Resonance of Transverse Capillary and Longitudinal Waves as a Tool for Monolayer Investigations at the Air/Water Interface

Periodical surface density fluctuations, Le., longitudinal waves, and transverse capillary waves are inherently coupled at fi-covered liquid-gas interfaces. A resonance occm for a particular surface dilatational elasticity where both waves have the same propagation velocity. In this paper, periodical surface density fluctuations are excited in polymer monolayers via the generation of transverse capillary waves. Without contacting the monolayer these transverse ripples are excited by an electrocapillary deflection of the interface (100 Hz to 1 IrHz) and detected by specular reflection of a laser beam from the water surface. The surface pressure fluctuations are investigated by superposing two transverse capillary waves perpendicularly to each other under resonance. In addition, the surface pressurejarea isotherm as well as the propagation characteristics of transverse capillary waves are given for a poly(viny1 acetate) monolayer.