University of Bristol

Maybe it won't be unlocking doors any time soon, but a sonic technique can grip and manipulate tiny particles suspended in water. And there's probably a "Doctor Who" fan or two on the team, as it's affectionately referred to by its inventors as a "sonic screwdriver."

Researchers at the University of Bristol's Department of Mechanical Engineering and Northwestern Polytechnical University in China have demonstrated in a paper published May 29 in the journal Physical Review Letters that acoustic vortices act as a sort of sonic tornado, causing particles to rotate and collect at the core of the vortex.

The concept of an acoustic vortex having an effect on particles has been demonstrated previously. In 2001, a team of researchers determined that acoustic vortices could be used to trap particles, but the size of the particle in question has an effect on how it acts inside the vortex.

"Previously researchers have shown that much larger objects, centimetres in scale, could be rotated with acoustic vortices," explained co-author Dr ZhenYu Hong of the Department of Applied Physics at Northwestern Polytechnical University in China.

The team created the vortices using an array of small, ultra-sonic loudspeakers arranged in a circle. A number of differently sized particles were then introduced into the water. What they found was that smaller particles -- less than one micron, or a millionth of a metre -- rotated slowly around the core of the vortex.

Larger particles, on the other hand, behaved differently. When household flour was introduced into the water, the particles either spun at high speeds or became stuck in a series of rings, held in place by acoustic radiation forces.

"We have now shown that these vortices can rotate microparticles [particles between 0.1 and 100 microns in size], which opens up potential applications such as the creation of microscopic centrifuges for biological cell sorting or small-scale, low-power water purification," said Univesity of Bristol's Professor of Ultrasonics, Bruce Drinkwater, study co-author.

"If the large-scale acoustic vortex devices were thought of as sonic screwdrivers, we have invented the watchmaker's sonic screwdriver."