Scientists Use Sound to Generate and Shape Water Waves

A group of international researchers have developed a way to use sound to generate different types of wave patterns on the surface of water and use those patterns to precisely control and steer floating objects. Though still in the early stages of developing this technique, the scientists say further research could lead to generating wave patterns to help corral and clean up oil spills and other pollutants. Further out, at the micrometer scale, light waves based on the research could be used to manipulate cells for biological applications, and by scaling up the research to generate water waves hundreds of times larger using mechanical means, optimally designed water wave patterns might be used to generate electricity.

The team conducted laboratory experiments that generated topological wave structures such as vortices, in which the water swirls around a center point; Möbius strips that cause the water to twist and loop around in a circle; and skyrmions, where the waves twist and turn in 3D space.

“We were able to use these patterns to control the movement of objects as small as a grain of rice to as large as a ping-pong ball, which has never been done before,” says Yijie Shen, an assistant professor at Nanyang Technological University in Singapore who co-led the research. “Some patterns can act like invisible tweezers to hold an object in place on the water, while other patterns caused the objects to move along circular or spiral paths.”

Commenting on the findings, Usama Kadri, a reader in applied and computational mathematics at Cardiff University, in Wales, noted that “the research is conceptually innovative and represents a significant development in using sound to generate water waves.”Kadri, who is researching the effects of acoustic-gravity waves (sound waves influenced by gravity and buoyancy) added, “The findings can be a bridge between disciplines such as fluid dynamics, wave physics, and topological field theory, and open up a new way for remote manipulation and trapping of particles of different sizes.”

3D-Printed Structures for Wave Generation

The lab setup consisted of carefully designed 3D-printed plastic structures based on computer simulations, including a hexagonal structure and a ring-shaped structure. Each is partially submerged in a tank of water. Rubber tubing from individual off-the-shelf speakers is attached to precisely sited nozzles protruding from the tops of the structures and are used to deliver a continuous low frequency 6.8-hertz sound to the hexagonal device, or a 9-Hz sound to the ring device. The sounds cause the surface of the water to oscillate and create desired wave patterns. A particular sound’s amplitude, phase, and frequency can be adjusted using a laptop computer, so that when the waves meet and combine in the tank, they create the complex patterns that have previously been worked out using computer simulations. The findings were published in February in Nature.

The wave patterns apply forces similar to those seen in optical and acoustic systems, including gradient forces that change in intensity. They can attract objects toward the strongest part of the wave, like leaves moving to the center of a whirlpool. Radiation pressure pushes objects in the same direction the wave is moving.

“The wave patterns we generated are topological and stable, so they keep their shape even when there is some disturbance in the water,” says Shen. “This is something we want to study further to better understand what’s happening.”

Shen’s previous research was in using light waves to create complex patterns that could be used to trap and move tiny particles of metal. He wondered whether the same phenomenon might be possible with water waves. After discussing the idea with Konstantin Bliokh, a theoretical physicist at Donostia International Physics Center, in Spain, Shen collaborated with experimental physicists at Fudan University and Henan University, both in China, to develop the structures and test the idea.

“This is just the first step, so there is a lot of work to do, starting with investigating whether 3D topological wave patterns can be generated beneath the water and not just on the surface,” acknowledges Shen. “But this research opens the way to possible new applications in wave-induced mechanics.”

But operating the technology in the ocean is an entirely different proposition from conducting lab experiments, says Kadri. Generating and sustaining structured waves at large scales in the open sea will face many environmental challenges, such as noise and sound dissipation that can interfere with wave propagation and its structure, he points out. And when dealing with accidents such as oil spills, “such spills can break up and so require individual manipulation, leading to the wave patterns interfering with each other.”

Nevertheless, Kadri notes, the research prepares the groundwork for a new approach to controlling water waves. And he adds, “It will also likely encourage the validation of other theoretical mechanisms, including acoustic gravity waves, for manipulating moving particles in the deep ocean and so could help direct nutrients to marine life and control the spread of pollutants there.”