A quantitative study of some nonlinear aspects of drop-shape oscillations in a liquid–liquid system has been completed. The results suggest a soft nonlinearity in the fundamental resonant mode frequency as the oscillation amplitude is increased. Indications of an increase in the rate of decay have also been obtained. A study of the internal flow fields has revealed patterns of circulation not present at low amplitude.

An ultrasonic levitation device operable in both ordinary ground-based as well as in potential space-borne laboratories is described together with its various applications in the fields of fluid dynamics, material science, and light scattering. Some of the phenomena which can be studied by this instrument include surface waves on freely suspended liquids, the variations of the surface tension with temperature and contamination, the deep undercooling of materials with the temperature variations of their density and viscosity, and finally some of the optical diffraction properties of transparent substances.

Steady-state acoustic streaming flow patterns have been observed during the operation of a variety of resonant single-axis ultrasonic levitators in a gaseous environment and in the 20–37 kHz frequency range. Light sheet illumination and scattering from smoke particles have revealed primary streaming flows which display different characteristics at low and high sound pressure levels. Secondary macroscopic streaming cells around levitated samples are superimposed on the primary streaming flow pattern generated by the standing wave. These recorded flows are quite reproducible, and are qualitatively the same for a variety of levitator physical geometries. An onset of flow instability can also be recorded in nonisothermal systems, such as levitated spot-heated samples when the resonance conditions are not exactly satisfied. A preliminary qualitative interpretation of these experimental results is presented in terms of the superposition of three discrete sets of circulation cells operating on different spatial scales. These relevant length scales are the acoustic wavelength, the levitated sample size, and finally the acoustic boundary layer thickness. This approach fails, however, to explain the streaming flow-field morphology around liquid drops levitated on Earth. Observation of the interaction between the flows cells and the levitated samples also suggests the existence of a steady-state torque induced by the streaming flows.

Measurements of the characteristics of small-amplitude shape oscillations of drops immersed in a host liquid have been carried out by acoustical means. The resonance frequencies of the first few modes have been measured, as well as the damping constant for the fundamental mode, as functions of the drop radius and viscosities of the two liquids. A qualitative photographic study during steady oscillations has revealed a simple internal fluid-particle flow field with no circulation. The theory available at the present time has been found to provide results which are in general agreement with experimental findings for low-viscosity liquids.

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