and mesoscale simulation techniques, and subsequently the obtained results will be discussed.
Equipment and simulation tools developed.
We also built a flexture-based microgap rheometer which enabled studying fluids in either bulk flows or in confinement. For bulk flows, the setup has the advantage that it can be combined with X-ray scattering from the vorticity plane, i.e. the plane in which deformation and orientation of the micro and nanostructures can be observed.
Flow of complex systems is closely linked to the rheology of nanoparticle based suspensions and nanocomposites. Within Nanodirect two aspects related to the rheology were studied. First rheologically complex fluids were used as media in which substantial normal stresses develop. The idea is that flow induced anisotropy can be used to direct structuring was investigated. Secondly, the use of rheology for densen suspensions of nanocomposites required us to adapt tools, both in terms of measurment geometry and methodologies. For example, A cone-partitioned-plate fixture has been developed and validated in order to be able to reliably measure nonlinear shear rheology in highly elastic nanocomposite systems.
To study dense nanosuspensions; and in particular to understand how these systems rearrange and structure over long times (related to shelf life), the use of large amplitude oscillatory shear (LAOS) has received quite some attention, Rheology is blind, but offers the advantage that it does not suffer from the requirements for optical contrast (either too little or too much). Monitoring the higher harmonics in the stress signal by Fourier-Transform (FT) rheology may provide useful insight on the progressive transition from linear to nonlinear viscoelastic response. However, the physical interpretation of FT-rheology data is still not obvious. For example the process of yielding in a colloidal glass formed by star-like nanoparticles was studied and the underlying mechanisms are cage breaking and reformation as well as stress storing and relaxation within the period of oscillatory shear which are affected by an interplay between shear and Brownian motion and thus relate with Peclet number variation with strain and frequency An entirely new way of analyzing linear and nonlinear oscillatory material has also been developed. A new quantitative sequence of physical processes (SPP) method views generic oscillatory responses within the Frenet-Serret frame as sequences of planar 2D curves embedded in the 3D space defined by the strain, strain rate, and stress axes. This offers a possibility to understand in more detail the physical processes taking places when the deformation amplitude is increased and the microstructure is altered by flow.
Finally, to study the hydrodynamic forces in complex fluids, which is important to understand the DSA by viscoelastic forces, a standard AFM was modified in order to take a closer look at the effect of fluid rheology on lubrication forces.
MESOSCALE SIMULATION TECHNIQUES: The properties of soft matter- nanoparticle systems are mainly determined by their mesoscopic structure. Due to the large separation of length and time scales between the atomic scale of the solvent and the mesoscale of the solute, the simulation of the dynamical behavior of such composite soft matter isparticularly challenging. Traditional molecular dynamics simulations are often inappropriate because of the unaffordable simulation time, especially when hydrodynamics needs to be taken into account. For a colloidal suspension in a viscoelastic solvent, e.g. proteins in a cell, or spherical or rod-like colloids dispersed in polymer solutions the situation is even more complex. The solvent itself is now a complex fluid too. To be able to address and unravel dynamical and rheological properties of such complex systems requires a simplified mesoscopic model of a viscoelastic solvent, preferentially in combination with a mesoscopic hydrodynamic simulation technique such as the multiparticle collision dynamics (MPC) approach or the and Responsive particle dynamics.
The techniques differ in the level of coarse graining and in the size of the systems that can be simulated. The MPC technique obtains a higher efficiency in reproducing the viscoelasticity of the solvents using molecular models, or at least being able to incorporate more of physical insight into the real system. The silulations are computatonially extremely efficient, and here is even an on-line real time demo of the code which can be tried out at:
http://www.fz-juelich.de/ics/ics-2/EN/Forschung/HydrodynamicsSimulation/_node.html(opens in new window)
Conclusion: Several techniques, mainly related to the rheological properties, were further developed and adapted to nanoparticle systems, but are more generally applicable. The use of LAOS has proven to be of use mainly for dense colloidal systems, where simple physical processes underly the response dynamics. Specific tools (to address issues in the experiments on nanocomposite systems were developed. A major effort has been the development of mesoscale simulation methods, required to bridge the wide range of length and time scales in soft matter composites containing nanoparticles. Two simulation approaches were explored, one deterministic and particle collision based (MPCD), the other based on Brownian dynamics and coarse grained. MPCD is a tool oriented at a fundamental understanding of the phenomena, whereas Rapid can be sued to explore a broader range of systems.
Directed self-assembly of rod like particles
Suspensions of rodlike nanoparticles and their solidlike response at extremely low volume fractions can be exploited technologically. Understanding the link between the physicochemical parameters such as size, aspect ratio, volume fraction, and interparticle forces with the resulting microstructure and the subsequent rheological response remains challenging. Within Nanodirect, suspensions of monodisperse rodlike virus particles, whose surface is modified by grafting with a thermoreversible polymer poly(N-isopropylacrylamide) in WP.2 are used as a model system. The repulsive and attractive contributions to the total interaction potential can be changed independently by varying the ionic strength and the temperature. The anisotropic nature of the electrostatic potential may result in different self-assembled microstructures. T. Rheological measurements of the near critical gel properties as a function of concentration and ionic strength proved to be more sensitive compared to scattering in resolving the structural differences, as characterized by a gel strength S, and a structural parameter n. A percolating structure is formed at very low volume fractions (we believe to be the world record holder for gel formed on earth, zero gravity experiments having gone lower), which show a weak dependence on the ionic strength with the anisotropy of the repulsive interactions playing the main role in creating more open structures. Surprisingly, the intrinsic stiffness of the rodlike particles does not affect the moduli of the gel states very strongly.
For stable rodlike suspensions, the use of orientable rod like particles as viscosity modifiers may well be more interesting as a tool to modify the dynamics of the suspensions. This was investigated by again using fd-virus nanoparticles in extensional flow. Another way to strengthen the efficiency of flow is to increase the concentration of particles. Real space confocal studies have shown for the first time the effects of flow on the organization of rodlike particles subjected to flow. To date, these results have not yet been confirmed for nanoparticles, but the governing parameter has been shown to be the rotational Peclet number, and everything can be expected to scale with the rotational diffusivity. This explains how shape anisotropy and size will intervene, and this method will be applicable for nanoparticles.
Directed self-assembly by viscoelastic stresses
Experiments on directed self-assembly in viscoelastic liquids were carried out to investigate the usefulness. As reported sporadically in the literature, due to the changes in hydrodynamic interactions, string-like structures can be observed in sheared viscoelastic fluids. To clarify the influence of the fluid rheology and to understand the mechanism of chaining formation, optical microscopy observations as well as small angle light scattering (SALS) experiments were performed using parallel plate geometry. Suspensions of monodisperse polystyrene spheres were dispersed in shear thinning viscoelastic liquids. Aligned strings of particles were observed when shearing faster then a critical shear rate, which was found to be independent of particle size. However, the role of particle migration was found to be of prime importance and migration effects were quantified.
To understand the fundamental underlying the chaining, three types of experiments have been performed. On the one hand, studies on single particle dynamics have been used both experimentally and using the MPCD simulations, as well as standard continuum mechanical calculations. Depletion interactions were induced to measure the interaction strengths and AFM measurements to quantify the hydrodynamic interactions. All these results combined suggested that the degree of shear thinning is the controlling parameter, although this awaits experimental verification. The mesoscale simulation methods were also used to explore certain aspect further in detail. For example, the effect of using bidispersed sized particles and effects of polydispersity were investigated. These showed effects of segregation according to size, potentially this could be exploited in size sorting method.
Directed assembly in emulsion droplets
By producing a monodisperse emulsion by means of a viscoelastic monodisperse emulsion, nanoparticle clusters with controlled morphology could be produced. For nanoparticle assembly using emulsion droplets the following approach was taken. A nanoparticle-loaded oil is emulsified in water using a surfactant. The emulsification process breaks up initially large droplets into smaller ones, each carrying their nanoparticle payload. In a final step the hexane oil is slowly removed by evaporation, forcing the particles to assemble into clusters, the size of which can be controlled via droplet size and nanoparticle loading. Clusters were made containing as many as hundreds of nanoparticles, or as few as two. Control over cluster size will be afforded by a judicious choice of the viscoelastic properties that control droplet breakup. In a later stage this approach will be combined with a controlled shear to try to obtain exquisite control over the cluster uniformity.
Conclusion: Flow strength constitutes an important and processing related parameter to direct the structuring of nanoparticle suspensions. For rodlike particles, orientation can be induced by flow and the interplay between directional interactions and flow structuring can be used to tailor the microstructure and the resulting properties at will. These systems have potential as rheology modifiers, either to impart solid like rheological properties at low volume fractions or to affect the extensional flow properties. For nanoparticles these aspects could then be achieved while maintaining optical transparence.
Selected References resulting from the work of WP3:
Zhang, Z.K. N. Krishna, M.P. Lettinga, J. Vermant, and E. Grelet, "Reversible Gelation of Rod-Like Viruses Grafted with Thermoresponsive Polymers", Langmuir, 25(4), 2437-2442 (2009)
R.Pasquino F.Snijkers N.Grizzuti and J.Vermant Directed self-assembly of spheres into a two-dimensional colloidal crystal by viscoelastic stresses, Langmuir, 26 (5), pp 3016-3019 (2010)
S. Rogers, P. T. Callaghan, G. Petekidis and D. Vlassopoulos,Time-dependent rheology of colloidal star glasses, J. Rheology, 54, 133-158, (2010)
Effect of Viscoelasticity on the Rotation of a Sphere in Shear Flow F. Snijkers, G. D Avino, M. Hulsen F. Greco, Pier Luca Maffettone,J. Vermant, J. Non. Newt. Fluid. Mech., 166, 363-372 (2011).
Anke Kuijk, Alfons van Blaaderen, and Arnout Imhof Synthesis of Monodisperse, Rodlike Silica Colloids with Tunable Aspect Ratio, J. Am. Chem. Soc., 133 (8), pp 2346-2349 (2011)
S. A. Rogers, B. M. Erwin, D. Vlassopoulos, and M. Cloitre, -Oscillatory Yielding of a Colloidal Star Glass, J. Rheol., 55, 733-752 (2011).
S.C. Ji, R. Jiang, R.G. Winkler, and G. Gompper, Mesoscale hydrodynamic modeling of a colloid in shear-thinning viscoelastic fluids under shear flow.J. Chem. Phys. 135. 134116 (2011).
I. S. Santos de Oliveira, A. van den Noort, J. T. Padding, W. K. den Otter and W. J. Briels. Alignment of particles in sheared viscoelastic fluids. J. Chem. Phys. 135, 104902 (2011)
Frank Snijkers and Dimitris Vlassopoulos , Cone-partitioned-plate geometry for the ARES rheometer with temperature control J. Rheol. 55, 1167 (2011).
G. D'Avino F. Snijkers R. Pasquino M.A. Hulsen F. Greco P.L. Maffettone J. Vermant Migration of a sphere suspended in viscoelastic liquids in Couette flow: Experiments and simulations Rheoogica Acta, 51: 215-234 (2012)
Simon A. Rogers, A sequence of physical processes determined and quantified in LAOS: An instantaneous local 2D/3D approach, J. Rheol. 56, 1129 (2012)
Naveen Krishna Reddy, Zhenkun Zhang, Jan Vermant, M. Paul Lettinga and Jan K. G. Dhont, Probing Structure in Colloidal Gels of Thermo-Reversible Rod-Like Virus Particles: Rheology and Scattering Journal of Rheology, 56, 1153 (2012)
3. Directed self assembly by electric fields.
During the four years of NanoDirect, the work within WP4 has focused on the using mainky AC electric fields for directed self-assembly.
We studied the phase behavior and pattern formation of colloidal particles in AC electric fields as this does not lead to electrophoresis of the particles, which would merely deposit them on one of the electrodes (electrodeposition). Furthermore, we now have two parameters, field strength and field frequency, to vary. Another advantage of using an AC electric field is that it acts by dielectric polarization, which will work on any kind particle, or by electric double layer polarization, which will act on most particles. Various new methodologies were developed to probe the assembly of colloids in electric fields: We used various light scattering techniques, confocal microscopy, and computer simulation. New instruments were developed, in which electric fields can be applied and particle assembly can be monitored. Directed assembly of particles makes use of the following three principles:
1.Orientation of (nonspherical) particles by alignment of their induced dipole moment with the external electric field.
2.Alignment of particles by the mutual dipole-dipole interaction of their induced dipole moments.
3.Migration of particles by the action of a field gradient on the induced dipole moment (dielectrophoresis).
Experimental methods to study the assembly in electric fields by light scattering and optical (confocal) microscopy have been adapted and developed. The samples cells consisted of ITO coated glass slides which are connected to external function generators; Structure formation taking place inside the cell in response to the electric field can be observed by placing the cell on a microscope, or interrogating it using scattering techniques such as dynamic light scattering (DLS) or birefringence
Simulation tools: Theory and simulations tools were developed to describe the structures observed using electric field directed self-assembly, as well as to predict that should be obtainable in the large parameter space of the experiments. Simulations were done using the Monte Carlo method in the NVT ensemble, at low packing fractions. To handle the long-range dipolar interactions, we use Ewald summations with conducting boundary conditions. To improve equilibration and sampling speed in the systems with strings, cluster moves were introduced to move particles in a cylindrical volume collectively
Results of DSA by electric fields.
Experiments were done to study the assembly of particles of various shapes (rods, plates, and in some cases spheres) directed by external fields. For Rods, the fd-virus nanoparticles from WP2 were used and Various phases, patterns and dynamical states have been found At low field amplitudes there is equilibrium between non-chiral nematic and isotropic regions; the N-phase. At larger field amplitudes there is a transition to coexistence between a non-chiral nematic and chiral nematic; the N-phase. The large non-chiral domains break up to smaller, disconnected domains in the N*D-phase. The chiral texture disappears and the non-chiral nematic domains melt and form within the dynamical state Ds. In the dynamical state Df, melting and forming of nematic domains is very fast. At high frequencies, in the H-phase, the system becomes homogeneous, and the rods are aligned along the electric field. The N-phase directly transforms to the H-phase at high frequency on increasing the field amplitude and the H-phase transforms to a dynamical state at higher field amplitudes.
For shorter silica rods or dumbells in an AC field, more detailed insight into the nature of the microstructures could be obtained. Here the true power of directed self-assembly was shown. Without an electric field the particles did not assemble into any ordered phase on any appreciable time scale. To assemble the colloidal dumbbells a high frequency electric field to induce dipolar interactions between them. This readily led to ordering of the particles. Similar, albeit less striking results were obtained with the silica rods. Another experimental model system is one that consists of very thin (20 nm) gold plates. Although the plates are not very monodisperse in diameter (between 500 and 1000 nm), they readily form stacks due to the secondary minimum in the DLVO potential. Using an electric field, columns can be formed regardless of ionic strength. Here, the columns run perpendicularly to the electric field. This means that the individual plates are aligned parallel to the field with their long axis. After some time, the columns continue to assemble side by side into broad bands. This still requires only modest field strengths of around 10 V/mm. The strong plasmon absorption of the plates causes a strong polarization dependent extinction by these structures. Turning off the field causes the platelets to rapidly disperse.
Combination of electric bottle and shear flow
A system of rodlike silica particles with an aspect ration of 6 was studied in shear flow at a number of different volume fractions. The particles had a diameter of 550 nm and a length of 3300 nm, were labelled with a layer of fluorescent dye (fluorescein, FITC), and were refractive index matched in a mixture of water and DMSO (dimethylsulfoxide) in which they are charge stabilized. The electric bottle incorporated into a parallel plate shear cell offers a convenient way to increase the volume fraction gradually, such that the shear can be applied to the same system in the same experiment.
The behavior of this system was studied at increasing volume fractions where a different phase is found in the absence of shear. Upon increasing the shear rate we found the following progression of shear induced structures:
Isotropic to paranematic
Nematic to oriented nematic
smectic to oriented smectic to oriented columnar to small Sm/Col domains to paranematic
Clearly, the most elaborate succession of structures is found in the smectic. Snapshots of the structures in the plane of zero velocity illustrate this. On the whole, the order in the structure diminishes with increasing shear rate, but with interesting new structures being formed. The results demonstrate the utility of the instrument in studying structures in a dispersion of rodlike particles directed by shear flow.
The work carried out in Nanodirect Work package 4, demonstrated means to assemble of both spherical and nonspherical colloidal particles by applying external electric fields. Instruments were developed to study this assembly using confocal microscopy and light scattering techniques. The sample cells apply AC electric fields of the desired geometry to make use of polarization of the particles by the electric field, which orients them parallel to the field. The induced dipole moments subsequently lead to modified mutual interactions that may induce order among particles. In many experiments the particles used were around a micron in length and several hundred nanometer in length so that they could be studied in more detail. For these particles use can be made of the dielectric polarization of the particles in the external field, which is proportional to the particle volume. For particles below about 100x100x100 nm3 in size, however, electric double layer polarization is still strong enough to allow their assembly to be directed by external fields, as is shown by the experiments on fd virus. This is effective at field frequencies in the range of 10Hz - 10kHz.
A comparison with theory and computer simulations shows that interactions can be modeled well by the theory given, even though it is approximate for interacting particle systems. Also the nanoparticle assemblies are reproduced well in the simulations. With this understanding it becomes possible to predict the nanostructures that could be obtained. This is important because the experimental work shows that the number of different ordered structures in relatively simple shaped particles is already very large.
Selected References resulting from the work of WP4:
Kang, K., Dhont, J. K. G., Eur. Phys. J. E 30, 333 (2009).
Kang, K., Dhont J. K. G., Soft Matter 6, 273 (2010)
Demirörs, A. F., Johnson, P. M., van Kats, C. M., van Blaaderen, A., Imhof, A., Langmuir 26, 14466 (2010).
Kuijk, A., van Blaaderen, A., Imhof, A., J. Am. Chem. Soc. 133, 2346 (2011).
Kuijk, A. , Byelov, D. V., Petukhov, A., van Blaaderen, A., Imhof, A., Faraday Disc. (accepted); DOI: 10.1039/C2FD20084H
4. Directed Self-assembly by walls and interfaces
During the four years of NanoDirect, the work within WP5 has focused on the use of interfaces and wall to template or direct self-assembly.
Particles at interfaces
Equipment developed: One of the intriguing features of nanoparticle laden interfaces is the appearance of significant mechanical properties imparted to a monolayer. Typical measurement techniques for measuring the properties of complex fluid-fluid interfaces have been designed for relatively simple fluid surfaces and often focused on surfactants. Particle laden interfaces turned out to behave as insoluble and elastic interfaces. To study these, properly designed surface rheometers measure both the stress and deformation history for fluid interfaces at which the constitutive property is not known a priori.
Sensitive techniques for measuring the shear rheology of (nano)particle-laden interfaces have been developed a constitute an important part of the toolbox for studying DSA at fluid-fluid interfaces. They have been validated, theoretically underpinned and tested by different members of the consortium. There is still a need for developing adequate tools for dilatational rheology, although some progress has been made. Also future work should focus on developing adequate constitutive models to describe the performance of these interfaces in products or during processing steps. Yet the basic elements of the toolbox are available.
Directed self-assembly at fluid-fluid interfaces.
Capillary forces, arising due to the presence of a surface or interfacial tension can effectively pin particles to an interface and make their expulsion difficult. The presence of an interface adds significant complexity to the colloidal interactions. For example, compared with similar particles in a bulk liquid, the electrostatic repulsion between charged particles is greatly enhanced at a water low dielectric medium interface. At the interface a particle has an asymmetric counterion distribution that results in a dipole-dipole interaction through the low dielectric constant phase. Within nanodirect the nature of the electrostatic interaction was investigated and it was argued that the effect of the finite ion sizes of the hydrated ions on the water side need to be taken into account to arrive at quantitative predictions of the electrostatic forces.
The versatility of the technique was subsequently further explored to different types of particles. Using rough particles also enables one to generate strong interfaces, but even more spectacular results could be obtained with nanosheets. Graphite Oxide (GO) sheets were selected to be studied as surface-active particles to produce technologically relevant materials. GO nanoparticles consist of single or few layers of carbon obtained by strong oxidation of graphene. Their lateral size (approximately 10m) and thickness gives very high aspect ratios and surface to volume ratios. GO particles have excellent mechanical properties and can be reduced in situ to restore the high electrical conductivity of pure carbon sheets. Such properties make GO sheets a good candidate for the design of complex high-performing materials through interface-driven assembly. Within Nanodirect we demonstrated how these sheets assemble at the interface forming compact films of nanometer thickness. These films are highly resistant to deformation (both in compression and shear) and at the same time are extremely deformable, resembling the behavior of elastic membranes. When graphene oxide sheets are introduced in a mixture of simple liquids undergoing phase separation, particles segregate at interfaces forming a strong film that can arrest phase separation. This allows to arrest the liquid system into metastable structures, for example co-continuous emulsions stabilized by particles. This process yields 3D complex architectures of particles that can be separated from the fluids to be used for electronic applications (conductive foams, light supercapacitors, etc.)
Conclusion: Exploiting nanoparticle self-assembly at fluid-fluid interface is a very power novel method to create 1D, 2D or 3D structures. The use of lateral capillary forces to create strong assemblies has shown to be extremely useful. This method is very robust has potential to be combined with relatively classic processing methods such as coating techniques, although some work is required;.
Particles near a solid wall and directing by confinement.
Equipment developed. Two main techniques were either further developed or adapted to the use of nanoparticles for the study of nanoparticles both of them relying on the properties of the evanescent wave and how it is being scattered. We have developed the experimental tools to investigate the translational and rotational dynamics of rod like particles in the ultimate vicinity of a hard wall by evanescent wave dynamic light scattering (EWDLS) with polarization analysis. Experiments were performed on aqueous solutions of rod shaped gold particles. Anew velocimetry technique based on EWDLS to measure the near wall velocity of nano particles showed as yet unparalleled resolution.
A second technique adapted to nanoparticles was total internal reflection microscope (TIRM) was adapted and tested in order to use fluorescense instead of scattering. The advantage of using fluorescense is that much smaller colloids can be studied, that mixtures can be probed, where the component of interest is fluorescently labeled, and that self-dynamics can be studied in concentrated dispersions. We applied this method to study particle-wall interactions with anisotropic depletants In a standard TIRM experiment the scattered intensity, Is, from a colloidal sphere in the ultimate vicinity of a solid surface, which is illuminated with an evanescent wave is recorded. By analyzing the fluctuations of Is it is possible to determine the interaction potential between the sphere and the wall. Alternatively the fluorescent intensity, IF, from a fluorescently labelled sphere might be detected and analyzed accordingly. The advantages of using a fluorescent probe would be a significantly increased sensitivity and a reduced signal to noise ratio, as compared to the scattering probe. Such that the technique, which is so far limited to probes spheres with a minimum radius of about 500 nm, could also be used with much smaller spheres. However the advantages of the TIRFM concept might be counterbalanced by the fact that fluorescent dyes tend to bleach.
Conclusion: Two techniques for studying near wall-dynamics have been evaluated for use to Nanoparticles within nanodirect. Setups for EWDLS and TIRF have been adapted tested for the use with nanoparticle systems. The techniques are suitable to measure near all dynamics of both spherical and non-spherical nanoparticles. They are very sensitive but non-trivial to use. To study the interplay between walls and flows, these techniques can also be complemented with a flexure microgap rheometer, which moreover was adapted for combined use with X-rays, to study the interplay between the nanosuspension structure , flow and the effects of confining walls.
Directed self-assembly near solid walls Near Wall Dynamics of concentrated suspensions of spherical nano-particles have been studied using EWDLS and TIRF. On increasing concentration, suspensions of spherical nano particles are known to self assemble into a crystalline structure at a volume fraction of close to 0.5. For spheres, the effects of the hydrodynamic interactions with the wall reamin important even down to the nanometer scale. shaped particle assemble into a nematic phase at a volume fraction. As a first step to study the dynamic pathway of this shift, we investigated the translational and orientational near wall dynamics of dilute suspensions of gold nano rods. For this purpose the evanescent wave instrument had to be equipped with additional features to analyse the polarization dependence of the scattered light. For the microscopic understanding of flow or shear driven self assembly, it is crucial to know the velocity profile and the local shear rate in the vicinity of the interfaces confining the suspensions. We have set up a new evanescent wave velocimetry technique for the experimental determination of these parameters with an unprecedented resolution in the ten nanometer range.
To analyze the results for rods near walls, theoretical and simulation work was carried out and the effect of the walls on slowing down of the deposition rates (due to an enhanced hydrodynamic friction) were analyzed in detail, showing that even in the nanoscale regime the hydrodynamic forces are dominating. To further direct the assembly also under the influence of flow, the effective interactions between a single colloid and the solid wall in bi-disperse systems were investigated. Of special interest are mixtures where so-called depletion interactions are important, pushing the particles to the wall. Such interactions between relatively large colloids and the wall are induced by a second species of many small colloids or polymers. The potential application of such systems towards wall-induced self assembly lies in the easy tuning of the potential by means of the concentration and the size of the depletant. The resulting anisotropic, directed attraction towards the wall can induce structures at high concentrations of the large colloids. As a first step in the direction of wall-assisted self assembly, we investigated the depletion attraction of colloidal spheres as induced by disk- and rod-like colloids, also under the influence of shear flow.
Conclusion: Whereas directed self-assembly by solid walls may be of practical importance in coating and deposition processes, overall the effects are weak. The detailed studies presented here show that even in the nanoparticle regime the hydrodynamic forces dominate the near wall dynamics. Depletion forces may help to control the interaction, but flow weakens the effects, due to alignment and organization of the depletants. Confinement effects also help to promote near wall aligment or structuring, at reasonable values of the flow strength. Overall, the use of solid walls to promote self-assembly is a method with limited potential. Most possibly, walls with controlled surface energy would provide better control, but this complicates matters even more.
Selected References resulting from the work of WP5:
B.Cichocki E. Wajnryb, J.K.G. Dhont, P.R. Lang, The intensity correlation function in evanescent wave scattering, J. Chem. Phys., 132, 074704/1-12, 2010. and in Padding, J. T.; Briels, W. J. J. Chem. Phys. 132, 054511 (2010).
Steven Vandebril, Aly Franck, Gerald G. Fuller, Paula Moldenaers and Jan Vermant A double wall-ring geometry for interfacial shear rheometry, Rheologica Acta, 49:131-144 (2010).
Masschaele K, Park BJ, Furst EM, Fransaer J., Vermant J.,
Finite Ion-Size Effects Dominate the Interaction between Charged Colloidal Particles at an Oil-Water Interface â?¨ â?¨ PHYSICAL REVIEW LETTERS, 105 Article Number: 048303 (2010)
July, C.; Kleshchanok, D.; Lang, P.R. Depletion interactions caused by polydisperse platelets
Soft Matter, 7, 6444 - 6450 (2011)
Zhenkun Zhang, Patrick Pfleiderer, Andrew B. Schofield, Christian Clasen, and Jan Vermant, Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles J. Am. Chem. Soc., 133 (3), pp 392-395 (2011)
P. Pfleiderer, S.J. Baik, Z. Zhang, E. Grelet, C. Clasen, J. Vermant, A sliding-plate microgap rheometer for simultaneous use with small-angle x-ray scattering (SAXS) in the vorticity direction, Rheologica Acta, submitted.
Laurence de Viguerie, Rabea Keller, Ulrich Jonas, Rodiger Berger, Christopher G. Clark, Jr., Christopher Klein, Thomas Geue, Klaus Mullen, Hans-Jorgen Butt, Dimitris Vlassopoulos; "Effect of the Molecular Structure on the Hierarchical Self-Assembly of Semifluorinated Alkanes at the Air/Water Interface", Langmuir 27 (14), 8776-8786 (2011)
Tom Verwijlen, Paula Moldenaers, Howard A. Stone and Jan Vermant, Study of the Flow Field in the Magnetic Rod Interfacial Stress Rheomete, Langmuir, 27, 9345-9358 (2011)
Editorial: dynamics and rheology of complex fluidâ??fluid interfaces Gerald G. Fuller and Jan Vermant Soft Matter, 2011,7, 7583-7585, See entire issue - Themed issue initiative edited by nanodirect.
Luna Imperiali, Ken-Hsuan Liao, Christian Clasen, Jan Fransaer, Christopher W Macosko, and Jan Vermant, Interfacial rheology and structure of tiled graphene oxide sheets, Langmuir, Volume 28, 7947-8280 (2012)
July C, Kleshchanok D, Lang PR. Shear-affected depletion interaction. Eur Phys J E Soft Matter. 35(7):60 (2012)
Fuller, G. G., and Vermant, J. Complex Fluid-Fluid Interfaces: Rheology and Structure. Annual Review of Chemical and Biomolecular Engineering, 3(1), 519-543 (2012).
Potential Impact:
Nanodirect was in line with the objectives of the NMP theme in trying to improve the competitiveness of European industry in the area of nanomaterials. The project generated a knowledge-based approach to the rational design of new, nanoparticle based materials. The project was intended to be 'enabling science' which has potential applications in a variety of disciplines. With the proposed toolbox companies and research institutes can now go beyond the current state of the art, and this in several areas, to present the European industry with a competitive edge. Nanodirect did not promise to bring a quantum leap in technology. The research evolved from the work on colloidal materials, yet going well beyond the current state if the art. Many of the results will also be useful in more traditional areas of colloid science and technology, which are sometimes classified as 'soft nanotechnology'
As laid out in the work program, the objectives were threefold:
- Different roadmaps or strategies for directed assembly and novel materials design have been compared, with an assessment on their potential for scale-up. As discussed in the S&T sections, some methods such as the electric field assembly, while easy to use in the colloidal domain (100 nm to 1 micrometer remain cumbersome to use for nanoparticles (well below 100 nm). However, some other methods have surpassed the expectations, such as the use of liquid interfaces for driving self-assembly.
- Integrated building blocks, directing tools and experimental tools to test and software tools to for assessing directed self-assembly and enabling a rational basis for nanoparticle based materials design, development and processing. In order to make sure these can be used by researchers in the ERA, five out of six partners participate in an Fp7 infrastructure initiative called ESMI European soft matter infrastructure (http://www.esmi-fp7.net(opens in new window)) were the different elements of the toolbox (synthesis, simulation and experimental tools) for the different directing tools (chemistry, Flow, E fields and walls and interfaces) are made available under the infrastructure initiative of the EU.
- Creating durable partnerships with research groups and selected industrial partners: This was first achieved by establishing a users platform, consisting of leading EU companies, active over a broad range of areas. Secondly, there were exchanges of young and senior researchers between the companies and the research institutions. And finally, several of the company representatives active in the users platform took up a part time positions in some of the participating universities (1 day/week), typically delegated by their company, as this proved to be an efficient route for exchanging information and disseminating results. In some cases the companies requested more of a consulting activity for the future interactions with the nanodirect benificiaries.
The potential impact may occur in different sectors of industrial activity. The results within Nanodirect show the roadmaps for generating specific structures, mainly with tailored optical or mechanical properties. Currently, there are some specific applications being discussed with some members of the users platform and also some ideas are being further explore with companies outside of the users platform, after the first right for refusal was exercised. However, the impact of nanodirect can mainly be expected in the longer run, as the toolbox starts being used. Nanodirect research was focused on creating structures and the canopy of possibilities can now be used for designing materials with tailored properties.
Main dissemination activities
As nanodirect was aimed to be enabling science, disseminating research results through publications and conference contributions was one of the most important aspects. There was a special effort to have joint publications between partners to emphasize to the outside world the coordinated, collaborative effort. Also young researchers were given the ability to attend international conferences. More than 100 papers were published or are in press and several more can be expected in the next few months. More than 20 (and in the end probably more than 30 papers will be joint publications) Often these results appeared in high-impact factor journals (Nature, JACS, PRL, Ang. Chem., ACS Nano, Nanoscale, J. Materials chemistry, Langmuir).
As special highlights form nanodirect dissemination we could cite:
A joint review was written between 2 partners of Nanodirect on directed self-assembly of nanoparticles in the leading journal ACS Nano. It was one of the top 3 most read, most downloaded papers in both 2010, 2011 and is an ISI highly cited paper with already close to 200 citations.
There was a themed issue in SOFT matter on WP5 related topics : Dynamics and Rheology of Complex fluid-fluid interfaces, with 3 Nanodirect contributions ( Editorial: dynamics and rheology of complex fluid fluid interfaces , Gerald G. Fuller and Jan Vermant Soft Matter, 2011, 7, 7583-7585) and a themed issue on Bridging the Gap between Hard and Soft Colloids has been published. (Editorial bridging the gap between hard and soft colloids Dimitris Vlassopoulos and Michel Cloitre Soft Matter, 2012,8, 4010-4013)
From the workpackage 5 there will be a textbook on complex fluid-fluid interfaces: rheology and structure. This book will be published with Oxford University press, and the manuscript will be submitted in the spring of 2013.
Main public events organized by Nanodirect
1. Workshop on Synthesis and Surface Modification of Nanocolloids a Topical Meeting WP2, Baiona, Spain - 18-20 February (2009): The aim of the workshop was to create an opportunity to discuss research and future work perspectives in the field of colloidal synthesis of nanoparticles and their applications.
2. International Workshop on Rheology and Structural Design of Complex Fluids, Leuven, February 7th and 8th, 2011, The meeting took place in the Arenberg Castle on the Campus of the K.U. Leuven and attracted 60 participants, with half of the oral contributions from academia and the other half from industry.
3. A summer school, with participation of all nanodirect on "Directed assembly of Nanoparticles". The aim of the short course was to give participants an overview of the different approaches. The fundamentals and a critical analysis of the approaches were offered. Lecturers included L. Liz-Marzan (U Vigo), D. Vlassopoulos (FORTH), C. Clasen (KU Leuven),J. Vermant (KU Leuven), J.K. Dhont (FZ-Jülich), G. Gommper (FZ-Jülich), W. Briels (Twente) and A. Imhoff Utrecht). The meeting was held July 10-11th 2012 in Baiona, Spain.
Overall, there were more than an impressive 60 invited conference contributions by Nanodirect members, 50 oral contributions and more than 20 posters. 2 senior researchers and 1 junior researcher received an ERC grant and there were 2 conference awards and a publication award related to Nanodirect research. The results were actively disseminated, at the highest levels of our field and with impressive quantitative and qualitative output.
Exploitation of results
Nanodirect provides the European industry with a toolbox for structuring materials in new way. These materials are becoming the first step in increasing the value of products and their performance, rather than focusing the production steps. Multifunctional surfaces and materials with tailored properties and predictable performance based on directed self-assembly of nanoparticles can be achieved. Focus during the last 2 years was on identifying potential applications. During the annual project meetings and the workshops, the members of the users platform presented valuable input. Nanodirect as a project focused heavily on creating structures with controlled anisotropy and identifying pathways to achieve those structures. Work is still needed in translating these results on structures in specific end user properties, but this was not the focus of the participants within nanodirect. Nevertheless, the following areas of applications have been identified:
- Photonic, plasmonic and other optical applications, mainly using the metallic nanoparticles or ordered suspensions.
- Rational formulations strategies for tailoring the rheological properties using non-spherical particles, in shear and extensional flows
- Methodologies to address the long time dynamics and the shelf life of concentrated products.
- Interface templating of graphene based materials to make conductive coatings or 3D structures.
- Using interfacial rheology and controlling interaction between particles as a tool to rationally design materials such as emulsions, foams and high internal phase emulsions.
Several of the tools (though not all) developed within nanodirect have been made available to the wider public through the FP7 infrastructure project ESMI, coordinated by FZ-Julich and to which 5 of the 6 partners participate. The synthesis, experimental platform and the theory and simulation tools are made available to a wide audience, within academia and outside. In particular for the industrial interest within the framework of ESMi there have been several companies, which have used the tools developed in WP5 to look at complex fluid interfaces.
In conclusion, Nanodirect has achieved its goals as presenting enabling science and in identifying and structuring the tools and methods to direct self-assembly of nanoparticles. The speed at which the international research community has picked up research results from Nanodirect is impressive, and similar funding initiatives to Nanodirect have appeared in the US and some of the Asian countries. Nevertheless, challenges remain in translating the control that can be obtained over structures, to generating materials with specific and desired properties. The latter requires the knowledge generated within Nanodirect, which is focused at the methodological level, to be combined with expertise on the material properties and challenges in specific applications. Interested parties can always contact the coordinator or specific partners within Nanodirect to explore routes for future participation.
List of Websites:
Address of the project public website, if applicable as well as relevant contact details
http://www.nanodirect.eu(opens in new window)
Coordinator:
Prof. Dr.ir. Jan Vermant.
Department of Chemical Engineering
W. De Croylaan 46, B-3001 Leuven
KU Leuven, University of Leuven