Direct observation of microbubble oscillations

The study of microbubble oscillations is important for the understanding of sonoluminescence phenomena. Bubble dynamic behavior is conventionally detected with light scattering techniques. This method generally gives little information about the shapes of a bubble during its oscillation. In order to observe the bubble shapes, a direct imaging system was set up. The shape of a bubble levitated in a liquid is magnified and displayed on a TV screen. The bubble is illuminated with an LED lamp which is strobed at a frequency slightly different from the driving sound field. This technique can slow the moving image of the bubble, allowing one to observe the shape of a bubble oscillating between 5 to 100 μm in diameter. Experiments show that when a bubble oscillates with sonoluminescence, it keeps a closely spherical shape during the entire oscillation period. However, with the increase of gas concentration in the host liquid, sonoluminescence disappears and higher mode shapes are developed during the bubble’s shrinking period. Experiments also show that the asymmetric shape of the bubble may cause the levitated bubble to become unstable. [Work supported by NASA through JPL, contract 958722.]