The History Of Ultrasound: The Cavitation

People have come up with many ways to clean the surfaces of various contaminants. Now the list is replenished with ultrasound. Ultrasonic cleaning either replaces or complements traditional treatment methods and techniques – from manual operations with different solutions to jet washing machines.

One of the main advantages of ultrasonic cleaning over other methods is its unsurpassed level of efficiency, furthermore it is much easier to clean parts with a complicated shape, difficult places, narrow slit, small holes and cavities. Ultrasonic cleaning efficiency allows replacement of flammable or expensive organic solvents with aqueous solutions of alkali metal salts, liquid freon and other less dangerous and less expensive substances. The industrial application is gaining momentum with the emergence of innovating businesses like Hilsonic – one of the most advanced ultrasonic cleaner manufacturers in UK.

Cavitation story

How do you explain the high efficiency of ultrasonic cleaning? The answer to this question is connected with a very interesting physical phenomenon called cavitation (from Latin ‘cavitas’ means ‘empty’).

In theory, the existence of the phenomenon is known since then as the Petersburg academician Leonhard Euler proved the possibility of breaks formation in the fluid (voids) due to local pressure reduction followed by slamming any cavities. But in fact Euler predicted cavitation and never saw it in action.

Cavitation was encountered massively much later during the 20th century, when side paddle wheels were removed from the ships in favor of the screws rotating at high speed. Captains began to notice that the speed of their vessels with the passage of time gradually falls for no apparent reason. But the reason was there, and it was quite visible. When examining screws one of the ships set out in the dock for repairs, its blades looked like slightly damaged across their surface. This phenomenon, of course, was immediately studied, as shipbuilders and hydraulic turbines manufacturers were puzzled about the ways to deal with this formidable and implacable enemy, how to protect the propeller blades and turbines from the damaging effects of clouds of cavitation bubbles, which have been found.

The paradox of bridling the destructive nature of cavitation has arisen relatively recently – from the time when the phenomenon of ultrasound was studied in the labs. And this is where it takes its roots.

So how it works?

Cavitation bubbles appear not only in rotation and screws turbines. They appear if the fluid to emit ultrasonic vibrations. Cavitation that occurs under the influence of ultrasonic vibrations, sometimes called ultrasonic cavitation. In accordance with the alternating field ultrasonic vibrations form high and low frequency pressures in liquid. In rarefied zone the hydrostatic pressure is reduced to such an extent that the forces acting on the molecules of the liquid becomes larger than intermolecular cohesion forces. The sudden change of hydrostatic equilibrium liquid is broken, generating numerous bubbles of gases and vapors, before being dissolved in a liquid state.

In the next moment, when the fluid comes a period of high pressure, the bubbles formed earlier slam. A shock wave with very high local instantaneous pressure several hundred atmospheres. These countless microexplosions cavitation bubbles and tear off the surface of the workpiece dirt, grease, scale and often even rust.

Cavitation easily detected on the nebulous cloud in the ultrasonic field. At high intensities, cavitation noise is reminiscent of a boiling kettle hiss.