Project “CoPEC” (Colloidal Particles evolving in Elasto-Capillary fields)
The “CoPEC” project is a European fundamental research project (Program H2020, MSCA-IF-GF-2017, ID 794837) designed to advance knowledge in the field of soft matter Physics, and more particularly in the area of colloids and complex fluids interfaces. It is carried out in strong collaboration between the University of Bordeaux and the University of British Columbia (UBC) in Vancouver (CA). The main task of the proposal is to investigate the physical properties of colloidal particles evolving in elasto-capillary fields, which can be realized by considering, e.g., particles attached to liquid crystal (LC) fluid interfaces. Such systems remain largely unexplored today and represent a new type of material whose properties are anticipated to be mainly governed by the coupling between capillary and elastic phenomena. The former originates from the surface tension inherent to any fluid interface, whereas the latter is intrinsic to all liquid crystal phases.
These phenomena have always been considered separately in previous works, and consequently, examining their intimate coupling is one the salient and innovative aspects of the “CoPEC” project. Indeed, elasto-capillary couplings may lead to novel colloidal interactions and the subsequent discovery of new collective properties which could be exploited to design materials with yet unknown important functions. Potential areas of impact include optical applications such as the next generation of ‘smart’ PDLC (Polymer Dispersed Liquid Crystals) windows or drug delivery systems at the micro- or sub-micrometer scale for the pharmaceutical industry.
The project is carried out using numerical simulations based on continuum theories. The overall objective is to develop computer models to gain basic knowledge on the behavior of colloidal particles adsorbed at liquid crystal interfaces. Understanding collective phenomena , which are essential to make predictions on potentially interesting applications, would constitute a major achievement and one of the final scientific targets of the project. But prior to addressing multi-particle systems, general knowledge on (i) the behavior of a single particle, and (ii) pair interaction potentials, should first be harvested. To fulfill these goals, we consider model systems consisting of solid micron-sized particles attached to both planar and curved nematic LC interfaces. We vary the numerous system parameters (e.g., boundary conditions) in a systematic way and consider both static and dynamic situations. In the latter case, particular attention is paid to the two-way coupling occurring between the fluid flow and the order parameter of the nematic LC. In all cases, we take great care to validate the codes in a progressive manner on well-known benchmark examples before addressing the situations of interest for the project.
Figure 1 : Particle attached to a liquid crystal/isotropic liquid interface. Result of a two-dimensional drag force simulation in the presence of a deformed interface (magenta solid line in the middle). The solid green lines represent the fluid flow passing by the particle (white disc) in both phases. The colors in the liquid crystal (LC) phase indicate the degree of molecular order (red: perfect alignment along the horizontal direction. Blue: perfect alignment along the vertical direction). Because of specific boundary conditions imposed at the interface and the particle surface, the LC order is perturbed and a topological defect, seen as a small point very close to the particle, is formed in the LC phase. We see that this defect is swept to the right by the incoming flow.