A Flat Interface and its Unfolding Bifurcations

Abstract

The experimental setup used is a classical directional solidification one where a movable sample holder is displaced at a controled velocity along a temperature gradient provided by two thermally regulated copper blocks. The sample consists of a thin layer of liquid crystal (here the 4,4’-n-octylcyanobiphenyl, 8CB) sandwiched between two glass plates. The temperatures are adjusted such that the liquid crystal undergoes a phase transition above the gap between the two blocks. The 8CB is isotropic down to 40.5°C where it becomes nematic through a weakly first order transition. The sample is properly moved along the gradient so that the amount of the more ordered phase (here the nematic one) is increased with respect to the less ordered one (the isotropic one); hence the name directional ordering experiment. The experimental setup is mounted on an optical microscope to observe the interface and eventually record it for later analysis (Oswald et al., 1987; Bechhoefer, 1988; Simon et al., 1988; Simon and Libchaber, 1990).