Sound Waves Can Be Used To Move Objects Like A Tractor Beam, Even Around Obstacles

Over the past several decades, scientists have developed optical and acoustic tweezers. Using light or sound waves, they can lift and control the motion of a small object, like a tiny tractor beam . These are exciting developments, but have very stringent requirements to work. Researchers have now developed a new way to move and manipulate objects at a distance, still using sound, that allows them to move on complex paths.

The method is called wave momentum shaping, and it is indifferent to the physical properties of the object and the environment. The team demonstrated it by moving a floating ping pong ball around a series of obstacles.

“Optical tweezers work by creating a light ‘hotspot’ to trap particles, like a ball falling into a hole. But if there are other objects in the vicinity, this hole is difficult to create and move around,” senior author Romain Fleury, from Ecole Polytechnique Fédérale De Lausanne, said in a statement . “In our experiments, instead of trapping objects, we gently pushed them around, as you might guide a puck with a hockey stick.”

The design of the system is ingenious. Speakers and microphones were placed around the water tank, and the speakers produced audible soundwaves that would push the ping-pong ball around. The mics registered the feedback from the waves hitting the ball. This is known as the scattering matrix.

They added a camera to the mix that could see the motion of the ping pong ball, and they were able to calculate the optimal momentum to deliver to the ball to make it move around the obstacles successfully.

“The method is rooted in momentum conservation, which makes it extremely simple and general, and that’s why it’s so promising,” Fleury added.

The team played around with fixed and moving obstacles. The goal is to simulate environments that can change without warning. The team hopes that this method might be used in biomedical applications, so the successes that this approach has achieved are exciting.

“Some drug delivery methods already use soundwaves to release encapsulated drugs, so this technique is especially attractive for pushing a drug directly toward tumor cells, for example,” Fleury continued.

It might also help to manipulate cells or biological tissue outside of a body, since you wouldn’t use instruments that might damage or contaminate the object of study. It might also have an application in 3D printing, and it might be possible to use light as well as sound.

But the team's first step is going from the macro to the micro and see how it would work there.

A paper describing the results is published in the journal Nature Physics .