Viscoelastic Properties of Molecularly Thin Liquid Crystal Films by Surface Light Scattering Spectroscopy*

We present a surface light scattering spectroscopy study of the surface properties of molecularly thin films of 8CB (4′-Octyl-4-biphenylcarbonitrile) at an air-water interface as a function of increasing temperature, covering the range over which the bulk phase transition from smectic to nematic occurs. The spectrometer analyzes light scattered from thermocapillary waves to determine surface viscoelastic properties, knowing surface tension, which is measured independently with a Wilhelmy plate. Historically, this experiment has been difficult. Enhanced design has improved signal quality, increasing accessible wave vectors and ease of use. With this much-improved signal, we reproduced previous measurements from a monomolecular layer of pentadecanoic acid at an air-water interface. We also compare the visco-elastic properties deduced from the power spectrum of capillary waves of a smectic liquid crystal at an air-water interface with equivalent properties of a free-standing smectic liquid crystal film in air.