Ultrasonic cavitation

NPL has developed a sensor capable of measuring localised ultrasonic cavitation - the implosion of bubbles in a liquid when a high-frequency sound wave is applied.

Cavitation is used throughout the NHS by doctors and dentists to clean and disinfect surgical instruments. A high-frequency sound wave is passed through a disinfecting liquid to create bubbles that implode. The force of each implosion removes contaminate particles from surrounding materials.

Cavitation is one of the most effective cleaning processes. But until now there has been no accurate method of identifying how much cavitation takes place at different locations in a cleaning system, and therefore no measurable way to ensure the cleaning process is effective.

Previously, the only way to measure cavitation rates has been to lower a piece of aluminium foil into the liquid and count the number of 'dents' caused by bubble implosion. NPL's new sensor takes a different approach by monitoring the acoustic signals generated when the bubbles implode. It listens to the bubbles as they collapse and uses the sound to identify how much cavitation is taking place at a given location.

'To spark cavitation we use ultrasonics to "shout" at a liquid. Our sensor then listens to the response and tells us how much cavitation is taking place as a result of using that particular stimulus,' explained Mark Hodnett, a senior research scientist at NPL.

Sonic Systems has purchased one of NPL's sensors and said that it fills an important gap in the market. 'There is nothing else like the NPL sensor available to sonic equipment manufacturers. We use it as part of our product development process. It has enabled us to verify the cavitation fields inside some of our more complex systems. This has given us the confidence to confirm to customers that our equipment is truly optimised.'

The device recently won the annual Outstanding Ultrasonics Product award from the Ultrasonic Industry Association.