Ultrasonic characterization of the marine microlayer

The authors have developed an ultrasonic method to excite and detect transient capillary waves on the surface of water. The measurement is used to determine the surface tension and viscoelastic properties of thin surfactant films on the surface of the ocean. A focused ultrasonic transducer is used to excite a high-amplitude wave whose radiation pressure lifts the surface of the water and allows a capillary wave packet to propagate from the focus of the transducer. Another ultrasonic transducer, which is part of a phase-measuring acoustic microscope, measures the variation in the location of the water surface as the capillary waves pass over the transducer. An optical confocal microscope is used to detect capillary waves, and since it is a noncontacting measurement, the capillary wave evolution can be measured at different distances from the excitation point. The authors have developed a theoretical model to predict the shape of the capillary wave packet; the results of simulations of the evolution of the capillary wave in time and space are shown. The measured capillary wave amplitude is inverted to evaluate the surface tension of the water. Also shown are the results of measurements on sea water samples, which indicate that surfactants float up to the surface of the water and reduce surface tension as a function of time