At the start of the project, we completed a survey of the pattern formation mechanisms of liquid crystal phases with periodic ground states, including the chiral liquid crystals that are of interest in the EXCHANGE_inLCs project. This review brought into the focus the role of overall system geometry (for instance, film thickness and confinement curvature) and the relevant length scales in determining the system’s surface patterns. An underappreciated mechanism of pattern formation, called the Helfrich-Hurault instability, arises in layered systems that experience geometrical frustration from either incompatibility of the layer spacing with the system shape or external fields. This work will be published in Reviews of Modern Physics next month.
In our experiments, to avoid nanoparticle aggregation that often occurs with changes in the particle surface chemistry, we focused on three systems that span a range of sizes. For inclusions, we look at lipids with varying hydrocarbon tail lengths. We also look at nanoparticles that are formed in situ via photopolymerization. We lastly look at micron-sized colloids with varying surface chemistry. These inclusions are studied in confined liquid crystals with varying geometries: on emulsions, on thin films with a free interface, and in the liquid crystal bulk sandwiched between two glass slides.
For lipid assembly on chiral liquid crystal emulsions, we observed changes in the surface patterning of the emulsions with varying emulsion size and lipid concentration. The lipids are fluorescently labeled and introduced from within the liquid crystal phase. We also observed that the longer the lipid hydrocarbon tail, the lower the concentration is needed to access a specific pattern, in line with previous work. We have difficulty in controlling the emulsion stability with varying lipid concentrations, so we are now changing our experimental set up to work with thin films, where we begin to work with lipid mixtures. The work is still in progress, to be written up for publication in the next year.
For in situ production of nanoparticles in a nematic liquid, we saw that the nanoparticles located at the interface for large enough particle sizes, likely at sizes large enough to induce significant distortions in the liquid crystal bulk. The interfacially adsorbed nanoparticles assembled into a hexagonal lattice due to long-range elastic interactions from distortions in the liquid crystal. Notably, a steady evaporation of the monomer was necessary for the photo-polymerized particles to be uniform in size. Using 2D simulations of particles with long-range, repulsive interactions, we found that only the isotropic and hexagonal crystals are stable phases, consistent with what is observed experimentally. We aim to refine our work by improving the experimental set up to have a flat interface, to remove the influence of curvature on the particle surface density. This work is still to be completed for publication.
For micron-sized colloids confined between two glass plates, we observed that the type of defect formed by the colloids, either point or ring defects, varied with the ratio of the particle size to the chiral liquid crystal pitch. We used Landau-de Gennes energy minimization to map where the defect transition takes place with varying particle radius and pitch. In experiments, we set up a laser tweezer to locally melt the liquid crystal around a colloidal particle, then releasing it to allow the elastic field to relax. From repeating this, we gather statistics for defect types observed for systems with fixed particle radius to pitch ratios. We are still working on mapping these statistics to our simulations, with the goal of publishing these results next year.
