Direct observation of microbubble oscillations

A direct imaging system is introduced to study the oscillations of microbubbles acoustically levitated in water. The bubble is illuminated with a light emitting diode (LED) which is strobed at a frequency different from the driving sound field. This technique slows the moving image of the bubble, allowing one to observe the shape of a bubble oscillating between 5 to 100 μm in diameter. The bubble shapes are recorded on video tape and then analyzed by an automatic image analysis system which gives the variations of bubble diameter versus time. Experiments were performed with and without the bubbles sonoluminescing. Asymmetric shapes of the bubbles were observed in some cases.