Periodic Reporting for period 1 - EXCHANGE_inLCs (EXamining CHemistry and Nanoparticle Geometry Effects at the INterface of Liquid CrystalS)
Reporting period: 2020-11-01 to 2022-10-31
Arranging nanometer-sized objects into designed configurations with submicron precision is a major challenge in the development of nanotechnology. Adjusting the positioning of these objects tunes the overall macroscopic properties of the system. Designing scalable nanotechnology requires self-assembly, where components arrange themselves to minimize the system energy. Liquid crystals, materials renowned for the modern display industry, have emerged as an attractive medium for ordering nanoscale objects, due to the medium’s ability to form complex patterns. The dielectric and elastic properties of the constituent rod-like molecules of liquid crystals enable pattern formation that is controllable with confining surfaces and externally applied fields. Foreign inclusions, such as large molecules and particles, can disrupt the organization of liquid crystalline molecules, generating defects – local regions of disorder. In order to minimize the distortion in the medium, the foreign inclusions come together, self-assembling into shapes dictated by the liquid crystal defects.
In order to develop new bottom-up approaches for designing nanomaterials, the EXCHANGE_inLCs project seeks to elucidate particle-liquid crystal interactions at the submicron scale, by EXamining CHemistry and Nanoparticle Geometry Effects at the INterface of Liquid CrystalS. To achieve this aim, the objectives of the project are to:
1) Vary the system GEOMETRY to explore confinement, particle size, and shape on assemblies, and
2) Vary the system CHEMISTRY to clarify the behavior of certain chemical species around particles.
By varying system length scales and types of surface treatments, key interactions could be isolated. To probe both the effect of system geometry and chemistry, we studied several systems. First, we examined the assembly of lipids with varying hydrocarbon tail lengths at the interface of emulsions and thin films. The emulsion size was variable with the use of microfluidic techniques. Then, we investigated the assembly of nanoparticles that were synthesized in situ within thin films of liquid crystals via photo-induced polymerization. From comparing the interfacial assembly of particles to simulation results, we characterized the effective interparticle interactions. Third, we studied the role of particle size compared to the pitch of chiral liquid crystals in stabilizing two different types of defects: either point or ring defects, using both simulations and experiments. We lastly published a comprehensive review on how geometrical frustration from applied fields or boundary shapes can influence the pattern formation of liquid crystals with periodic ground states.
In order to develop new bottom-up approaches for designing nanomaterials, the EXCHANGE_inLCs project seeks to elucidate particle-liquid crystal interactions at the submicron scale, by EXamining CHemistry and Nanoparticle Geometry Effects at the INterface of Liquid CrystalS. To achieve this aim, the objectives of the project are to:
1) Vary the system GEOMETRY to explore confinement, particle size, and shape on assemblies, and
2) Vary the system CHEMISTRY to clarify the behavior of certain chemical species around particles.
By varying system length scales and types of surface treatments, key interactions could be isolated. To probe both the effect of system geometry and chemistry, we studied several systems. First, we examined the assembly of lipids with varying hydrocarbon tail lengths at the interface of emulsions and thin films. The emulsion size was variable with the use of microfluidic techniques. Then, we investigated the assembly of nanoparticles that were synthesized in situ within thin films of liquid crystals via photo-induced polymerization. From comparing the interfacial assembly of particles to simulation results, we characterized the effective interparticle interactions. Third, we studied the role of particle size compared to the pitch of chiral liquid crystals in stabilizing two different types of defects: either point or ring defects, using both simulations and experiments. We lastly published a comprehensive review on how geometrical frustration from applied fields or boundary shapes can influence the pattern formation of liquid crystals with periodic ground states.
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.
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.
Overall, we find that the inclusion size compared to the system size significantly impacts the elastic distortions and subsequent inclusion assembly. These results broaden the potential of liquid crystals as mediums for directed self-assembly, with implications for advancing liquid-crystal-based optical and sensing devices.