A novel order transition inside the nematic phase of trans -4-hexylcyclohexane-1-carboxylic acid discovered by image processing

Abstract

We have observed by polarizing microscopy a novel order transition inside the nematic phase and definitely below the clearing point of trans -4-hexylcyclohexane-1-carboxylic acid (C). The substance is known to exhibit the phase sequence Cr 32 C SmB 47 C N 96 C I (where Cr crystal, SmB smectic B, N nematic, I isotropic). The order transition, very smooth, was recognized both on cooling and on heating of the sample, consisting of a sandwich cell made by two glass plates unidirectionally rubbed, with a gap around 1mum. The nematic 'subphase' below the order transition is better ordered (with smaller and more regular domains) than the other 'subphase' above this transition. The data are compared with those obtained for 4- n -heptyloxybenzoic acid (HOBA) and already discussed in terms of a surface transition, and are interpreted as due to the presence of smectic cybotactic clusters. To enhance the detection sensitivity, we applied a method of image processing recently introduced by us and also able to reveal soft structural changes in the image frame. Since the method sensitivity is at least 10 times higher with respect to standard techniques, the order transition, even though very smooth (especially on heating), was easily detected, without the necessity for special cell treatment to align the director perfectly in the high temperature nematic phase. 6