DIRECT MEASUREMENT OF THE SPATIAL DAMPING OF CAPILLARY WAVES AT LIQUID-VAPOR INTERFACES

We measured the spatial damping of low-frequency surface waves at air- water interfaces using a novel heterodyne light-scattering technique. For pure water the measured damping agrees well with linear hydrodynam ic theory. For interfaces covered with a monolayer of pentadecanoic ac id, we find a 5-fold increase in damping at a surface concentration of 2.2 molecules nm-2, near the high-density end of the gas/liquid-expan ded coexistence region. The behavior of the damping as a function of s urface concentration cannot be explained by existing theories.