Channels mechanical energy in the preferred direction
(Nanowerk News) A research group led by scientists from the RIKEN Center for Emergent Matter Science have developed a unique material, based on nanofillers embedded in hydrogels, which can channel mechanical energy in one direction but not the other, acting in a “nonreciprocal” way. Using a composite material — which can be made in a variety of sizes — the team was able to use a vibrational, up-and-down motion to make liquid droplets rise within a material. Thus, using a material can allow the use of random vibrations which are useful for moving the material in the desired direction.
The research has been reported in Science (“Mechanical non-reciprocity in uniform composite materials”).
Channeling energy in a favorable direction is an important property that truly makes life possible. Many basic biological functions such as photosynthesis and cellular respiration are made possible by channeling random fluctuations in nature in a non-reciprocal way, to keep systems away from entropy, like Maxwell’s famous demon. Devices that allow energy to move are preferentially used in many fields, such as electricity, where they allow AC current to be converted to DC current, as well as in photonics, magnetism, and sound. However, it is proving more difficult to fabricate devices that transmit mechanical energy, which could also have many potential uses.
Now, the RIKEN-led group has developed an extraordinary yet uniform material that is relatively easy to manufacture and can perform this function. To make it, the group used a hydrogel — a soft material made mostly of water — made of a polyacrylamide network and embedded graphene oxide nanofiller into it, at an angle. The hydrogel is fixed to the floor, so that the upper part can move when subjected to a shear force but not the bottom. And the filler is tilted, so it tilts clockwise from top to bottom.
When a shear force is applied to the left, from the tilted direction of the nanofillers, they tend to warp and therefore lose their resilience. But in the other direction, where they face away from the force, the applied shear force only makes them stretch longer, and they retain their strength. This allowed the sheet to deform in one direction but not the other, and in fact the group measured this difference, finding that the material was roughly 60 times more resistant in one direction than the other.
As an experiment to demonstrate what this could actually do, they made blocks of the material and placed them on vibrating stands. Depending on the way the material is designed and oriented it can funnel through the material to make the droplets move right or left, or even up through the grid but not down. They can also use vibrational motion to actuate circular motions which can be controlled either clockwise or counterclockwise. In the case of vertical orientation, when the colored liquid droplets are injected into the hydrogel, they move upward as if by magic, with an alternating, normally useless, vibrational motion channeled to create a clean motion.
Finally, as a further test, the group, in collaboration with researchers from the RIKEN Hakubi Fellows program, placed C. elegans worms on the material, and although their movements were usually random, they eventually migrated to one or the other side of the hydrogel, depending on their orientation.
According to Yasuhiro Ishida of the RIKEN Center for Emergent Matter Science who led the research, “It was an amazing and surprising result, seeing how mechanical energy can be channeled in one direction in such a clear way, and using materials that are somewhat easy to manufacture and are quite scalable. In the future, we plan to find applications for this material, with the hope that we can use it to effectively utilize vibrational energy that has hitherto been considered waste.”
