2-Dimensional Elastic Constant in Asymmetric Films of Tilted Smectics

Abstract

We report on what we believe to be the first measurement of the surface elastic constant, K₁. This new elastic constant is manifested through the formation of a purely elastic instability. The instability is observed to form in thin smectic films, which are induced at the free surface of a liquid crystal droplet. Under polarising microscope, the instability is viewed as an array of interference fringes, which denote variation of both the orientation and the amplitude of the c director. The instability corresponds to a continuous rotation of the c director coupled to modulations of the molecular tilt angle θ. We propose that this mechanical instability is produced by the surface elastic term, K₁c²divc. This new term to consider in the free energy density is found to be consistent both qualitatively and quantitatively with the experiments.

Key Words:

• smectic C_A
• instability
• disclination walls
• elastic constant
• asymmetric films
• striped state