Abstract:

It is generally recognized that in SBSL, the bubble is displaced slightly above the antinode of the acoustic standing wave which drives the bubble oscillations. This is because the acoustic radiation force must balance the average buoyancy of the bubble. For scientific purposes and to simulate SBSL in microgravity, it would be desirable to reposition the bubble at the antinode. One strategy to achieve that repositioning where the buoyancy force would be balanced entirely by the average optical radiation pressure of a downward propagating laser beam has been numerically investigated. Optical levitation of small bubbles has been previously demonstrated [B. T. Unger and P. L. Marston, J. Acoust. Soc. Am. 83, 970--975 (1988)]; however, it is not trivially applied to SBSL; with cw illumination of a quiescent bubble, the laser power required increases as the cube of the bubble radius. To simulate the large-radius phase of SBSL bubble oscillations, a modified Rayleigh--Plesset equation was numerically integrated. The required power for cw and pulsed laser beams was estimated. The average power is significantly reduced if the beam is pulsed to coincide with the large-radius phase of the oscillations so as to maximize the relevant optical cross section.