THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

16.3 Cavitation 453Figure 16.2. Variation of threshold intensity with frequency in aerated and gas-free waterat room temperature (20 ◦ C).intensity reduces in value as the pulse length is increased to an upper limit, beyondwhich it becomes independent of pulse length. This frequency-dependent upperlimit would be of the order to 20 ms for a frequency of 20 kHz.The amount of energy released by cavitation depends on the kinetics of thebubble growth and collapse of the bubbles. This energy should increase withsurface tension at the bubble interface and lessen with the vapor pressure of theliquid. Water has a comparatively high surface tension, so it can be a very effectivemedium for cavitation. It can be made even more effective by the addition of 10%alcohol—this results in an appreciable increase in vapor pressure but at the costof a decrease in the surface tension, but the former effect outweighs the lattereffect.Weak emission of light has been observed in cavitation. This phenomenon isknown as sonoluminescence. Frenzel and Schultes first observed its effects in waterin 1934 (Frenzel and Schultes, 1934). Two separate forms of sonoluminescenceare thought to exist: multiple-bubble sonoluminescence (MBSL) and single-bubblesonoluminescence (SBSL). When a sufficiently strong acoustic field propagatesthrough a liquid, placing it under dynamic stress, preexisting microscopic inhomogeneitiesserve as nucleation sites for liquid rupture. Most liquids such as waterhave thousands of potential nucleation sites per milliliter, so a cavitation field canharbor many bubbles over extended space. This cavitation, if sufficiently intense,will produce sonoluminescence of the MBSL type. It was more recently discoveredthat under certain conditions, a single, stable oscillating gas bubble can beforced into large amplitude pulsations that it produces sonoluminescence emission