Tao Shi
Robert E. Apfel
Dept. of Mech. Eng., Yale Univ., New Haven, CT 06520-8286
The interfacial instabilities between two fluids under the action of gravity, interfacial tension, and acoustic radiation pressure is examined, with particular attention given to instabilities at a flat interface. As an example of the instability at a curved surface, the break-up of a liquid drop levitated in air in a standing acoustic field has been studied using the boundary integral method. The nature of the instabilities at a flat interface is a more fundamental question. The theory to predict the threshold of the instability has been derived. Unlike results for static electromagnetic fields, the threshold depends not only on the properties of fluids but also on the frequency of acoustic waves. Experiments on the instabilities at the interface of carbon-tetrachloride and water were performed with a PZT disc transducer with a resonant frequency of 2.11 MHz. At a critical ``threshold'' level, a disturbance on the interface quickly grows into several sharply pointed peaks, a phenomenon similar to ``Taylor's cones'' observed in an electric field. [Work supported by JPL through Contract No. 958722.]