'Watchmaker's sonic screwdriver' manipulates microparticles

Using acoustics to force microparticles to spin in water could improve water purification and biological cell sorting, according to researchers at Bristol University. Aptly-named research director Bruce Drinkwater has dubbed the technique “the watchmaker’s sonic screwdriver”.

Working with physicists at the Northwestern Polytechnic University in China, ultrasound specialist Prof Drinkwater developed a system of ultrasonic loudspeakers arranged in a circle to create an acoustic vortex that, like a minute tornado, force microparticles to spin and move towards the centre of the vortex, as the team describes in the journal Physical Review Letters. The effect depends on the size of the particle: while submicron particles rotated slowly around the eye of the vortex, grains of flour were drawn to the centre and either spun on the spot or moved in tight rings.

“Previously researchers have shown that much larger objects, centimetres in scale, could be rotated with acoustic vortices, proving that they carry rotational momentum,” commented ZhenYu Hong, who directed the team in China.

“We have now shown that these vortices can rotate microparticles, which opens up potential applications such as the creation of microscopic centrifuges for biological cell sorting or small-scale, low-power water purification,” Drinkwater said. “If the large-scale acoustic vortex devices were thought of as sonic screwdrivers, we have invented the watchmaker’s sonic screwdriver.”