On the modulation of capillary surface waves

Abstract

The amplitude-distance curves of capillary waves at the aqueous solution-air interface of aqueous solutions of SDS, SDS-1-tetradecanol, SDS-monoglyceride, monoglyceride, sodium caseinate and sodium caseinate monoglyceride, together with the phase angle (i.e. the phase difference between the detected and wave generating signal) as a function of the distance were determined by an electromechanical method. It is shown that in several cases a non-trivial amplitude modulation exists (i.e. not due to reflections as in the case of pure water). For the SDS-1 -tetradecanol system the observed modulation was clearly due to the presence of a longitudinal capillary wave whose amplitude is modulated by transverse capillary waves also present as found before by De Voeght and Joos. In the other cases investigated, where a similar modulation phenomenon was observed, the relation with a longitudinal wave of known type could not be checked directly, probably because the complex surface modulus is too low in these cases. A quantitative phenomenological explanation is offered for these modulation phenomena and a method is suggested for the determination of dynamic surface properties of surfaces of intermediate elasticity. In those cases a direct determination of the longitudinal wave is not possible, because the transverse waves are not damped strongly enough. The latter seems to be the case for the monoglyceride system.