Phase transition in the conductive and dielectric regimes of liquid crystals

Abstract

A phase transition is the transformation of a thermodynamic system from one phase to another. The distinguishing characteristic of a phase transition is an abrupt sudden change in one or more physical properties. Typically the phase transition is driven by a change in the temperature of the system. However, change of state may be executed by a change in an intensive variable like, for instance, the voltage. For liquid crystals in a sandwich cell the two dynamics can be executed. In the conduction regime at low excitation frequencies, one observes the director and flow fields changes (pattern dynamics). In the high-frequency, dielectric regime, we investigate the molecular dipole dynamics. So, in the conductive regime (detected via electroconvection measurements) the phase transition can be executed by voltage while in the dielectric regime (detected via dielectric spectroscopy) by temperature. The Low Frequency Dielectric Spectroscopy diagram can be used as a classical thermogram. In planarly and homeotropically oriented liquid crystal cells electroconvection experiment results in the morphological phase diagram. In this article we demonstrate that for certain excitation (ac voltage and/or temperature) the liquid crystal film system's dynamic response can by interpreted in terms of the phase transition.