Ultrasonics – It’s All About Implosion

Preceding blogs have discussed the inappropriateness of using units of measure such as watts per gallon in describing the power and possible overall effectiveness of an ultrasonic cleaning system.  It is all about energy, but only the energy that results in cavitation bubbles that catastrophically implode really counts!  Before I go further, it will be helpful for the reader to understand what happens to all that energy we deliver to ultrasonic cleaning tanks.  The vast majority of it ends up as heat with the remainder (a minute fraction of 1%) radiated as sound.  Heat, although it can be helpful in cleaning, is not our primary goal in an ultrasonic cleaning system.  Cavitation and implosion is one path for the conversion of sound waves into heat but there are other not as useful paths as well.

Conversion of energy to heat in an ultrasonic cleaning system starts with the ultrasonic generator.  The circuitry in the ultrasonic generator utilizes components including transformers, inductors, capacitors, resistors and solid state switching devices all of which, having limited efficiency, result in heat being radiated.  In most cases, heatsinks (sometimes with a fan), are used to remove heat by conduction and radiation.  Although every attempt is made to minimize energy loss in the generator, some is inevitable.

Next after the generator comes the transmission line to the ultrasonic transducer.  As frequency increases, internal energy loss is inevitable as high frequency electricity is conducted in somewhat differently than typical household current at 60 cycles.  Coaxial cables reduce losses but, again, some loss occurs before the energy reaches the ultrasonic transducer.  Most of this lost energy is expended as heat as well.

Ideally, the ultrasonic transducer converts electrical energy to mechanical energy as sound waves.  In doing so, however, additional energy loss is inevitable.  Things including internal (mechanical) friction, electrical resistance (capacitance and inductance) and the transmission of mechanical energy through various mechanical interfaces and bonds creates additional heating and a resultant loss in energy.

The next step, of course, is getting the vibration produced by the transducers into the cleaning liquid.  This, although it would seem simple, is much more difficult than it would at first appear.  For ultrasonic vibrations to be effectively transmitted into the liquid, the liquid must remain in contact with the vibrating transducer surface.  If this coupling is lost, the result is the formation of voids directly at the transducer surface.  This situation is often called surface cavitation or decoupling.  Although cavitation bubbles are being produced that may implode, since this effect does not occur at the surface of the part being cleaned it does not contribute to effective cleaning.

Once the vibrations reach the cleaning liquid, not all cavitation activity produced contributes to cleaning.  Some of the cavitation bubbles created never reach a sufficient size to forcefully implode.  Cavitation bubbles that form but are re-absorbed into the liquid without the violent collapse of implosion represent stable cavitation and result in heating of the liquid due to its internal friction but do not contribute significantly to cleaning.  Cavitation bubbles that form and collapse in implosion represent transient cavitation.  It is transient cavitation that produces the useful cleaning effect.  Transient cavitation, just like all of the above, eventually ends up as heat.

As one can see from the above, there are many routes that electrical energy supplied to the ultrasonic generator to take to the inevitable production of heat. Successful ultrasonic cleaning relies on as much of that energy as possible taking the route through transient cavitation in the liquid.

 – FJF –

Leave a Reply