Electrohydrodynamic instabilities in nematics with small positive anisotropy and smectic C order

Threshold and electro-optical characteristics are studied at domain instability in the nematic phase, with small positive anisotropy or the dielectric constant ( epsilon a >0) and smectic order, for the seventh, eighth and ninth homologues of p-n-alkyloxybenzoic acid. The Orsay parameter zeta 2 is found to be negative for the seventh and eighth homologues and approximately zero for the ninth homologue. The dependence of the domain formation threshold, the Fredericksz threshold the elastic modulus and the Orsay parameter zeta 2 on temperature show a characteristic change in the middle of the nematic phase, due to a change from the classic nematic structure to the planar short range order structure. The molecular director reaction and relaxation and the break-up and formation of domains are also studied.