Abstract:
An underwater bubble is well known to grow in response to a strong negative acoustic pulse and then collapse because of inertial forces. Here it is shown that adding an auxiliary positive pulse after collapse begins intensifies the collapse. The negative-then-positive pulse sequence is produced by two ellipsoidal reflectors, each with an electrical spark at its near focus f[inf 1] and beamed so that they share a common second focus f[inf 2]. The negative pulse is produced by a polyurethane (pressure release) ellipsoid, the positive by a brass (rigid) ellipsoid. A timing circuit controls the delay between the pulses. Cavitation is recorded by pitting (caused by bubble collapse) of an aluminum foil membrane, which is centered at f[inf 2] and lies coplanar with the two crossed beams. When the brass reflector is fired alone, a narrow path of ~1-mm diameter pits appears. Firing the polyurethane reflector alone yields more widespread, ~0.1-mm diameter pits. When both are fired, a pitted X pattern shows the position of the two beams. If the delay between the two firings is 2--6 (mu)s, the intersection of and the centerline between the paths erupts with deep pits. [Work supported by ONR.]