The study of the interaction of bubbles with shock waves and ultrasound is sometimes termed ‘acoustic cavitation'. It is of importance in many biomedical applications where sound waves are applied. The use of shock waves and ultrasound in medical treatments is appealing because of their non-invasiveness. In this review, we present a variety of acoustics–bubble interactions, with a focus on shock wave–bubble interaction and bubble cloud phenomena. The dynamics of a single spherically oscillating bubble is rather well understood. However, when there is a nearby surface, the bubble often collapses non-spherically with a high-speed jet. 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. The presence of a shock wave complicates the bubble dynamics further. We shall discuss both experimental studies using high-speed photography and numerical simulations involving shock wave–bubble interaction. In biomedical applications, instead of a single bubble, often clouds of bubbles appear (consisting of many individual bubbles). The dynamics of such a bubble cloud is even more complex. We shall show some of the phenomena observed in a high-intensity focused ultrasound (HIFU) field. 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 conclude the article with some comments on the challenges ahead.

Most real life bubble dynamics applications involve multiple bubbles, for example, in cavitation erosion prevention, ultrasonic baths, underwater warfare, and medical applications involving microbubble contrast agents. Most scientific dealings with bubble-bubble interaction focus on two similarly sized bubbles. In this study, the interaction between two oscillating differently sized bubbles (generated in tap water) is studied using high speed photography. Four types of bubble behavior were observed, namely, jetting toward each other, jetting away from each other, bubble coalescence, and a behavior termed the ``catapult'' effect. In-phase bubbles jet toward each other, while out-of-phase bubbles jet away from each other. There exists a critical phase difference that separates the two regimes. The behavior of the bubbles is fully characterized by their dimensionless separation distance, their phase difference, and their size ratio. It is also found that for bubbles with large size difference, the smaller bubble behaves similarly to a single bubble oscillating near a free surface.

One way to focus the diffuse energy of a sound field in a liquid is by acoustically driving bubbles into nonlinear oscillation. A rapid and nearly adiabatic bubble collapse heats up the bubble interior and produces intense concentration of energy that is able to emit light (sonoluminescence) and to trigger chemical reactions (sonochemistry). Such phenomena have been extensively studied in bulk liquid. We present here a realization of sonoluminescence and sonochemistry created from bubbles confined within a narrow channel of polydimethylsiloxane-based microfluidic devices. In the microfluidics channels, the bubbles form a planar/pancake shape. During bubble collapse we find the formation of OH radicals and the emission of light. The chemical reactions are closely confined to gas–liquid interfaces that allow for spatial control of sonochemical reactions in lab-on-a-chip devices. The decay time of the light emitted from the sonochemical reaction is several orders faster than that in the bulk liquid. Multibubble sonoluminescence emission in contrast vanishes immediately as the sound field is stopped.

We present a study on achieving intense acoustic cavitation generated by ultrasonic vibrations in polydimethylsiloxane (PDMS) based microfluidic devices. The substrate to which the PDMS is bonded was forced into oscillation with a simple piezoelectric transducer attached at 5 mm from the device to a microscopic glass slide. The transducer was operated at 100 kHz with driving voltages ranging between 20 V and 230 V. Close to the glass surface, pressure and vibration amplitudes of up to 20 bar and 400 nm were measured respectively. It is found that this strong forcing leads to the excitation of nonlinear surface waves when gas–liquid interfaces are present in the microfluidic channels. Also, it is observed that nuclei leading to intense inertial cavitation are generated by the entrapment of gas pockets at those interfaces. Subsequently, cavitation bubble clusters with void fractions of more than 50% are recorded with high-speed photography at up to 250 000 frames/s. The cavitation clusters can be sustained through the continuous injection of gas using a T-junction in the microfluidic device.

The scaling relations for bubbles induced by different external sources are investigated based on a modified Rayleigh model and experimental observations. The equations derived from the modified Rayleigh model are presented to describe the collapse of bubbles induced by the different external sources such as electrical spark, laser, and underwater explosion. A scaling law is then formulated to establish the scaling relations between the different types of bubbles. The scaling law reveals the fact that the characteristic length scale factor differs from the characteristic time scale factor for the different types of bubbles. It is then validated by our experimental observations of the spark- and laser-generated bubbles as well as the bubbles induced by underwater explosions from previous published reports. With the present scaling law, studies on spark- or laser-generated bubbles as well as their applications (for example, in industrial or biomedical related applications) can benefit from the experiences and information built up over the years in underwater explosion bubbles. Conversely, it is possible to substitute a spark- or laser-generated bubble for an underwater explosion bubble in the study of a large-scale and complex physical problem.

We report on an efficient ultrasound based technique for lysing Escherichia coli and Pichia pastoris with oscillating cavitation bubbles in an integrated microfluidic system. The system consists of a meandering microfluidic channel and four piezoelectric transducers mounted on a glass substrate, with the ultrasound exposure and gas pressure regulated by an automatic control system. Controlled lysis of bacterial and yeast cells expressing green fluorescence protein (GFP) is studied with high-speed photography and fluorescence microscopy, and quantified with real-time polymerase chain reaction (qRT-PCR) and fluorescence intensity. The effectiveness of cell lysis correlates with the duration of ultrasound exposure. Complete lysis can be achieved within one second of ultrasound exposure with a temperature increase of less than 3.3 °C. The rod-shaped E. coli bacteria are disrupted into small fragments in less than 0.4 seconds, while the more robust elliptical P. pastoris yeast cells require around 1.0 second for complete lysis. Fluorescence intensity measurements and qRT-PCR analysis show that functionality of GFP and genomic DNA for downstream analytical assays is maintained.