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Computational study of assisted assembly of colloids

Final Report Summary - COSAAC (Computational study of assisted assembly of colloids)

The assembly of particles into ordered structures is subject of great scientific and technological interest. When the constituent particles of a material are randomly distributed in a fluid phase the material has usually little interest. However, when particles assemble into some specific pattern a material with interesting properties may arise. This is the case, for instance, of colloidal particles, that can form fluids with no particular functionality or crystals with optical properties that can be employed for photonic applications. The disorder-order transition is also important in the molecular world. For example, many drugs are presented as a solid crystallised from a solution where the solute molecules are randomly distributed.

The main idea of the COSAAC project is to investigate the possibility to aid the formation of ordered structures by using external fields or by dynamically changing the thermodynamic conditions at which the constituent particles assemble to form the ordered structure. Molecular simulations is the main research tool of this project.

The assisted formation of crystalline structures has been the main research topic of the project. The process of formation of crystalline solids has been assisted in several ways. One is the use of seeds of the target solid structure. This strategy enables lowering free energy penalty required for the formation of the crystal and, when combined with a theoretical analysis of the simulation data, gives valuable information on the crystallization process. In this way, the crystal nucleation rate of hard-sphere-like colloids has been obtained for the widest available range of pressures so far. A set of hard spheres is the simplest non-trivial system that shows a fluid-crystal transition, which is the reason why this system has been widely studied both by means of computer simulations and by experiments with spherical colloidal particles stabilised against aggregation in such way that they interact like hard-spheres. The high interest raised by this archetypal system makes the results obtained in this project highly valuable. The seeding approach has also been applied to molecular systems. The most important studied system has been water. By assisting the formation of ice via seeds it has been possible to predict the ice nucleation rate for a wide range of temperatures. Due to technical limitations, the ice nucleation rate was measured so far only in a narrow temperature window (-35 to -40 Celsius). With the results of this project we now have a simulation prediction of the ice nucleation rate available for a wide temperature range, which is important for making climate change models that take into account the important factor of ice formation in tropospheric clouds.

Another possibility that has been explored to assist the assembly of crystals is the use of potential energy wells that confine the particles in the fluid and induce the formation of crystalline regions. This sort of strategy could be achieved experimentally using optical tweezers to trap colloidal particles. This trick has been exploited as a means to measure the crystal-fluid interfacial free energy in computer simulations. This parameter controls crystal nucleation and growth. Unfortunately, it is very difficult to measure the ice-liquid interfacial free energy experimentally, so computer simulations can be used to provide reasonable estimates. A few methods were already available to do that, but that developed in this project based on the assisted formation of crystal slabs via potential molds has proven more accurate and efficient than existing alternative approaches. On the other hand, the use of potential energy molds has been exploited for the calculation of crystal nucleation free energy barriers. This issue has been focus of intense research activity. The advantage of using potential energy wells over previous approaches to the calculation of nucleation free energy barriers is that one can study arbitrarily complex crystal structures without increasing the methodological complexity. This novel approach has been conceived and developed during the project. Its use has been illustrated with archetypal systems like hard-spheres or Lennard Jones and molecular systems like water.

Both the seeding and the potential mold approaches were combined to study the effect of external fields like pressure or electric fields in the crystallization of fluids composed of dipolar molecules (such as water). By assisting the formation of the crystal with these tools it has been possible to unveil the underlying physics behind the decelerating effect of pressure on ice nucleation. This is effect is currently being exploited in the cryopreservation of biological samples, but a physical explanation that will allow for the development of the current technology was lacking. Moreover, by combining the developed methodologies, it has been proposed in the project that applying pressure in conjunction with electric fields may be a promising strategy to hinder ice formation.

There are materials, like gels or glasses, for which a disordered arrangement of particles is required in order for the material to be functional. In the project it has been investigated whether assisting the formation of these materials by means of a modulation of the thermodynamic conditions under which they are formed may improve their properties. In particular, the effort has been devoted to glasses composed of hard sphere particles, a system that can be realised experimentally with colloids. A rapid change of the pressure in the formation of the material can change the resistance of the resulting glass to crystallize. Crystallization of glasses is known as de-vitrification and it is an undesired process because the mechanic properties of the material are changed when it crystallizes. Therefore, crystallization needs to be avoided by building materials resistant to de-vitrification. The ability of glasses to crystallize when formed assisted by specific compression protocols designed to minimize the crystal content has been assessed. Furthermore, the mechanism of devitrification in colloidal glasses has been elucidated. Understanding how devitrification takes place is the first step required to try to avoid it. The investigations on de-vitrification were performed in collaboration with the Soft-Condensed Matter groups in The University of Edinbugh and in La Sapienza University in Rome.

The impact of a magnetic field in the arrangement of magnetic colloidal particles in two-dimensional sheets has also been studied. Such arrangements can be used to build colloidal vesicles or membranes of tunable permeability. This part of the project has been carried out in collaboration with the experimental group of colloids of the Physical-Chemistry department to study the kinetics of disassembly of 2D colloidal crystalline clusters after a magnetic field is applied. A remarkably good agreement between the experiments and the simulation model was observed and a mechanism for the sublimation of 2D colloidal crystalline clusters was proposed.

The use of heterogeneous seeds for the crystallization of colloidal systems was also investigated. Heterogeneous seeds are those made of a material different from that of the parent phase. In that case the assembly of the material is assisted by a heterogeneous seed whose role is usually that of lowering the interfacial free energy between the ordered and the disordered phases. There is a long-standing controversy in the comparison between experimental and simulation results of the crystallization rate of hard-sphere-like colloidal particles. The possibility that the crystallization is assisted by heterogeneous seeds has been disregarded. This issue was investigated in the context of the assisted formation of ordered structures of the COSAAC program. The results show that, indeed, the controversy is sorted by taking into account the role of heterogeneous seeds. The discrepancy mentioned above was interpreted by some scientist as an inability of computer simulations to correctly describe the process by which a fluid transforms into an ordered crystal lattice. Hopefully, the fact that the long-standing discrepancy was sorted in the COSAAC project will bring confidence in computer simulations as a good scientific tool to investigate phase transitions.

In summary, in this project different possible ways to externally affect and assist the ordering of randomly located building blocks have been investigated. Strategies like the use of pressure, potential molds, seeds, impurities or magnetic and electric fields have been considered. The results of this work have been published in 15 peer-reviewed international physical-chemistry journals.