Quenched random disorder studies in liquid crystal + aerosil dispersions

This thesis presents a series of studies of quenched random disorder (QRD) on liquid crystals. We have used high-resolution AC-Calorimetry, high-resolution X-Ray Diffraction (XRD). X-Ray Intensity Fluctuation Spectroscopy (XIFS), Turbidity, Integrated Low-Angle Light Scattering (ILALS), as well as Polarizing Microscopy to characterize the effects of a nano-colloidal dispersions of aerosils in the phase transitions of several liquid crystals. The aerosil (SIL) is made of 70 A diameter SiO2 particles coated with hydroxyl (-OH) groups. The coating allows the SIL particles to hydrogen-bond together, to form a very low density gel in an organic solvent. This provides the quenched random disorder. The liquid crystals of interest are: octyloxycyanobiphenyl (8OCB), 4- n-pentylphenylthiol-4'-n-octyloxybenzoate (8¯S5), 4'-transbutyl-4-cyano-4-heptyl-bicyclohexane (CCN47), and octylcyanobiphenyl (8CB). Studies have been carried out as a function of aerosil concentration and temperature spanning the following phase transitions, Isotropic to Nematic (I-N ), nematic to smectic-A (N-SmA), smectic-A to smectic-C (SmA-SmC ), and crystallization.;A double heat capacity feature is observed at the I-N phase transition with a SIL independent overlap of the heat capacity wings far from the transition and having a non-monotonic variation of the transition temperature. A crossover between low and high SIL density behavior is observed. These features are generally consistent with those on the 8CB + SIL system. Differences between the 8CB + SIL and our systems lie in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover density between the two regimes of behavior; these indicate a liquid crystal specific effect. Calorimetry, light scattering, and microscopy data coherently combine to allow for an accurate determination of the temperature dependence of the onset of the nematic state. The nematic order develops through two distinct processes while the nematic correlation length mildly decreases. We understand the doubling of the phase transition as due to a cross-over between a random dilution regime---where the silica gel couples to the scalar part of the nematic order parameter---to a random field regime---where the coupling induces distortions in the director field.;High-resolution XRD has been carried out on SIL dispersions in the liquid crystal 8OCB. The measurements were made over a temperature range around the bulk N-SmA transition temperature. The random gel leads to observable broadening of the x-ray reflection from the smectic layers. The structure factor is well described by modeling the effect of the SILs as a quenched random-field. Dispersed silica surfaces are thought to pin both the direction of the translational ordering and the position of the smectic layers. The latter appears to have the greatest effect on the x-ray lineshape. We show that the SIL gel surface area, as verified by small angle scattering, equates to the variance of the random field. Calorimetric results reveal substantial changes to the specific heat peak associated with the N-SmA transition. (Abstract shortened by UMI.).