A composite material can focus mechanical energy in a desired direction.

A one-of-a-kind material based on nanofillers embedded in a hydrogel and developed by researchers from the RIKEN Center for Emergent Matter Science can “nonreciprocate” mechanical energy in one direction but not the other.

The team was able to use vibrational up-and-down movements with this composite material, which can be made in a variety of sizes, to make liquid droplets rise against gravity within a material. Utilizing this material could consequently make it conceivable to utilize irregular vibrations and move matter in a favored bearing.

The group’s exploration paper is distributed in the journal Science.

Being able to direct energy in a particular direction is a crucial quality that actually makes life possible. By channeling random fluctuations in nature in a non-reciprocal manner, like the well-known Maxwell’s demon, to steer a system away from increasing entropy, many fundamental biological functions like photosynthesis and cellular respiration are possible.

“It was a remarkable and surprising result to see how mechanical energy could be channeled preferentially in one direction in such a clear way, and using a material that is relatively simple to make and quite scalable.” In the future, we want to identify applications for this material in order to make effective use of vibrational energy, which has previously been seen as waste.”

Yasuhiro Ishida of the RIKEN Center for Emergent Matter Science,

In electronics, for instance, devices that enable the conversion of AC current into DC current allow energy to move preferentially. In the fields of photonics, magnetism, and sound, similar devices are utilized. Nonetheless, regardless of the numerous likely purposes, making gadgets that channel mechanical energy has shown to be more troublesome.

Now, a group led by RIKEN has come up with a remarkable, uniform material that can do this and is easy to make. The team used a hydrogel, a flexible substance primarily composed of water and a polyacrylamide network, and inserted graphene oxide nanofillers into it at an angle. Because the hydrogel is attached to the floor, only the top part can move when subjected to a shear force. From top to bottom, the fillers are positioned at a counterclockwise angle.

The leaning nanofillers have a tendency to buckle and lose their resistance when subjected to a shear force from right to left. However, if the force comes from the opposite direction and the nanofillers are facing away from it, the applied shear simply makes them stretch even further while maintaining their strength. The sheet can deform in one direction but not the other because of this. The group measured this difference and discovered that the material was about 60 times more resistant in one direction than in the other.

They made a block of the material and put it on a vibrating stand as an experiment to see what this could do. The material was able to channel the vibrational energy through the material, causing the droplets to move to the right or left, depending on the tilt direction of the embedded nanofillers. They could also use the vibrations to move in a clockwise or anticlockwise direction in a circular motion. Drops of colored liquid that were placed on the hydrogel moved upward against gravity as if by magic when the vibrating stand was set up vertically. This way, net motion was created by channeling alternating vibrational movements, which are typically of no use.

Finally, as a further test, the group placed Caenorhabditis elegans worms on the material in collaboration with researchers from the RIKEN Hakubi Fellows program. Although the worms typically move at random, they all ended up moving to one side or the other of the hydrogel, depending on the tilt direction of the embedded nanofillers.

“It was a remarkable and surprising result, seeing how mechanical energy could be channeled in one direction preferentially, in such a clear way, and using a material that is rather easy to make and quite scalable,” said Yasuhiro Ishida of the RIKEN Center for Emergent Matter Science, who was in charge of the project. We hope to find uses for this material in the future, using it to make efficient use of vibrational energy that has been considered waste up until now.”

More information: Xiang Wang et al, Mechanical non-reciprocity in a uniform composite material, Science (2023). DOI: 10.1126/

Bohan Sun et al, A mechanically one-way material, Science (2023). DOI: 10.1126/science.adh3098

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