New Technique Uses Sound Waves to Steer Objects Underwater

Researchers from EPFL have managed to guide floating objects through an aquatic obstacle course using only sound waves. The unique approach, influenced by optics, has much potential for use in biomedical applications, like non-invasive targeted drug delivery. The study has been published in Nature Physics.

The experimental setup, with speakers and microphones at either end of a water tank, and vertical scattering objects at the center. Image Credit: LWE EPFL

The invention of optical tweezers - laser beams that can be used to manipulate microscopic particles - won Arthur Ashkin the 2018 Nobel Prize in Physics. Despite having numerous biological uses, optical tweezers necessitate highly regulated, stable environments to function effectively.

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.

Romain Fleury, Head, Laboratory of Wave Engineering, School of Engineering, EPFL

For the past four years, Fleury and postdoctoral researchers Bakhtiyar Orazbayev and Matthieu Malléjac have been attempting to use sound waves to move objects in dynamic, uncontrolled environments. The team’s technique, known as wave momentum shaping, actually does not care at all about the surroundings or even the physical characteristics of an object. The position of the object is all that is needed; the sound waves carry out the remaining functions.

In our experiments, instead of trapping objects, we gently pushed them around, as you might guide a puck with a hockey stick.

Romain Fleury, Head, Laboratory of Wave Engineering, School of Engineering, EPFL

The novel approach was made possible by funding from the Swiss National Science Foundation (SNSF) Spark program. Scientists from the Vienna University of Technology in Austria, the University of Bordeaux in France, and Nazarbayev University in Kazakhstan collaborated on the study.

Very Simple, Very Promising

In Fleury's analogy, if soundwaves are the hockey stick, then a floating object, such as a ping-pong ball, is the puck. The ball was floating on the water’s surface in the lab's experiments, and an overhead camera recorded its location. The ball was guided along a preset path by audible sound waves coming from speaker arrays at either end of the tank. Meanwhile, a second array of microphones “listened” to the feedback as it bounced off the moving ball and formed a scattering matrix.

The researchers were able to determine the ideal momentum of the soundwaves as they nudged the ball along its path in real-time by combining this scattering matrix with the positional data from the camera.

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

Romain Fleury, Head, Laboratory of Wave Engineering, School of Engineering, EPFL

Wave momentum shaping, he continues, is the first application of the concept to move an object; it is inspired by the optical method of wavefront shaping, which is used to focus scattered light. Furthermore, the team’s technique can be used to control rotations and move more intricate floaters like an origami lotus in addition to moving spherical objects along a path.

Mimicking Conditions Inside the Body

After successfully guiding a ping-pong ball, the scientists carried out more tests with moving and stationary obstacles to introduce more inhomogeneity into the system. The ball was able to navigate around these scattering objects successfully, proving that wave momentum shaping could function well in dynamic, uncontrolled environments like the human body. According to Fleury, sound holds great promise for use in biomedical applications because it is safe and non-invasive.

Fleury said, “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.”

The technique might revolutionize tissue engineering or biological analysis applications where handling cells directly would result in contamination or damage. Fleury envisions further uses of wave momentum shaping in 3D printing, such as organizing microscopic particles before their solidification into an object.

In the end, the researchers think that their approach might also be effective with light, but their next objective is to move from macro- to micro-scale sound-based experiments. The researchers have already been awarded funds by the SNSF to conduct research using ultrasonic waves to move cells around under a microscope.

Moving an object with sound despite disorder

Moving an object with sound despite the disorder. Video Credit: EPFL

Journal Reference:

Orazbayev, B., et al. (2024) Wave-momentum shaping for moving objects in heterogeneous and dynamic media. Nature Physics.

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