Acoustic defoaming liquids

Acoustic transducers operating at 10 and (or) 20 kHz are capable of defoaming liquids provided the acoustic source is placed above the liquid surface upon which the foam is being generated. 

Komarov et al. [61] for the viscous glycerin-water mixtures is reproduced in Figure 7.19. Defoaming at a given frequency is seen to occur at a threshold value of the acoustic pressure. That threshold increases with the frequency but decreases with the increase in viscosity accompanying an increase in the proportion of glycerin in the liquid mixture. 

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. 

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