Engineers create microscopic sonic 'screwdriver'
29 May 2015
A team of engineers has created tiny acoustic vortices and used them to grip and spin microscopic particles suspended in water.
The researchers, from the University of Bristol's Department of Mechanical Engineering and Northwestern Polytechnical University in China, have shown that acoustic vortices act like tornados of sound, causing microparticles to rotate, drawing them to the vortex core. Like a tornado, what happens to the particles depends strongly on their size.
Bruce Drinkwater, Professor of Ultrasonics in the Department of Mechanical Engineering and one of the authors of a paper on the research, published in Physical Review Letters, said the work could lead to potential applications such as the creation of microscopic centrifuges for biological cell sorting or small-scale, low-power water purification.
"If the large-scale acoustic vortex devices were thought of as sonic screwdrivers, we have invented the watchmakers sonic screwdriver," he adds.
The research team used a number of tiny ultrasonic loudspeakers arranged in a circle to create the swirling sound waves. They found that when a mixture of small microparticles (less than 1 micron) and water were introduced they rotated slowly about the vortex core. However, larger microparticles (household flour) were drawn into the core and were seen to spin at high speeds or become stuck in a series of circular rings due to acoustic radiation forces.
"Previously researchers have shown that much larger objects, centimetres in scale, could be rotated with acoustic vortices, proving that they carry rotational momentum," adds Dr ZhenYu Hong, of the Department of Applied Physics at Northwestern Polytechnical University in China.