The Theory of Cavitation
When a diaphragm or solid object is vibrated rapidly in a liquid, compression and rarefaction waves propagate outward from the radiating surface. At high enough intensity these alternating pressure and vacuum waves cause micron-sized bubbles to form. Since the liquid temperature is below the boiling point, there is insufficient energy to sustain the vapor phase of these microbubbles, and as they condense back to the liquid phase the surrounding molecules rush in to fill the void, in affect colliding and rebounding as a shock wave, this is termed ‘Cavitation’. Shock waves are discontinuities in pressure and temperature, which in a collapsing microbubble may be on the order of 15K – 150K psi and 5K – 10K degree C respectively, these values are more than enough to generate ions and create free radicals. The cavitation bubble ideally is a bubble of the vapor of the liquid being sonicated, without any air (gasses), While the cavitating bubble contains the gas-phase of the parent liquid (which is steam in aqueous solutions), any other gasses dissolved or suspended as microbubbles will be forced out of the solution and into the cavitating bubble. When the vapor condenses to the liquid phase, these gasses will remain behind in this bubble. The evacuated bubble will then take in any gases of the parent liquid, and upon collapse of the void, these outgassed materials will form visible bubbles. Continued sonication will cause these bubbles to coalesce and rise out thus degassing and improving cavitational intensity. Solvents with high gas absorption coefficients (Freons) will only degas to a limited extent at atmospheric pressure and will show limited cavitational intensity. Degassing can be achieved more quickly in aqueous solutions by adding small amounts of surfactant, this will lower the cavitation threshold by reducing surface tension on the cavitating bubble, but may ultimately decrease intensity by reducing the sound propagation velocity in this solution. In general, increasing the cavitational threshold increases intensity once cavitation is reached, so that the more difficult it is to produce cavitation, the higher the shock wave (intensity) will be if there is enough power available to overcome the hydrostatic pressure.

Hydrostatic Pressure
The cavitation threshold is directly proportional to the hydrostatic pressure applied to the liquid. The threshold of a liquid can be increased if the hydrostatic pressure is maintained long enough for gas to diffuse out of the nucleus.

Surface Tension
The cavitation threshold varies inversely with surface tension.

The cavitation threshold varies inversely with temperature. The decrease in threshold with increasing temperature is linear, although near the boiling point the threshold drops to zero.

Solid Contaminates
The cavitation threshold increases with decreasing numbers and size of solid contaminates. Theoretically pure water would require impractically high power levels (>20K psi) to initiate cavitation. Because water is never really pure, less than 18 psi acoustic pressure will cavitate most tap water.

Dissolved Ion Concentration
The cavitation threshold as a function of dissolved ions is not simple or straightforward, however, in general as the concentration increases, the threshold, relative to extremely low concentrations also increases.