Scientists use sound waves to keep objects afloat and move around in air

NEW DELHI: Scientists have devised a machine that can keep objects afloat in air, and even move them around, using sound waves. It can handle several objects at a time.

The system was made by researchers at the Swiss Federal Institute of Technology (ETH) in Zurich. It is reported in the scientific journal Proceedings of the National Academy of Sciences.

Dimos Poulikakos, a mechanical engineer at the ETH used the acoustic device to keep water droplets, coffee granules, fragments of polystyrene and even a toothpick afloat in the air by sound waves.

Usually, magnetic forces have been used to levitate objects. Although it was known that sound waves too could counter gravity, there was no practical application since objects could not be moved around.

Poulikakos and his colleagues built sound-making platforms using piezoelectric crystals, which shrink or stretch depending on the voltage applied to them. Each platform is the size of a pinky nail, the journal Nature reported today, describing the discovery.

"The platforms emit sound waves which move upward until they reach surface lying above, where they bounce back. When the downward-moving reflected waves overlap with the upward-moving source waves, the two 'cancel out' in the middle, at so-called node points. Objects placed there remain stuck in place because of the pressure of sound waves coming from both directions," Nature wrote.

Objects can be moved around by adjusting the nodes. The platforms themselves can be arranged in different arrays to provide the movement or position required. By adjusting the position of the nodes, the researchers can tow the objects between platforms. In one demonstration involving a T-shaped array of platforms, the researchers joined two droplets introduced at separate locations then deposited the combined droplet at a third location.

This provides an method of starting chemical reactions with no contact with the container, thus minimizing risk of contamination. Poulikakos suggests that the system could be used to safely try out hazardous chemical reactions. "We had fun demonstrating the idea by colliding a lump of sodium with some water, which is obviously an aggressive reaction," he told Nature.

Peter Christianen, a physicist who works on electromagnetic levitation at Radboud University in Nijmegen, the Netherlands, told Nature that he's impressed with the invention. "I really like it; this is a very versatile platform - almost anything you want to manipulate, you can," he said.