Abstract

When surfactant molecules are spread at a liqud surface, a two-dimensional monolayer is formed at the interface. It has been known since antiquity that the presence of a surface-active material increases the damping of capillary waves, yet little is known about the mechanism responsible for this enhancement. Since the advent of the laser, damping measurements have been carried out using light scattering techniques. The temporal damping coefficient is obtained from the width of the Brillouin peak in the power spectrum after deconvolution with. complex instrumental corrections. As the scattered light has very small intensity, it is difficult to obtain accurate results using this technique. We report here direct measurements of the spatial damping coefficient of capillary waves in the presence of a surfactant monolayer using a double-beam phase-matched Fourier transform heterodyne technique. This technique allows one to obtain accurate damping coefficients without any calibration or instrumental deconvolution.

© 1992 IQEC