When bubbles go bad.
Air bubbles in water can sometimes oscillate, rhythmically expanding and contracting at a characteristic rate. But what happens to the oscillations when sound waves of a given frequency hold a single bubble in place while driving its pulsations? Experiments carried out at the National Center for Physical Acoustics in Oxford, Miss., and the Technische Hochschule Darmstadt in West Germany show that such a bubble, instead of remaining spherical, shifts back and forth between a spherical shape and one resembling a triangle, a cube or some other lobed figure. Moreover, the oscillations occur erratically. There seems no way of predicting precisely how long a driven, oscillating bubble would remain in its contorted form before returning to a spherical shape.
Using bubbles about 80 to 90 microns in diameter and a sound-wave frequency of approximately 24.k kilohertz, the researchers took high-speed photographs to capture the bubble shapes. The photo above shows a spherical bubble (left) and its corresponding contorted form (right). The shape of the contorted form seems to depend on the bubble's radius. The researchers also recorded the intensity of laser light scattered from an oscillating bubble to determine when the shape changes occurred.
"We didn't know that bubbles would indeed oscillate chaotically," says mechanical engineer R. Glynn Holt of Yale University in New Haven, Conn., who described the research at last month's Acoustical Society of America meeting in State College, Pa. "We were just seeing how bubbles would oscillate, hoping to get some sort of fuel to add to the fire to get theoretical work going on the subject."
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|Title Annotation:||oscillations of air bubbles in water|
|Date:||Jun 16, 1990|
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