Study of structure and interactions in nematic liquid crystal suspensions

The influence of surface-induced structure and dynamic interactions in liquid crystal (LC) suspensions is investigated by microdielectrometry, optical, and rheological measurements. We apply the theories developed by the currently-accepted continuum description of LC structure to determine if it can adequately represent such systems of particle suspensions.; We first investigate the forces generated between two neighboring surfaces with incommensurate anchoring boundary conditions. We monitor the gravity-driven approach of a thin glass plate towards a bottom plate of larger dimensions submerged in a nematic LC. With boundary conditions on the inner surfaces such that a twist distortion is created in the interstitial fluid, theory predicts final settling of the top particle at a finite, non-zero equilibrium separation. The separation between the two plates is experimentally monitored by measuring the dielectric permittivity from a capacitive sensor photolithographically patterned at the bottom surface. (We demonstrate this dielectric technique to be sensitive to separations of the order of a micron.) The results confirm the attainment of a non-zero equilibrium separation between the plates. A mathematical model, based on Stefan's equation for squeezing flow with an additional term describing Frank repulsive interactions is employed to decipher the experimental transients.; Our second approach to study dynamics in LC particle suspensions utilizes polarized optical microscopy to observe the interactions between spherical particles and the nematic matrix. We observe the positions of the spheres in nematic drops (which have well defined orientation fields) and compare results of particles that are untreated to those treated with lecithin, a surfactant that imparts homeotropic alignment on the surface. The observations show significant differences between the preferred positions of the treated and untreated particles, and thus indicate that the molecular alignment on the surface of the particles is important in particle-matrix dynamics.; Finally, we examine the effects of particle surface anchoring on the structure and rheology of spherical particle suspensions. Dynamic oscillatory measurements of suspensions of both treated and untreated particles of three different concentrations are performed for both the isotropic and nematic phases. The experiments reveal that as the samples are sheared, the coating on the treated particles aids in preserving the initial structure of the matrix, which indicates a strong coupling between matrix microstructure and surface conditions.