Electric Field-Induced Flow Instabilities in Low Molecular Weight and Polymeric Nematics

Abstract

We compare the electrohydrodynamic instabilities in the thermotropic nematic phases of T2/60, a co[poly-(ethylene terephthalate)-1,4-benzoate] containing 60 mole % of p-oxybenzoyl groups, and in p-azoxyanisole (PAA). Due to the difference in their viscosities, the formation time of Williams domains in a d.c. field is on the order of an hour for the polymer, as compared to tenths or hundredths of a second for PAA. With a.c. at frequencies below 10 Hz., the threshold voltage for both materials falls significantly below the d.c. threshold value. Application of a d.c. voltage to very thin samples of both materials produces the variable grating mode pattern parallel to the rubbing direction. A similar pattern forms in PAA with low frequency a.c., but at 3 kHz the pattern changes to chevron-like domains perpendicular to the rubbing direction, and at 5 kHz dark snake-like regions move laterally across the field of view.