How Ultrasonic Cavitation Bubbles Scrub

Photomicrograph of an imploding cavitation bubble

Pardon us for the take-off on a well-advertised household cleaner.  We have many posts describing how the implosion of cavitation bubbles in an ultrasonic cleaner solution removes contaminants from wetted surfaces. Here we’ll take a look at what happens at the bubble/surface interface in an ultrasonic cleaner.

Airless Bubbles

Let’s start by saying that unlike the household cleaner bubbles cavitation bubbles have no “air” inside.  Instead they have a vacuum.  The nitty-gritty of how is not as important as what happens.

Except to say in brief, cavitation bubbles are created by ultrasonic transducers bonded to the bottom and/or sides of the ultrasonic cleaner tank.  The generator-driven transducers cause a vibration on the tank surface to produce sound waves in the cleaning solution.  This causes compression and expansion in the liquid leading to cavitation bubbles that implode violently and produce shock waves radiating from the site of the collapse.

What is violent?  We’re looking at temperatures in excess of 10,000°F and pressures in excess of 10,000 psi at the implosion site.  Yet the process is so fast that there is little heat buildup and no damage to parts being cleaned.

Transducer vibration is measured in kilohertz (kHz) or thousands of cycles per second and the higher the kHz the smaller the bubbles.  Larger bubbles create more vigorous cleaning action.  Smaller bubbles penetrate smaller openings and provide gentler cleaning action on highly polished or delicate surfaces.

Got that? Now we turn to the topic at hand.

How Ultrasonic Bubbles Scrub

Part contamination can be soluble and/or insoluble.  Cleaning is done at the interface of the ultrasonic cleaning solution and the surface of the contaminant on the part being cleaned.

In the process contaminants are attacked by the cavitation bubbles.  They are dissolved and/or displaced from surfaces and carried into the cleaning solution.   This “carrying away” is necessary because otherwise a saturated layer develops at the interface and either slows or stops the cleaning action.  The carrying away process can be enhanced by agitating the parts in the cleaning solution. This is especially helpful when cleaning small diameter tubes or holes in machined parts.

Contaminants that are removed either rise to the surface to be skimmed off or sink to the bottom of the tank for removal when the cleaning solution is replaced.  For detailed information on this topic see our post on oil skimmers and solution filters.  This applies primarily to large industrial ultrasonic cleaners, but solution maintenance is important for ultrasonic cleaners of every size.

For additional information see our page on how these cleaners work.

Post-cleaning rinsing removes solution residues from the parts and can be done with plain water, DI or distilled water. A separate rinse tank is an option and it may incorporate ultrasonic energy for very thorough rinsing.   On the other hand some parts may not require rinsing.  More details on the process can be found in our post on ultrasonic rinsing tips.

Ultrasonic Cleaning Frequencies

Earlier we mentioned that the size of the bubble size is controlled by ultrasonic frequency, and that the lower the frequency the larger the bubble and the more vigorous the cleaning action.  As an example the radius of a cavitation bubble produced at 37 kHz is approximately 88 microns.  At 80 kHz it is 41 microns.

If you’d like more information on this topic please visit our post on choosing the correct ultrasonic cleaning frequency.

To conclude, in just about every instance ultrasonic cleaning is far more efficient and safer than manual scrubbing using wash tank solvents and sprays.  That’s because no amount of manual scrubbing can reach tiny cracks, crevices and blind holes in complex-shaped parts.   Knowing how cavitation cleans is important but more important is the selection of the correct ultrasonic cleaner size and model, the operating frequency/frequencies, the correct ultrasonic cleaning solution and establishing an orderly cleaning/rinsing procedure.

The scientists at Tovatech are ready to help you in all aspects of the process.