FLUCTUATIONS OF A DEFECT LINE OF MOLECULAR-ORIENTATION IN A MONOLAYER

Microscopy at the Brewster angle is used to study the phase transition that occurs during the formation of an adsorbed sodium myristate film at the surface of an aqueous solution. The formation and evolution of structures in which the tilt of the molecules creates an observable optical anisotropy are studied. Structures consisting of curved stripes of a given width are observed. Across each stripe the tilt direction of the molecules turns continuously in the plane by about 95-degrees; two neighboring stripes are separated by curved defect lines, across which the tilt direction jumps back by about 95-degrees. The position of these defect lines fluctuates, indicating a very low line tension. Under strong repulsions between domains, these defect lines are strongly distorted. This stripe structure can be explained by a continuum elastic model for ''locked tilted mesophases,'' and two constants of the model can be estimated from our data.