Dynamics of supercooled liquids and liquid crystals: Optical Kerr effect experiments and mode coupling theory analysis

Optically heterodyne-detected optical Kerr effect (OHD-OKE) experiments are applied to study the orientational dynamics of supercooled liquids and liquid crystals over a wide range of times from sub-ps to mus. The studies on ionic supercooled liquids reveal complex dynamics with relaxation involving several power law decays followed by a final exponential decay. One ionic liquid displays features of the boson peak near its glass transition. Mode coupling theory (MCT) analysis indicates that the theory can explain the experimental data very well for a range of temperatures above the critical temperature Tc, but, as might be expected, there are some deviations from the theoretical modeling at temperatures close to Tc. The temperature-dependent OHD-OKE data of the ionic liquids are consistent with the molecular liquids, previously studied using the identical technique. The long-range Coulomb interactions in the ionic liquids do not appear to play a role of producing distinct dynamical characteristics in the bulk liquids.; The nematogens in the isotropic phase exhibit OHD-OKE decays of a functional form identical to those of supercooled liquids. Landau de Gennes (LdG) theory can explain the long time scale dynamics, but not the short time scale. In addition, the MCT model that fits the supercooled liquid data can not reproduce the full shape of the nematogen decays. In this work a combined MCT-LdG model is proposed. This model is capable of reproducing the temperature-dependent nematogen OHD-OKE data very successfully.; OHD-OKE studies on the nematic phase of liquid crystals reveal relaxations that are very different from the isotropic phase. A temperature-independent power law t-z with z ∼ 0.5 is observed on the longest time scale. On shorter time scales, an intermediate power law is observed with an exponent that depends on the nematic order parameter.; A discotic liquid crystal is studied in the isotropic phase above the columnar-isotropic phase transition. The OHD-OKE decays are very similar to those of supercooled liquids. The MCT model for supercooled liquids can describe the dynamics over the full range of times. The results indicate that the columnar liquid crystal in its isotropic phase behaves like a normal liquid.