Electrowave Ultrasonics Corporation
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 out-gased 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 degass 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.  

Temperature
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.

Bubbles Hotter Than The Sun
Sound consists of altemating expansion and compression cycles traveling through a medium. Compression cycles push molecules together, while expansion cycles pull them apart. In a liquid the expansion cycle of ultrasound can generate sufficient negative pressure to create bubbles (cavities) in the liquid. Ultrasonic cleaning is based on the effects caused by the creation and collapse of these bubbles.

The bubbles form at microscopic points where gases are either dissolved in the liquid or are trapped in other contaminating particles. The negative pressure wave causes the gas to expand, creating a bubble, as the expansion cycle continues, the liquid surrounding the bubble begins to vaporize into the negatively pressurized cavity.

The importance of cavitation is not so much how the bubbles form; rather it is what happens when they collapse. As the bubble reaches the size where the internal negative pressure is greater than the external force which created it, and can no longer absorb energy from the sound wave, it implodes.
The rapid compression of gases and vapors inside the bubble creates enormous temperatures which can be measured at the surface on the order of >5000°C, similar to the surface of the Sun and internally may be as hot as the core of a star. At the same time the pressure inside the bubble is approximately 1000 atmospheres, equivalent to pressures in the Mariana Trench, the deepest point in the worlds oceans.
Because the bubbles are so small compared to the volume of the surrounding liquid the heat dissipates rapidly and the ambient conditions are essentially unaffected. It is estimated that the cooling following the collapse of a cavitation bubble is on the order of 10 billion °C/s, by comparison, plunging red-hot steel into liquid nitrogen produces cooling of only a few thousand °C/s.

The consequences of cavitation vary depending on the materials involved. The presence of a solid renders the implosion asymmetric, with a jet of liquid forming on the side of the bubble opposite the solid. This microjet blows through ttre bubble at speeds of approximately 350 ft/s, a force strong enough to puncture metals. In addition to extreme temperatures and pressures, cavitational collapse produces shock waves in the surrounding liquid which may be on the order of 10,000 atmospheres and capable of slamming particles into fussion-type reactions, as evidenced by metal slurries with melting temperatures up to 3000°C which show this effect. This combination of high temperatures, high pressures and rapid cooling produces conditions unattainable by any other method.
© Electrowave, Inc. 2008