A.C. Shear Elastic Constant of A Smectic B Liquid Crystal

Abstract

We have measured the low frequency A.C. complex shear modulus of a smectic B liquid crystal, parallel to the smectic layers for thin samples. The apparent elastic modulus C₄₄ is very small (∼ 10⁶ dyn/cm²). It increases monotonically versus the frequency (from 10⁻¹ to 5.10³ Hz), following a logarithmical law. At a given frequency (10³Hz) the elastic constant is measured close to a transition to a smectic A phase and shows a pre-transitionnal softening. These results compare well with ultrasonic data from Thiriet and Martinoty demonstrating a large relaxation effect between 10⁴Hz and 10⁸Hz.