An ultrasonic phase sensitive method for surface velocity measurements in a liquid

A highly sensitive ultrasonic method for measuring the surface velocity in a liquid is described. The method is based on sensitive detection of the phase of a high frequency continuous ultrasonic wave (probe beam) reflected from the moving surface. A simple analysis shows that the interaction, through the acoustic nonlinearity parameter B/A of the fluid, between the reflected carrier wave and the low frequency pressure wave transmitted by the moving surface in the liquid, produces a phase-shift of the carrier proportional to the surface velocity and to the time delay undergone by the probe beam. Results of experiments carried out in water with a 30-MHz focused transducer probe are in good agreement with the analysis. Surface velocity smaller than 0.1 mm/s, i.e. mechanical displacements smaller than 5 pm can be detected in a 5 MHz bandwidth. Lateral resolution of 0.4 mm has been achieved. Compared to optical techniques, this method has the advantages of compactness and low sensitivity to surface roughness.