Gas bubbles in liquid generally rise due to buoyancy but can be forced to move downward or to be stably levitated by subjecting the liquid to vibration (Jameson, 1966; Hashimoto and Sudo, 1980; Leighton et al., 1990; Brennen, 1995; Ellenberger and Krishna, 2007). We have performed experiments in a quasi-two-dimensional test cell in which the motion of bubbles can be observed and measured. This paper presents observations and data regarding the generation and motion of levitated bubbles when the vibration conditions are varied.
While bubble downward motion due to the Bjerknes force is well known at acoustic frequencies close to the bubble resonant frequency, these experiments, like those of Ellenberger and Krishna (2007), demonstrate that such effects can be observed at relatively low frequencies as well. Experiments were performed primarily in one of several thin, quasi-two-dimensional rectangular acrylic boxes partially filled with polydimethylsiloxane (PDMS) silicone oil or deionized water with ambient air above. The apparatus was subjected to sinusoidal axial vibrations for frequencies of 100–300 Hz, displacements up to 200 microns and accelerations up to 35 times standard gravity. Bubbles generated either by direct injection deep in the cell or by free-surface breakup into jets and droplets can, under appropriate vibration conditions, move downward in the test cell, where they are attracted together and merge to form a large coalesced bubble at the base of the cell. That large bubble can then rise until it reaches a location of stable levitation. The bubble damps free surface breakup above it. Under some vibration conditions, the levitated-bubble interface breakup is similar to the free surface breakup into jets and droplets.