Low Wavenumber Electroconvection Patterns in Nematic Phase 5 and 5A

摘要

Electroconvection (EC) in thin (d ～ 10-100μm) layers of nematic liquid crystals (LC) is a well known example of pattern forming phenomena in complex non-equilibrium systems. It provides a wide variety of patterns with easy tuning of control parameters (voltage, frequency, temperature, magnetic field) and orientation and it may also serve as an important model system for testing theoretical tools like the weakly nonlinear amplitude formalism (Ginzburg-Landau equations, GLE) which is often employed to describe the behavior slightly above the primary threshold. The nematic mixtures Phase 5 and 5A (Merck) are one of the favourite substances used for EC measurement due to their broad nematic range, chemical stability and known physical (elastic and viscous) parameters. Having a low cut-off frequency Phase 5 is ideal for measurements in the 'dielectric' regime, while Phase 5A is doped for higher conductivity and thus possesses 'conductive' regime in an extended frequency range. Now we present some recent experimental results obtained using these substances in planar and homeotropic cells. We focus mainly on two problems; the normal roll - abnormal roll transition and the patterns with a wavelength exceeding the sample thickness.