Bubble oscillations and motion under vibration

A large gas cavity can be stably “levitated” below a gas-liquid interface by strong vertical vibrations. The gas-liquid interface breaks up under strong vertical vibrations, and the bubbles generated there move downward against buoyancy to the bottom of the container.^1–3 Under proper conditions, these bubbles subsequently coalesce, and the gas cavity thus produced rises to an intermediate depth where it remains for all time. The vertical vibrations also cause the free surface of the gas cavity to break up into jets and droplets just like the gas-liquid interface above it. The downward bubble motion and the stable gas cavity below the gas-liquid interface are manifestations of the bubble Bjerknes force, which is well known at acoustic frequencies but also exists at low frequencies, for which the liquid is incompressible.³ 

Experiments are performed in a thin, quasi-two-dimensional rectangular acrylic test cell (0.6 cm thick, 7.6 cm wide) subjected to sinusoidal vertical vibrations (Fig. 1). The liquid in these experiments is polydimethylsiloxane (PDMS) silicone oil, and the gas is air. The levitated gas cavity spans the cell thickness, and the gas-cavity free surface breaks up into inward-traveling jets and droplets (Figs. 2 and 3 and video). Under certain conditions, the waves on the gas-cavity free surface and the gas-liquid interface are modulated 180° out of phase (Fig. 3). The gas-cavity free surface undergoes minimal breakup under these conditions, and gas-cavity lifetimes of hours have been observed.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.