Sound waves put levitation on the move

Levitating objects can spin, glide and collide together — no magnets or magic tricks required.

Using steady streams of sound waves, engineers maneuvered hovering toothpicks, coffee granules and water droplets through the air, a team from ETH Zurich reports July 15 in the Proceedings of the National Academy of Sciences. Scientists could use the touch-free technique to gently handle delicate or hazardous lab chemicals or to avoid contaminating cells in biological experiments.

“It’s a beautiful piece of work,” says Penn State bioengineer Tony Jun Huang, who has used sound to manipulate particles in liquid. In a single device, the study’s authors can move two airborne particles in different directions or make them converge. No one has done that before, Huang says.

Scientists have known for years how to use sound waves to hoist particles in the air, a process known as acoustic levitation. But moving the lifted bits around was more challenging. The sound waves tend to trap a levitated object in a fixed pocket of space.

The new technique moves the pockets around by deforming a field of sound waves, letting researchers transport trapped objects several centimeters, says study coauthor Dimos Poulikakos, a mechanical engineer at ETH Zurich.

“Before, it was like you had a beautiful car, but could only park it,” Poulikakos says. “Now you can drive the car.”

To achieve levitation, Poulikakos and colleagues vibrate aluminum blocks about the size of postage stamps up and down, like tiny jackhammers. The rapid buzz kicks up sound waves that sail upward until they hit a Plexiglas reflector and then bounce back down to the blocks.

When these falling waves run into the climbing ones, they can cancel out, creating a low-pressure pocket that can support an object’s weight.

By adjusting vibration rates to control the position of the pocket, the researchers could float particles across a chessboard of the aluminum blocks. The team used the technique to mix droplets of cells with DNA. They also glided a bubble of water into a globule of sodium metal to demonstrate how to safely work with hazardous materials from a distance. When the water struck the metal, the combo exploded, spewing flammable hydrogen gas. 

Poulikakos and colleagues also drifted together levitating drops of water and instant coffee granules to make tiny cups of joe. “That was just for fun,” he says. But the experiment highlights an advantage over magnetic levitation: The new technique doesn’t require the floating objects to be magnetic.

The method may also be an improvement over microfluidic chips, devices that mix tiny amounts fluids together in channels carved into plastic or glass, says Pierre Lambert a microfluidics engineer at the Université libre de Bruxelles.  Though the chips allow scientists to mix chemicals, the liquids still contact the devices’ carved channels. Levitation can keep the liquids from touching anything but air.

“Basically everything that is done on microfluidic chips could be done by levitation,” Lambert says.

Now, Poulikakos is scaling up his chessboard and working on moving heavier objects, such as steel pellets.