Fluctuations near a phase transition in liquid crystals

Abstract

The increase in molecular orientational fluctuations as a phase transition is approached is an interesting and dramatic phenomenon, but associated undergraduate experiments are rare. We present an advanced undergraduate experiment on light scattering from the molecular orientational fluctuations in liquid crystals. In the high-temperature, disordered phase of a liquid crystal, small clusters of parallel-oriented molecules form spontaneously and scatter light. The size of the fluctuations and the consequent light scattering increase dramatically as the temperature is reduced approaching the weakly first-order phase transition to the ordered liquid crystal phase. We discuss a simplified theory that captures the essential physics.

Fluctuating dynamics of nematic liquid crystals using the stochastic method of lines

“…We now compare theoretical and numerical results: In Fig. (2) we show the histogram of the a i (q) for a particular Fourier mode. This is normally distributed, as expected, with zero mean and variance as required by thermal equilibrium.…”

“…This approximation is adequate deep within the nematic phase, where the strength of nematic order is not significantly affected by thermal fluctuations. However, in the vicinity of the weakly first-order isotropic-nematic transition, significant fluctuations in the nematic order are observed, suggesting that the phase-only approximation embodied in Leslie-Ericksen theory is inadequate [2]. The study of nucleation in quenches from the isotropic to the nematic phase involves the growth of one phase within another, mandating the use of descriptions capable of describing both isotropic and nematic phases on the same footing.…”

Virtual isotropic‐nematic transitions in alkyl cyanobiphenyls

“…The sample tube was housed in a copper block with Teflon cladding for thermal isolation, resistively heated by a commercial temperature controller with ¡0.03uC stability. The apparatus for the light scattering experiments has been described previously (8). Briefly, a vertically polarised helium-neon laser was weakly focused on the sample cell, a square optical cuvette residing in a thermally controlled copper chamber.…”