WHAT do you get if you take a set of miniature silicon helicopter blades, drop them into a beaker of water and blast them with sound waves? A remote-controlled underwater "bubble rotor" that could be used to manipulate individual cells. The rotor, developed by Daniel Attinger of Columbia University in New York, consists of a piece of silicon, 60 micrometres wide, cut into two crossed blades. It can be made to spin by placing it near a bubble of air in water and hitting the bubble with ultrasound waves. Although the bubble rotor itself is 100 times bigger than ordinary...