By F. John Fuchs and William L. Puskas Abstract Selection of the proper frequency for an ultrasonic cleaning process and the use of more than one ultrasonic frequency in a single process have been identified as important variables in many ultrasonic cleaning applications. Ultrasonic cleaning effectiveness is enhanced by the use of the proper frequency or multiple frequencies. Recent advancements in ultrasonic hardware offer the user several hardware alternatives when more than one frequency is required in a process. This paper explores the hardware alternatives for producing multiple ultrasonic frequencies and their relative merits. Although three frequencies are used for illustration here, the concepts discussed here may be applied to any number of frequencies. Background Cavitation and Implosions Ultrasonic cleaning relies on the formation and violent collapse of microscopic cavities within a liquid. The cavities, called cavitation bubbles, are formed and grow in liquid under the oscillatory influence of rarefaction (negative pressure) and compression (positive pressure) zones within a traveling sound wave radiating from a vibrating source or transducer. Ultrasonic vibrations are those at frequencies above the limits of human audibility which is around 18,000 cycles per second. Figure 1 – Cavitation bubbles form and grow in response to the passage of sound waves radiating away from a vibrating source or transducer through a liquid. Once they reach an unstable size, they implode releasing a jet of energy in a shock wave. Cavitation bubbles, once formed, either continue to oscillate for a period of time and then degenerate or ultimately grow to a size that can not be sustained and collapse or “implode” releasing a shock wave which radiates in a “jet” from the point of collapse. It is the latter case that produces what is called “transient” cavitation. Those cavitation bubbles which only oscillate and do not implode produce an effect called “micro-streaming” which may provide micro agitation in their immediate vicinity but do not provide the intense shock wave and “jet” associated with the violent collapse of implosion. Only cavitation bubbles that violently implode produce the intense shock waves that do the work commonly associated exclusively with ultrasonic cleaning. Frequency and Bubble Size Relationship The intensity of the shock wave produced by the implosion of a cavitation bubble is directly related to the size of the cavitation bubble. A larger cavitation bubble will produce a stronger shock wave when it implodes. The size of cavitation bubbles produced by an ultrasonic sound wave is inversely related to frequency – larger cavitation bubbles are produced at a lower frequency. This is because a lower frequency generates wave fronts with a longer time interval between them thereby allowing more time for cavitation bubble growth. The number of cavitation bubbles produced increases with frequency. If ultrasonic input power remains constant, a low frequency will produce fewer cavitation bubble implosions each with higher energy while a higher frequency will produce more cavitation bubble implosions each with lower energy. Figure 2 – Two ultrasonic cleaning tanks. In this simplified illustration, the tank on the left, operating at 40 kHz, has three cavitation implosion events each with an energy content of 6e for a total of 18e overall. The tank on the right, operating at 104 kHz, has 9 cavitation events each with an energy content of 2e for a total of 18e overall. Although both tanks have the same overall power, the cleaning results will differ because of their different cavitation characteristics. Ultrasonic Cleaning The two basic mechanisms of cleaning are -
- chemical dissolution or emulsification of soluble soils such as oil
- physical displacement of non-soluble soils including particles
- There must be a continuous liquid in contact with both the source of the sound wave and the surface to be cleaned.
- Cavitation bubbles must form and implode releasing sufficient energy close enough to both the soil and the surface being cleaned to have the desired effect.