Defoaming Mechanism of Ultrasound

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. 

FIGURE 7.21 Velocity of sound in foam as function of gas volume fraction, (From Kann, K.B., Colloids Surf. A, 263,315, 2005.) For comparison, the velocity of sound in humid air at 20 kHz is 344.7 m s . (From Lide, D.R. ed., Handbook of Chemistry and Physics, 85th Edition, CRC Press, pp 14-43, 2004.) 

There would appear to be no knowledge of the specific acoustic impedance of foam. It seems probable, however, that the impedance at, for example, the top of 

FIGURE 7.22 Schematic diagram illustrating propagation of sound waves throngh foam in bubble column. Continuous lines represent progressive waves and broken lines represent reflected waves. (After Komarov, S.V., Kuwabara, M., Sano, M. ISIJ Int., 39, 1207, 1999. Originally published in ISIJ International.) 

We therefore have the possibility that the acoustic wavelength is of the same order as the dimensions of bubbles and therefore the lateral dimensions of foam films. This possibility is ignored in the theoretical treatment of acoustic defoaming by Nevolin [76]. That acoustic waves could cause symmetrical thickness oscillations in those films of a similar wavelength has in fact been suggested by Sandor and Stein [59]. This implies some kind of resonance where the lateral dimension of the film is some integer of the acoustic wavelength. It is perhaps easier to see how this could happen in the case of a film vertically oriented parallel to the sound pressure wave. Clearly, however, the actual orientations of films in a typical polydisperse foam are essentially mostly random, with, however, the exception of the dome-shaped films at the top of a foam column. 

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