Article Contents

1. Introduction
2. What is "Ultrasonics?"
3. Nature of Sound Waves
4. Cavitation and Implosion
5. Benefits of Ultrasonics
6. Ultrasonics Speeds Cleaning
7. Complex Contaminants
8. Ultrasonic Generators
9. Pulse and Frequency Sweep
10. Frequency and Amplitude
11. Magnetostrictive Transducers
12. Piezoelectric Transducers
13. Ultrasonic Cleaning Equipment
14. Maximizing the Cleaning Process
15. Maximizing Cavitation
16. Minimizing Dissolved Gas
17. Maximizing Overall Cleaning Effect (1)
18. Maximizing Overall Cleaning Effect (2)
19. Conclusion

Ultrasonic Cleaning: Fundamental Theory and Application
(p. 16)

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Maximizing the Ultrasonic Cleaning Process (continued)

Importance of Minimizing Dissolved Gas

During the negative pressure portion of the sound wave, the liquid is torn apart and cavitation bubbles start to form. As a negative pressure develops within the bubble, gasses dissolved in the cavitating liquid start to diffuse across the boundary into the bubble. As negative pressure is reduced due to the passing of the rarefaction portion of the sound wave and atmospheric pressure is reached, the cavitation bubble starts to collapse due to its own surface tension. During the compression portion of the sound wave, any gas which diffused into the bubble is compressed and finally starts to diffuse across the boundary again to re-enter the liquid. This process, however, is never complete as long as the bubble contains gas since the diffusion out of the bubble does not start until the bubble is compressed. And once the bubble is compressed, the boundary surface available for diffusion is reduced. As a result, cavitation bubbles formed in liquids containing gas do not collapse all the way to implosion but rather result in a small pocket of compressed gas in the liquid. This phenomenon can be useful in degassing liquids. The small gas bubbles group together until they finally become sufficiently buoyant to come to the surface of the liquid.

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