At first glance it appears to be a minuscule marble spinning around its vertical axis. Look closer, however, and you see a stationary spherical membrane of fluid, just 3 microns across. It is the stuff inside the droplet that is rotating. This self-contained centrifuge has been created by blasting a droplet of fluid with sound.
It doesn’t just look cool. The manipulation of drops of liquid using sound, known as acoustic microfluidics, is spawning a growing number of applications. Jonathan Cooper, of the University of Glasgow, thinks the technique could do away with bulky laboratory equipment, with all its finicky moving parts. This, Dr Cooper suggests, could radically cut the cost of diagnosing and treating diseases, or detecting harmful pathogens in hospitals, water sources or food-processing facilities.