A NOVEL ORDER TRANSITION INSIDE THE NEMATIC PHASE OF TRANS-4-HEXYLCYCLOHEXANE-1-CARBOXYLIC ACID DISCOVERED BY IMAGE-PROCESSING

We have observed by polarizing microscopy a novel order transition ins ide the nematic phase and definitely below the clearing point of trans -3-hexylcyclohexane-1-carboxylic acid (C-6). The substance is known to exhibit the phase sequence Cr --> 32 degrees C --> SmB --> 47 degrees C --> N --> 96 degrees C --> I (where Cr = crystal, SmB = smectic B, N = nematic, I = isotropic). The order transition, very smooth, was re cognized 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 1 mu m. The nematic 'subphase' below the order transition is better ordered (with smaller and more regular domains) than the ot her 'subphase' above this transition. The data are compared with those obtained for 4-n-heptyloxybenzoic acid (HOBA) and already discussed i n terms of a surface transition, and are interpreted as due to the pre sence of smectic cybotactic clusters. To enhance the detection sensiti vity, we applied a method of image processing recently introduced by u s 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 fo r special cell treatment to align the director perfectly in the high t emperature nematic phase.