CAPILLARY WAVE DAMPING IN HETEROGENEOUS MONOLAYERS

We studied capillary wave damping in heterogeneous monolayers of triglyceride at the air-water interface over a range of surface wavelengths (70-300 mu m) using a light scattering technique. In addition, we studied the monolayer morphology using a Brewster angle microscope. We found that the morphology has a strong effect on the capillary wave damping. In the gas/liquid expanded (G/LE) coexistence region the monolayer forms a two-dimensional foam structure, where ''bubbles'' of gas phase are separated by regions of Liquid expanded phase. If the width of the LE regions is smaller than the wavelength of the capillary wave, the 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 clean water surface. We attribute this increase to a rise in the dynamic dilational elasticity of the heterogeneous monolayer.