Levitating On Sound Waves
July 17, 2013

Researchers Use Sound Waves To Make Objects Levitate

Lee Rannals for redOrbit.com - Your Universe Online

Researchers at Eidgenossische Technische Hochschule Zurich (ETH) have found a way to use sound waves to make objects levitate.

The team wrote in the Proceedings of the National Academy of Sciences they were able to make objects like particles and liquid droplets fly in mid-air by letting them ride on acoustic waves. Not only were they able to make the objects levitate, but they also showed they could control the movements of objects.

The team said they were able to merge droplets, letting them react chemically or biologically, and even rotate a toothpick in the air. This research makes it possible to investigate processes while avoiding any disruptive contact with a surface. Some chemical reactions and biological processes are compromised by surfaces, so being able to manipulate them or mix chemicals without touching them opens the door to new areas of research.

Scientists have previously generated "contact-free" levitation using magnets, electrical fields and in liquids with the help of buoyancy. However, these methods 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.

The maximum diameter of an object being moved through sounds waves 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. 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 team generated as a standing wave between an emitter and a reflector that reverberates the acoustic waves. As the force of the acoustic wave pushes against the object, it prevents it from falling due to gravity.

"This method of moving levitated objects could have a wide range of possible applications," says Daniele Foresti, former doctoral student now a postdoctoral researcher at the Laboratory of Thermodynamics in Emerging Technologies.

Researchers will be able to use this technique to mix tiny amounts of substances and liquids in a step-by-step manner without any chemical changes arising due to contact with a surface. The team has already tested the method with droplets and particles of several millimeters in diameter.