Surface strain modulation of insoluble surface film properties

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Abstract
The complex dispersion relation of capillary waves on water has been measured in a wavelength regime (λ∼1–2 cm) of interest 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 surface, these sensors are compared to accepted standard techniques. The surface tension results are compared to a Wilhelmy plate while the elastic modulus results are compared to equations of state obtained by Langmuir trough techniques. The sensors are then employed to measure the response of surface film properties to surface areal strains for film pressures and strain rates, which are comparable to natural films in the presence of ship-generated surface currents. In the experiments reported in this paper, only insoluble surfactants are used. The monolayer surface films considered in this study are found to exhibit higher elastic modulus (∼8 mN/m) at small surface pressure (π≤1 mN/m) than expected from previous work reported in the literature. The response of these films to surface strain is consistent with our static surface measurements. The modulation of film parameters is found to be in phase with internal wave-generated oscillatory surface strain with periods on the order of 9 s. This implies that the film conforms to the strained water surface with negligible slippage in this regime. These measurements demonstrate the capability of this approach to continuously monitor surface films in situ with detector system response time of the order of 1 s. The technique is suited to large or deep containers and is applicable to sheltered ocean environments.