Liquid Crystals are extraordinary materials for the organization of fluids and nanoparticles. The director field wraps around colloidal inclusions and inevitably forms topological defects to meet far-field boundary conditions. Droplets of isotropic fluids which promote planar anchoring form boojums in the surrounding LC which cause bulk quadrupolar distortions. At an air-LC interface the droplets directly contact the air and one boojum can disappear leaving a solitary boojum pointing down. As the liquid crystal layer becomes thinner, the hybrid anchoring of the LC layer causes the boojum to tilt enabling the formation of dipolar chains. As the layer becomes thinner still, the bottom planar alignment forces the second boojum to reappear recovering the quadrupolar alignment. The elastocapillary force associated with droplets at an air-LC interface is a surprising many body force which can distort the layer thickness by up to 1 micron leading to the formation of rounded superstructures containing thousands of droplets. Polydisperse droplets can enable the formation of branched structures where a mix of dipolar and quadrupolar interaction allows for the formation of y-branches. The self-assembly properties of LC enable the formation of interesting emulsions and switchable hybrid materials.
Emulsions typically form with spherical boundaries due to the isotropic nature of surface tension. When a liquid crystal phase separates into two distinct phases highly non-spherical boundaries can be produced due to the elastic forces of liquid crystal and anisotropic surface tension which is director dependent. SEM imaging allows us to visualize the two distinct phases of liquid crystal and observe interesting responses to vacuum and electron beam excitation. Heating the sample also reveals the existence of two bubble phase transitions associated with the two liquid crystal phases. Utilizing the potential of liquid crystal phase separation can lead to new materials such as hexagonal blue phase, stable cholesteric tactoids and improve the design of other blue phase mixtures.
Cellulose Nanocrystals are readily produced from many plant and bacterial sources and have been studied extensively for low cost self-assembled optical elements. Nanocrystals are known to form a chiral nematic phase, which allows for production of films with chiral character. I will discuss recent advances in understanding how to control the chirality of cellulose nanocrystal solutions. The Debye length of a nanocrystal solution is typically around 4nm for pH between 1.5 and 10 and abruptly drops to around 1 nm for pH lower or higher than this range. With a Debye Length of 1nm, the solution can only form nematic phase, which is more useful for production of waveplates and other birefringent optics. Additionally I will discuss the behavior of cellulose solutions with amorphous material remaining in solution which form discotic- like systems.
Liquid crystal elasticity forms distortions and topological defects that allow for the controlled self-assembly of inclusions within liquid crystal media. Many groups have developed this idea for dispersion, alignment, and manipulation of a collection of individual particles to create plasmonic polarizers and others have studied colloidal crystallization processes within LC hosts. Extending this idea to consider more complex superstructures where a variety of forces can produce effects on larger lengthscales can expand the space of materials design and answer first principles questions about complex fluid dynamics. I will discuss results on colloidal inclusions at liquid-liquid and liquid- air interfaces where surface tension and capillary forces augment elasticity to provide the necessary tools for such hierarchical self-assembly.
Liquid crystal phase boundaries play an important role in self-assembly processes defining nanostructured complex fluid systems. Surface boundary conditions, director- dependent surface tension and topological defects produce elastic forces that combine to define the architecture of multiphasic anisotropic fluids. Developing a detailed understanding of these forces is necessary for a first principals understanding of anisotropic biological structures and producing the next generation of mesoscopic materials. We will discuss our recent experimental progress on thermotropic LC and colloidal LC consisting of cellulose nanocrystals and graphite.
Small molecule chiral nematic LCs contain a variety of defect lines which can cause aggregation of polymers, nanoparticles and small molecule materials such as fluorescent dyes. While normal dyes suffer aggregation induced quenching a class of propeller-like dyes display aggregation induced emission (AIE), which makes them an ideal material for cholesteric lasers. Nanoparticle dispersion is necessary for switchability, while aggregation can be used as a tool to produce assembled structures. Here we discuss strategies to promote or inhibit aggregation of various materials in chiral nematics.
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