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  Abbott Research Group - UW Chemical and Biological Engineering

Laboratory for Molecular Engineering
Department of Chemical and Biological Engineering

Directed Assembly of Microparticles and Nanoparticles at Liquid Crystalline Interfaces

Assemblies formed by solid particles at interfaces have been widely studied because they serve as models of molecular phenomena, including molecular self-assembly. Solid particles adsorbed at interfaces also provide a means of stabilizing liquid–liquid emulsions and synthesizing materials with tunable mechanical, optical, or electronic properties. Whereas many past studies have investigated colloids at interfaces of isotropic liquids, recently, new types of intercolloidal interactions have been unmasked at interfaces of liquid crystals (LCs): The long-range ordering of the LCs, as well as defects within the LCs, mediates intercolloidal interactions with symmetries that differ from those observed with isotropic liquids. Herein, we report the decoration of interfaces formed between aqueous phases and nematic LCs with prescribed densities of solid, micrometer-sized particles.Center-to-center, nearest-neighbor spacing of microparticles within chains of microparticles formed at the LC–aqueous interface The microparticles assemble into chains with controlled interparticle spacing, consistent with the dipolar symmetry of the defects observed to form about each microparticle. Addition of a molecular surfactant to the aqueous phase results in a continuous ordering transition in the LC, which triggers reorganization of the microparticles, first by increasing the spacing between microparticles within chains and ultimately by forming two-dimensional arrays with local hexagonal symmetry. The ordering transition of the microparticles is reversible and is driven by surfactant-induced changes in the symmetry of the topological defects induced by the microparticles. These results demonstrate that the orderings of solid microparticles and molecular adsorbates are strongly coupled at the interfaces of LCs and that LCs offer the basis of methods for reversible, chemosensitive control of the interfacial organization of solid microparticles.

(A) Center-to-center, nearest-neighbor spacing of microparticles within chains of microparticles formed at the LC–aqueous interface, plotted as a function of the concentration of SDS in the aqueous phase. Images are bright-field micrographs of microparticles at the nematic 5CB–aqueous interface with aqueous phase SDS concentrations of 150, 700, and 1,300 μM. (Scale bars: 5 μm.) (B) Center-to-center, nearest-neighbor spacing of microparticles, measured as a function of time, for microparticles assembled at the 5CB–aqueous interface in the presence of 700 μM SDS in the aqueous phase. (C) Probability of finding a given number of nearest neighbors for microparticles in regions free of compact aggregates when the SDS concentration in the aqueous phase was 1,300 μM. (D) Increase in the area fraction of the 5CB–aqueous interface (SDS concentration of 1,300 μM) occupied by regions of 2D microparticle assemblies with hexagonal symmetry, plotted as a function of the overall concentration of microparticles at the interface.