Devices such as ultrasonic cleaning baths and sonochemical processors can produce high acoustic pressures, resulting in acoustic cavitation. This can be described briefly as the growth and oscillation of microbubbles in response to a driving acoustic field. If the acoustic and medium conditions are conducive, the microbubbles can collapse, or implode, and the process is illustrated below:

A bubble initially at rest can grow during the rarefactional half-cycles of an applied acoustic field. Through rectified diffusion, gas and vapour are transported into the bubble, until it reaches a critical size and collapses. The bubble contents are compressed rapidly, resulting in extreme local conditions and a range of secondary effects that drive process, and may also be measured.

Extreme conditions occur at the collapse phase, perhaps with temperatures up to 5000 K and pressures of 1000 atmospheres, and these lead to physical and chemical effects that drive applications and processes: for example, ultrasonic cleaning (in which surface contaminants are removed by cavitation jetting and erosion), sonochemistry (improved reaction efficiency when ultrasound is applied through the generation of free radicals and enhanced mixing) and sonocrystallisation (in which collapsing bubbles form 'seed sites' from which crystals can grow in a controlled manner).

These secondary effects may also be detected and measured to provide information on the cavitation dynamics. Depending on the originating mechanism, however, each measured observable is likely to have a different dependence on the cavitation 'amount', 'activity level' or 'energy'. Some detection techniques may not be useable, or useful, in particular environments and applications.

The range of ultrasound applications that can generate cavitation is vast, and is shown pictorially below:

In some cases, such as diagnostic medical ultrasound, cavitation is an undesirable (and potentially hazardous) by-product of the application. Specified safety criteria exist to limit the acoustic output of these devices and minimise the likelihood of cavitation occurring. However medical applications such as lithotripsy and HIFU are designed for therapeutic benefit, and cavitation may be beneficial in breaking up kidney stones and treating cancerous cells.

To optimise processes and help ensure the safety of clinical applications, NPL is developing reference facilities and measurement techniques for acoustic cavitation. Much of the research involves collaboration with industry, and we are forming a Measurement Club to provide a forum for discussion.

To get involved, please contact Mark Hodnett