Periodic Reporting for period 5 - OMCIDC (Optical Manipulation of Colloidal Interfaces, Droplets and Crystallites)
Periodo di rendicontazione: 2023-02-01 al 2023-09-30
Interfaces and interfacial phenomena like nucleation, droplet coalescence and capillarity are amongst the most widely experienced phenomena in life, with relevance to areas as diverse as oil recovery, food and inkjet printing. The importance of interfacial phenomena has been further prompted by the recent and rapid emergence of micro- and nano-fluidics, because at these small length scales, interfacial phenomena are particularly dominant and interface dynamics strongly affect the transport and response of fluids in such devices. Actively controlling interfacial phenomena is therefore becoming increasingly desirable, both for providing new and fundamental insights into interfacial phenomena and for the successful implementation of micro- and nano-fluidic devices. The interfacial roughness in atomic and molecular systems is, however, typically in the (sub)nanometre range, making the relevant interfacial phenomena experimentally very hard to access. Colloidal systems, which consist of particles with a size between roughly one nanometre and several micrometres dispersed in a molecular solvent, provide a unique model system to explore interfacial phenomena in great detail due to their ultralow interfacial tension.
In this project, we have used optical tweezing and confocal microscopy to gain fundamental insight into interfacial phenomena by actively manipulating colloidal interfaces, droplets, crystallites and liquid crystalline droplets. The main scientific aims were:
1. Active deformation of interfaces in colloidal systems
2. Nucleation and evaporation of droplets, crystallites and tactoids
3. Coalescence and detachment of droplets, crystals and tactoids
4. Heterogeneous nucleation and capillary phenomena
The most important conclusions from this project are: (i) that colloidal nematic liquid droplets can be generated using optical tweezing of specially developed SU-8 colloidal rods; (ii) that banana-shaped colloidal particles can form a splay-bend nematic phase – and thus that this phase does exist, as this is the first experimental observation of this phase since its theoretical prediction more than 40 years ago; (iii) that polydispersity and entropy alone can result in hierarchical self-assembly (in a vortex phase); (iv) that solid-solid interfaces (grain boundaries) move according to an adaptation of the geometric framework called the O-lattice, which predicts the motion of both particles and dislocations during grain boundary migration; (v) that optical tweezing can be used to nucleate and manipulate colloidal liquid droplets and that we can study their interface fluctuations; (vi) that the emergence of interparticle friction is important in attractive colloidal systems and can be understood in terms of the coordination number of particles.
In this project, we have used optical tweezing and confocal microscopy to gain fundamental insight into interfacial phenomena by actively manipulating colloidal interfaces, droplets, crystallites and liquid crystalline droplets. The main scientific aims were:
1. Active deformation of interfaces in colloidal systems
2. Nucleation and evaporation of droplets, crystallites and tactoids
3. Coalescence and detachment of droplets, crystals and tactoids
4. Heterogeneous nucleation and capillary phenomena
The most important conclusions from this project are: (i) that colloidal nematic liquid droplets can be generated using optical tweezing of specially developed SU-8 colloidal rods; (ii) that banana-shaped colloidal particles can form a splay-bend nematic phase – and thus that this phase does exist, as this is the first experimental observation of this phase since its theoretical prediction more than 40 years ago; (iii) that polydispersity and entropy alone can result in hierarchical self-assembly (in a vortex phase); (iv) that solid-solid interfaces (grain boundaries) move according to an adaptation of the geometric framework called the O-lattice, which predicts the motion of both particles and dislocations during grain boundary migration; (v) that optical tweezing can be used to nucleate and manipulate colloidal liquid droplets and that we can study their interface fluctuations; (vi) that the emergence of interparticle friction is important in attractive colloidal systems and can be understood in terms of the coordination number of particles.
During the project we have studied the active manipulation of colloidal interfaces, droplets and crystallites and below is a list of the main achievements.
• We have achieved the active deformation of colloidal liquid-gas interfaces using optical tweezing, from we can determine the interfacial tension in the limit of small deformations.
• Using optical tweezing we managed to form (transient) colloidal nematic liquid crystalline domains in an isotropic phase.
• We have actively deformed grain boundaries (crystal-crystal interfaces) in 2D colloidal systems using optical vortices and successfully adapted O-lattice theory to fully predict both the particle and dislocation dynamics during GB migration (i.e. interface dynamics).
• We have implemented the use of electric fields to manipulate the liquid crystalline phases formed in both colloidal SU-8 rods and banana-shaped systems.
• We have used optical tweezing to successfully nucleate colloidal liquid droplets above the liquid-gas interface, i.e. in the bulk gas phase and study their evaporation dynamics.
• We managed to nucleate colloidal liquid crystalline droplets (tactoids) in a background isotropic phase in a system of colloidal SU-8 rods.
• We have studied the nucleation of crystals in supersaturated colloidal fluids and analysed the nucleus’ structure focusing on the role of (twin) grain boundaries during crystal growth.
• We studied various mechanisms in phase separating colloidal liquid-gas systems, where the mechanisms varied from arrested nucleation and growth to growth via the coalescence of liquid droplets to spinodal phase separation.
• We directly tracked the rotation of crystalline grains in polycrystalline materials, which is the basis of grain rotation induced coalescence.
• We have studied the break-up and fracture of colloidal single crystals under their own weight.
• We have analysed the thermal interface fluctuations of highly confined colloidal liquid and gas bubbles.
• We have established that the flow alignment of the colloidal particles with the walls in a dense fluid induces an instantaneous and very large single crystal.
• We observed that a crystal in contact with a glass can grow via either a fast growth mode or a via avalanche-like intermittent crystal growth ‘events’ depending on the glass was prepared.
• We have established how the motion of colloidal chains across optical potential energy landscapes is related to the emergence (and disappearance!) of friction upon modulating the potential and driving force.
• We developed colloidal banana-shaped particles with tuneable curvature. We established their phase behaviour, which included the elusive splay-bend nematic phase (first observation since its theoretical prediction over 40 years ago).
• We developed TPM-based colloidal spheres that contained an off-centre core, which allows for the simultaneous tracking of both the 3D positions and orientation of spheres and has enabled studying the emergence of friction in colloidal systems.
• We have achieved the active deformation of colloidal liquid-gas interfaces using optical tweezing, from we can determine the interfacial tension in the limit of small deformations.
• Using optical tweezing we managed to form (transient) colloidal nematic liquid crystalline domains in an isotropic phase.
• We have actively deformed grain boundaries (crystal-crystal interfaces) in 2D colloidal systems using optical vortices and successfully adapted O-lattice theory to fully predict both the particle and dislocation dynamics during GB migration (i.e. interface dynamics).
• We have implemented the use of electric fields to manipulate the liquid crystalline phases formed in both colloidal SU-8 rods and banana-shaped systems.
• We have used optical tweezing to successfully nucleate colloidal liquid droplets above the liquid-gas interface, i.e. in the bulk gas phase and study their evaporation dynamics.
• We managed to nucleate colloidal liquid crystalline droplets (tactoids) in a background isotropic phase in a system of colloidal SU-8 rods.
• We have studied the nucleation of crystals in supersaturated colloidal fluids and analysed the nucleus’ structure focusing on the role of (twin) grain boundaries during crystal growth.
• We studied various mechanisms in phase separating colloidal liquid-gas systems, where the mechanisms varied from arrested nucleation and growth to growth via the coalescence of liquid droplets to spinodal phase separation.
• We directly tracked the rotation of crystalline grains in polycrystalline materials, which is the basis of grain rotation induced coalescence.
• We have studied the break-up and fracture of colloidal single crystals under their own weight.
• We have analysed the thermal interface fluctuations of highly confined colloidal liquid and gas bubbles.
• We have established that the flow alignment of the colloidal particles with the walls in a dense fluid induces an instantaneous and very large single crystal.
• We observed that a crystal in contact with a glass can grow via either a fast growth mode or a via avalanche-like intermittent crystal growth ‘events’ depending on the glass was prepared.
• We have established how the motion of colloidal chains across optical potential energy landscapes is related to the emergence (and disappearance!) of friction upon modulating the potential and driving force.
• We developed colloidal banana-shaped particles with tuneable curvature. We established their phase behaviour, which included the elusive splay-bend nematic phase (first observation since its theoretical prediction over 40 years ago).
• We developed TPM-based colloidal spheres that contained an off-centre core, which allows for the simultaneous tracking of both the 3D positions and orientation of spheres and has enabled studying the emergence of friction in colloidal systems.
The development of our banana-shaped particles and off-centre core-shell spheres I consider to be progress that goes significantly beyond the state of the art.
i) We developed ‘banana-shaped’ colloidal particles with tuneable dimensions and curvature, whose structure and dynamics are accessible at the particle level. We fully elucidated the phase behaviour of differently curved colloidal bananas and found that they exhibit very rich phase behaviour, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the elusive splay-bend nematic phase, which was predicted 40 years ago and we have now experimentally observed for the first time. Our work opened the field of colloidal banana-shaped liquid crystals, which offers a host of opportunities to unveil how particle curvature affects the properties of liquid crystals.
ii) We developed compositionally uniform colloidal spheres with an off-centre, fully embedded core, which allowed us to directly access hydrodynamic and frictional coupling in colloidal suspensions up to arbitrarily high volume fractions. We reveal that nearly arrested particles exhibit a stick-slip rotational motion due to frictional coupling. We also elucidated the structural origin of the emergence of friction in attractive colloidal systems. Our off-centre core-shell colloidal spheres offer a unique way of detecting frictional coupling deep inside dense materials, which is crucial for elucidating the emergence of friction in a wide range of particulate materials such as colloidal fluids, gels, glasses and granular matter.
i) We developed ‘banana-shaped’ colloidal particles with tuneable dimensions and curvature, whose structure and dynamics are accessible at the particle level. We fully elucidated the phase behaviour of differently curved colloidal bananas and found that they exhibit very rich phase behaviour, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the elusive splay-bend nematic phase, which was predicted 40 years ago and we have now experimentally observed for the first time. Our work opened the field of colloidal banana-shaped liquid crystals, which offers a host of opportunities to unveil how particle curvature affects the properties of liquid crystals.
ii) We developed compositionally uniform colloidal spheres with an off-centre, fully embedded core, which allowed us to directly access hydrodynamic and frictional coupling in colloidal suspensions up to arbitrarily high volume fractions. We reveal that nearly arrested particles exhibit a stick-slip rotational motion due to frictional coupling. We also elucidated the structural origin of the emergence of friction in attractive colloidal systems. Our off-centre core-shell colloidal spheres offer a unique way of detecting frictional coupling deep inside dense materials, which is crucial for elucidating the emergence of friction in a wide range of particulate materials such as colloidal fluids, gels, glasses and granular matter.