Capillary waves on an asymmetric liquid film of pentane on water

Surface light-scattering measurements have been carried out on thin asymmetric films of pentane on water. We vary the film thickness l over a wide range (10(-9) m < l < 10(-5) m). Compared with the ripplons wavelength 1/q, thick films of pentane (l much greater than 1/q) display the same power spectrum as a pure pentane-vapor interface: a single peak. When thinning the film down to l approximate to 1/q, hydrodynamic coupling between the layers of the interface is revealed by the appearance of a second peak beside the first one. We describe the thickness dependence of the two coupled modes through variations of the positions, the widths, and the amplitudes of both peaks. (C) 2001 Optical Society of America.