Researchers from Japan's RIKEN Center for Emergent Matter Science, National Institute for Materials Science (NIMS) and Nagoya University have developed a material, based on graphene oxide nanofillers embedded in a hydrogel, that can channel mechanical energy in one direction but not the other, acting in a “nonreciprocal” way. Using the composite material - which can be constructed at various sizes - the team was able to use vibrational, up-and-down movements, to make liquid droplets rise within a material. Using the material could make it possible to use random vibration usefully to move matter in a preferred direction.
To create the unique material, the group used a hydrogel - a soft material made mainly of water - made of a polyacrylamide network and embedded graphene oxide nanofillers into it, at an angle. The hydrogel is fixed to the floor, so that the top part can move when subjected to a shear force but not the bottom. The fillers are set at an angle, so that they were angled clockwise from top to bottom. When a shear force is applied toward the left, from the direction the nanofillers are leaning, they tend to buckle and hence lose their resistance. But in the other direction, where they are facing away from the force, the applied shear merely makes them stretch even longer, and they maintain their strength. This allows the sheet to deform in one direction but not the other, and in fact the group measured this difference, finding that the material was approximately 60 times as resistant in one direction than the other.
As a demonstration, the team created a block of the material and placed it on a vibrating stand. Depending on the way it was designed and oriented material was able to channel the through the material to make droplets move to the right or left, or even upward through the network but not downward. They could also use the vibrational motion to drive a circular motion that could be controlled to be either clockwise or anticlockwise. In the case of vertical orientation, when drops of colored liquid were injected into the hydrogel, they moved upward as if by magic, with the alternating vibrational movements, which are usually not of any use, being channeled to create net motion.
Finally, as a further test, the group placed C. elegans worms on the material, and though their movements are normally random, they ended up all moving to one or another side of the hydrogel, depending on its orientation.
According to Yasuhiro Ishida of the RIKEN Center for Emergent Matter Science, who led the work: “It was a remarkable and surprising result, seeing how mechanical energy could be channeled in one direction preferably, in such a clear way, and using a material that is rather easy to make and quite scalable. In the future, we plan to find applications for this material, with the hope that we can use it to make effective use of vibrational energy that, up until now, has been seen as waste.”