FREE DECAY OF SHAPE OSCILLATIONS OF BUBBLES ACOUSTICALLY TRAPPED IN WATER AND SEA-WATER

Asymptotic results for the free decay of shape oscillations of viscous liquid spheres have been extended to include higher-order terms in th e ratios of the inner and outer viscous penetration lengths to the rad ius. The new expressions are shown to be important for studies in whic h the two fluids have dissimilar densities and viscosities such as air /liquid systems. The analysis also includes an expansion for the frequ ency of maximum response of driven oscillations. The calculations are supported by measurements of the small-amplitude quadrupole mode free decay of nearly spherical bubbles acoustically levitated in clean wate r. The bubble radii ranged from 400 mu m to 1400 mu m The results are interpreted in light of the initial-value problem. The lack of excess damping suggests that the interface behaves ideally for times up to tw o hours after bubble injection. Measurements were also carried out on bubbles in 0.5 M NaCl solution and in sea water. Larger bubbles (radiu s> 800 mu m) in clean water exhibit damping two to four times larger t han predicted by theory. The transition from this anomalous damping to theoretical damping is a very abrupt function of radius. All observat ions were carried out with similar acoustic fields for counteracting b uoyancy. The excess damping appears to be associated with some nonline ar response of the bubble.