  Bubbles under vibration can behave in unusual ways, e.g., moving downward
against the force of buoyancy. While the bubble downward motion due to the
Bjerknes force is well known at acoustic frequencies close to the bubble
resonant frequency, these experiments demonstrate that these effects can be
observed at relatively low frequencies as well. Experiments were performed in a
thin, quasi-two-dimensional rectangular acrylic box partially filled with
20-cSt PDMS silicone oil with overlying ambient air. The apparatus was
subjected to sinusoidal axial vibration that produced breakup of the gas-liquid
free surface, producing liquid jets into the air, droplets pinching off from
these jets, gas cavities in the liquid from impacts of these droplets, and
bubble transport below the interface. Vibration conditions for the attached
videos are 280 Hz frequency, 15 g acceleration, and 94 micron peak-to-peak
displacement. Behaviors shown in the videos include the following. 1. Free
surface breakup into jets and droplets, and formation of bubbles under the free
surface. 2. Bubbles thus generated moving downward in the cell. 3. Bubbles
attracted to the first bubble deep in the cell and eventually merging to form a
large bubble at the base of the cell. 4. Bubble cluster at the base of the cell
merging to form a larger bubble, which stabilizes at a levitated location below
the free surface and acts to damp out the surface breakup. 5. The levitated
bubble interface and its breakup are similar to the free surface breakup into
jets and droplets, but the jets in the bubble are facing downward. 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 DE-AC04-94AL85000.
