Initially, the bubble expands almost spherically ( figure 2 a). A typical example is given in figure 2, which shows the non-spherical collapse of a bubble near a rigid plate. In particular, the loss of spherical symmetry can result in the formation of high-speed liquid jets traversing the bubble. The bubble dynamics change when there is a nearby boundary. After its collapse at 7, the bubble can rebound, and may exhibit one or more oscillation cycles. The pressure is low (close to vapour pressure, a few thousand pascal) between 2 and 6. The pressure inside the bubble is high at instants 1 and 7. ( b) The corresponding bubble shapes at different times (indicated with 1–7) of its oscillation cycle. ( a) Radius–time curve of an oscillating bubble. The interested reader can refer to the basic spherical bubble dynamics theory in appendix A. the bubble spends much more time near its maximum size than near its minimum size. This bubble behaviour is strongly nonlinear, i.e. It then collapses (from 5 to 7) to a minimum volume ( figure 1 b). The bubble expands (from 1 to 3) because of a high initial internal pressure, and reaches a maximum size at 4. The radius–time curve shows the variation of the radius of the bubble in time as it oscillates ( figure 1 a). A schematic of the spherical oscillation of a cavitation ‘explosion’ bubble is shown in figure 1. They are found ubiquitously on moving ship propellers, underwater explosions and in shock wave lithotripsy treatment for the disintegration of kidney stones. They oscillate (usually strongly) in volume. In most cases, these bubbles also contain water vapour.Ĭavitation bubbles are not stationary. When there is an external excitation, such as a shock wave or an ultrasound field, cavitation bubbles are nucleated from gas pockets in the liquid. As noted by the ‘father’ of microscopy, Anthoni van Leeuwenhoek (paraphrasing in English), ‘…a large quantity of bubbles came out of the water, rose, and even more appeared when I gently tapped on the glass…’ when he described the use of an ‘air-pump’ to study the presence of air in water and blood. The cavitation bubbles in our bodily liquids are generated from the dissolved gas in the liquid. It is proposed that the ‘fluid cavitation is responsible for the cracking noise’. We conclude the article with some comments on the challenges ahead.ġ. Introduction-how bubbles can help the advance of biotechnologyĬavitation bubbles appear ‘naturally’ in the human body when our joints crack. The nonlinear nature of the sound field and the complex inter-bubble interaction in a cloud present challenges to a comprehensive understanding of the physics of the bubble cloud in HIFU. We shall show some of the phenomena observed in a high-intensity focused ultrasound (HIFU) field. The dynamics of such a bubble cloud is even more complex. In biomedical applications, instead of a single bubble, often clouds of bubbles appear (consisting of many individual bubbles). We shall discuss both experimental studies using high-speed photography and numerical simulations involving shock wave–bubble interaction. The presence of a shock wave complicates the bubble dynamics further. The direction of the jet depends on the ‘resistance' of the boundary: the bubble jets towards a rigid boundary, splits up near an elastic boundary, and jets away from a free surface. However, when there is a nearby surface, the bubble often collapses non-spherically with a high-speed jet. The dynamics of a single spherically oscillating bubble is rather well understood. In this review, we present a variety of acoustics–bubble interactions, with a focus on shock wave–bubble interaction and bubble cloud phenomena. The use of shock waves and ultrasound in medical treatments is appealing because of their non-invasiveness. It is of importance in many biomedical applications where sound waves are applied. The study of the interaction of bubbles with shock waves and ultrasound is sometimes termed ‘acoustic cavitation'.
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