Ultrasonic Power – Reaching the Limits?

Ultrasonic power has been a topic of much discussion ever since ultrasonic technology was first used for cleaning nearly a century ago. The quest has always been for more ultrasonic power, the thought being that higher power would produce better cleaning results.  As I have discussed earlier, higher ultrasonic power typically requires an increase in the amplitude of vibration of the ultrasonic transducer.  Historically, the major factors limiting the amplitude of vibration were the ability of the ultrasonic generator to produce sufficient energy and the ability of the transducer to translate that energy into the increased vibration amplitude required to achieve an increased power output.

Over the years, both generator and transducer technology have improved immensely. It occurs to me that we may now have the capability to produce vibration amplitudes that exceed the ability of the cleaning liquids to accept them.  So, what does this mean?

In order to be useful, the vibrations produced by the ultrasonic transducer must be transmitted efficiently into the cleaning liquid.  This requires that the liquid in contact with the transducer face stay in contact with it throughout its oscillatory cycle.  If contact is lost, energy is not transferred but, rather, lost at the interface.  As the amplitude of vibration increases, the acceleration as the transducer reverses direction from its extent of travel in one direction to the opposite direction must also increase.  Acceleration, in turn, produces strain in the bond between the liquid and the vibrating surface during the rarefaction part of the cycle.  Once this strain exceeds the strength of the bond between the liquid and the oscillating transducer face that bond is broken and contact is lost.

I hear you saying, “What did he say?”  Maybe relating ultrasonic energy to heat will help.

In the conduction of heat, the mechanism is to move heat from one place to another by contact between the heat source and the destination.  The factors that effect the rate of heat transfer are the temperature difference between the source and destination of the heat, the heat conductivity of the materials involved and the heat transfer surface area.  See Related Blog on Heat Transfer  The laws of heat transfer are well defined and accepted but there are limits to how much heat can be conducted across a given interface.  If heat can not be conducted away from the source fast enough, the temperature of the source will continue to increase uncontrolled.  If we relate heat to ultrasonic vibration, it makes sense that there might be a similar relationship when it comes to vibrational conductivity and surface contact area.  A logical conclusion would be that there is a limit to how much ultrasonic energy can be transmitted from a vibrating transducer into a liquid based on the contact surface area and the ability of the interface to transmit or couple vibrational energy.  Unfortunately, the rules for ultrasonic energy are not well defined as are those for heat but we must accept that there is likely a limit for ultrasonic conductivity just as there is for heat conductivity.  The pertinent question is, have we reached that limit with ultrasonic cleaning equipment today?  Is it now possible to generate more sound than can be transmitted into a liquid under normal conditions?

There are strong indications that we may have reached the limit.  One indication is the presence of transducer erosion which is more prevalent in high power systems and those operating at higher liquid temperatures.  It is thought that transducer erosion may be a result of surface cavitation which may, in turn, be a result of the bond being lost between the vibrating transducer surface and the liquid as the tension during the retracting portion of the vibratory cycle exceeds the forces of attraction between the two.  This loss of contact results in the two repeatedly crashing into each other as pressure is re-applied during the advancing portion of the vibratory cycle.

If we can prove the above, there could be serious consequences as we try to increase ultrasonic power above some practical limit at which the addition of more ultrasonic power does not produce a resulting benefit to the cleaning process.  It also raises the question of what might be done to increase the amount of ultrasonic energy that can be conducted across a given transducer/liquid interface.

FJF