Scientists Have Come up With a Way to Store and Release Mechanical Waves With No Energy Loss

Scientists have demonstrated, in a proof-of-concept experiment, that it is possible to capture and store a mechanical wave without energy loss and then guide it towards a specific location.

The breakthrough—described in the journal Science Advances—could improve our ability to manipulate waves, which would have implications for a broad range of fields.

The experimental setup consisted of a long, carbon steel bar with a cavity in the middle and two actuators—devices that turn energy into motion—at each end. The researchers produced two mechanical waves using these actuators that traveled in opposite directions.

"A mechanical wave is similar to a wave propagating on the surface of the ocean, but it carries vibrations that travel in solid materials," Andrea Alù, lead author of the study from the City University of New York, told Newsweek. "A good example is the wave traveling along a guitar string."

"Typically, storing energy in resonant cavities is inefficient, and it is challenging to accumulate signals and release it on-demand at a later time," he said. "Our experiment proves that it is possible to efficiently store energy in a given region and then release it on-demand towards a preferred direction."

In their research, the team realized materials that absorb energy can be designed to extract all the "impinging energy" coming from a source.

This phrase essentially means all of the energy being carried by a wave. Typically, if a wave is absorbed by the material upon which it's impinging—i.e. comes into contact with—some of the energy is lost.

"[Absorbing materials] transform this energy in other forms. Here, we wanted to mimic the absorption process in terms of efficiency, but using a system that preserves the energy in the same form, and can then release it on demand," Alù said. "Two years ago we showed, theoretically, that it is possible to achieve this goal by controlling and tailoring the waves' time evolution, so that when they came in contact with non-absorbing materials, they would efficiently pile up in the material without reflections, as if they were absorbed."

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Experimental setup, consisting of a waveguide bar with a cavity. The excitation of elastic waves traveling along the bar is provided by actuators placed at the two ends of the system. Giuseppe Trainiti, Georgia Tech

"We named this concept coherent virtual absorption. This method prevents the wave impinging on the structure from escaping, and it gets efficiently trapped inside as if it were being absorbed. The stored wave could then be released on demand," she said.

Essentially, by mimicking absorption, the researchers managed to capture all of the wave's energy and not convert into a specific form of energy—such as heat.

In the experiment, the team excited the carbon steel bar using the actuators—producing two mechanical waves traveling in opposite directions—and carefully controlled the time variations of each wave to ensure that the cavity would retain all of the impinging energy.

"Then, by stopping the excitation or detuning one of the two signals from the other, we were able to control the release of the stored energy and send it towards a desired direction," Alù said.

According to the researchers, the results of this study could lead to advances in several areas.

"Our experiment with mechanical waves shows a new degree of control of vibrations, which are commonly used for monitoring the integrity of structures like bridges, so our work can improve the efficiency of these integrity control systems," Alù said.

"In addition, we are currently exploring the application of similar concepts to other types of waves, including light. We envision efficient devices for optical computing and quantum computing based on similar principles, and improving the efficiency of wireless charging, memories and switches," he said.

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Stock photo: Illustration of a wave. iStock