Quadrupole resonance of drops driven by modulated acoustic radiation pressure—Experimental properties

Abstract

A new effect of acoustic radiation pressure was detected: the oscillating deformation of mm-radius liquid drops (para-xylene) acoustically levitated in a host liquid (water). A novel method is described for detecting μm amplitude deformations which utilized light scattered by the drop at the scattering angle normally associated with the rainbow. This rainbow photometry technique was used to measure the phase and relative amplitude of steady-state, low-frequency (?100 Hz) deformations induced by a modulated 217.5-kHz acoustic wave. Some of the predictions in ’’Shape oscillation and static deformation of drops and bubbles driven by modulated radiation stresses—Theory’’ [J. Acoust. Soc. Am. 67, 15–26 (1980)] agreed with the measurements. Deformations driven by the radiation pressure could greatly exceed the first-order displacement of the interface. The deformation amplitude varied as the square of the acoustic pressure, and phase had the expected dependence on the modulation frequency when data was normalized according to an experimentally determined damping parameter. The damping, however, was significantly larger than predicted for pure liquids. The interfacial tension inferred from the quadrupole resonance properties was 4% lower than static measurements. The apparatus was also used to levitate superheated drops and the technique may be useful for determining their interfacial tension.