P. L. Taylor

Freedericksz transition in an anticlinic liquid crystal

The Freedericksz geometry is used to show experimentally that a very-long-pitch, surface stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below the threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal, resulting in a nonzero polarization that couples to the applied field. Measurements of E(th) as a function of temperature are reported. Related quasielastic light scattering measurements demonstrate that acoustic Goldstone mode fluctuations are quenched by a dc electric field E>E(th). At high fields a transition to the synclinic phase occurs via solitary waves.

Liquid crystal anchoring transitions induced by thermal motion

Abstract The thermal motion of a substrate is shown to have strong effects on the orientation of liquid crystal molecules in contact with it. Using an invertedpendulum model, we find that the orientation of the liquid crystal molecules can have a sequence of transitions between planar and homeotropic orientations. Analytical expressions for stability conditions for the homeotropic orientations are found for both monochromatic and some multiple-mode thermal motions, and, in both homeotropic and planar anchoring cases. Numerical simulations confirm the analytical model calculations and show that strong interactions between molecules favour processes of dynamic stabilization and destabilization of the homeotropic orientation.

Entropic forces—making the connection between mechanics and thermodynamics in an exactly soluble model

Upper-level undergraduate texts in thermodynamics and statistical mechanics frequently describe a force as being of entropic origin if it represents the tendency of a system to evolve into a more probable state, rather than simply into one of lower potential energy. This concept is not easy to understand, as it arises in the context of systems in which no apparent force exists at zero temperature. It is only when thermal motion is introduced that an apparent driving force appears. This difficulty is compounded by the absence of any non-trivial soluble model. Here we provide insight into the nature of entropic forces by presenting an exactly soluble model in which the entropic force can be derived from both mechanical and thermodynamic analyses, with results that are shown to be identical.

Preliminary communication Nematic-substrate repulsion in the nematic-isotropic phase coexistence region

Observations of two types of nematic droplet in the nematic-isotropic phase coexistence region are reported. One type contains topological defects and is free to move within a thin, homeotropically treated cell; the other is defect free and appears to be pinned at the substrates. The freely moving droplet represents an apparently new liquid crystal-substrate repulsion, which depends on the director alignments at the substrate and at the surface of the nematic droplet.