Defoaming, ultrasound applications

As we have seen it has been known for more than 60 years that ultrasound can produce defoaming effects [62]. Despite this fact, authors are still forced to write in recent publications, for example, that the mechanisms of ultrasonic defoaming are not well known but it can be assured that they include the effects of the acoustic pressure, the radiation pressure, bubble resonance, streaming and liquid film cavitation [67]. This presumably reflects the difficult nature of the subject, although it may also reflect the limited extent of practical application of ultrasound in this context. It is even possible that these two factors mutually reinforce one another. 

There would appear to be no thorough systematic study of the defoaming effect of ultrasound (at frequencies > 20 Hz) as a function of acoustic pressure and frequency in the case of foams prepared from dilute aqueous surfactant solutions. This could be made with foam of different bubble sizes and low polydispersity to establish, for example, whether there exists a relation between frequency, bubble size, and defoaming. It should include consideration of the effect of foam age and therefore drainage. The application of the relevant method to additionally study the effects of the continuous phase viscosity could also be made. The mechanism of defoaming could be probed further if such studies could be combined with study of the effects of changes in the surface dynamic and rheological behavior of the surfactant solutions. 

For applications where chemical antifoam use is unacceptable, the text examines mechanical means of defoaming, such as the use of rotary devices and ultrasound. 

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