SURFACE STRAIN MODULATION OF INSOLUBLE SURFACE-FILM PROPERTIES

The complex dispersion relation of capillary waves on water has been m easure in a wavelength regime (lambda approximately 1-2 cm) of interes t to radar imaging. Two noninvasive sensors: a capacitive wave height antenna and a one-dimensional scanning laser slope gauge, are used to study the alteration of surface tension and surface elastic modulus in the presence of monomolecular surface films. On a static water surfac e, these sensors are compared to accepted standard techniques. The sur face tension results are compared to a Wilhelmy plate while the elasti c modulus results are compared to equations of state obtained by Langm uir trough techniques. The sensors are then employed to measure the re sponse of surface film properties to surface areal strains for film pr essures and strain rates, which are comparable to natural films in the presence of ship-generated surface currents. In the experiments repor ted in this paper, only insoluble surfactants are used. The monolayer surface films considered in this study are found to exhibit higher cla stic modulus (approximately 8 mN/m) at small surface pressure (pi less -than-or-equal-to 1 mN/m) than expected from previous work reported in the literature. The response of these films to surface strain is cons istent with our static surface measurements. The modulation of film pa rameters is found to be in phase with internal wave-generated oscillat ory surface strain with periods on the order of 9 s. This implies that the film conforms to the strained water surface with negligible slipp age in this regime. These measurements demonstrate the capability of t his approach to continuously monitor surface films in situ with detect or system response time of the order of 1 s. The technique is suited t o large or deep containers and is applicable to sheltered ocean enviro nments.