Phase transitions in Gibbs films: Star textural defects in tilted mesophases

Abstract

The formation of adsorbed films at the free surface of aqueous solutions of sodium hexadecanoate was observed by microscopy at the Brewster angle. A phase transition between a gas and a tilted phase occurs during these slow formations. The tilt of the molecules creates an observable optical anisotropy. We observed circular domains separated into regions of uniform tilt direction by straight defect lines, i.e., ‘‘star’’ textural defects very similar to those observed in freely suspended liquid crystal thin films and in Langmuir films of esters. The coalescence of domains leads to large regions of uniform tilt direction separated by straight or kink lines. A second transition towards a nontilted phase is observed. Quantitative information about the coefficients of the continuum elastic theory can be derived from our experiments.