Surfing on acoustic waves

ETH researchers are able to make objects such as particles and liquid droplets fly in mid-air by letting them ride on acoustic waves.

For the first time, they have been able to also control the movement of objects, merge droplets, letting them react chemically or biologically and even rotate a toothpick in the air.

A toothpick floating in mid-air without any support – this may sound like it involves hidden threads, magnets or other sleight-of-hand tricks from magicians. But the actual trick used by Daniele Foresti, former doctoral student now a postdoctoral researcher at the Laboratory of Thermodynamics in Emerging Technologies, is based on acoustic waves. Despite the appearance of “magic”, he and his colleagues realized and controlled the planar movement of floating objects in air, regardless of their properties, involving no sorcery but science. This is not simply an amusing trick: moving objects such as particles or droplets of a liquid freely in mid-air makes it possible to investigate processes while avoiding any disruptive contact with a surface. For instance, some chemical reactions and biological processes are compromised by surfaces, and certain substances disintegrate on contact with a surface.

Riding a stationary wave

Until now, scientists have been able to generate such a “contact-free” levitational state only with the help of magnets, electrical fields or in liquids with the help of buoyancy. These methods, however, limit the selection of materials that can be handled. “It is extremely difficult to levitate and precisely move a drop of liquid with a magnet. The fluid has to possess magnetic properties. In liquids, where buoyancy force supports levitation, you can only use immiscible liquids such as a drop of oil in water,” explains Dimos Poulikakos, Professor of Thermodynamics and head of the research project.

With acoustic waves, in contrast, it is possible to levitate various objects regardless of their properties. The limiting factor is the maximum diameter of the object, which must correspond to half the wavelength of the acoustic wave being used. An object reaches the stationary levitated state when all the forces acting on it are in equilibrium. In other words, the force of gravity that pulls the object in one direction is counteracted by an equally large force in the opposite direction. This force comes from the acoustic wave, which the researchers generate as a standing wave between an emitter and a reflector that reverberates the acoustic waves. The force of the acoustic wave pushes against the object and thus prevents it from falling due to gravity. It is conceptually similar to the air jet from a fan that keeps a ping-pong ball in the air.

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