CAPILLARY WAVE DAMPING IN HETEROGENEOUS MONOLAYERS

We studied capillary wave damping in heterogeneous monolayers of trigl yceride at the air-water interface over a range of surface wavelengths (70-300 mu m) using a light scattering technique. In addition, we stu died the monolayer morphology using a Brewster angle microscope. We fo und that the morphology has a strong effect on the capillary wave damp ing. In the gas/liquid expanded (G/LE) coexistence region the monolaye r forms a two-dimensional foam structure, where ''bubbles'' of gas pha se are separated by regions of Liquid expanded phase. If the width of the LE regions is smaller than the wavelength of the capillary wave, t he monolayer has no measurable effect on the damping of the capillary wave. When the width of the LE regions is larger than the wavelength, the capillary wave damping constant increases from its value for a cle an water surface. We attribute this increase to a rise in the dynamic dilational elasticity of the heterogeneous monolayer.