After we take a stroll on the beach, we walk on the sand with none trouble. The sand appears solid and is difficult to compress. After we put the identical sand grains in an hourglass, they behave very otherwise: the sand flows like a liquid. Granular materials like sand have many interesting properties. In lab-made granular materials, scientists have thus far been in a position to high quality tune their ‘liquid’ flow properties, however the ‘solid’ compressibility properties have remained fairly robust. Researchers from the University of Amsterdam and from Santiago in Chile have now managed to design recent granular materials that also can easily be compressed, a result which could have great potential in applications like shock dampening.
One can find all of them along the coast of Japan: tetrapods, enormous four-legged blocks of concrete that prevent erosion of coastal structures. Together, these tetrapods form a granular metamaterial: a granular material like sand, but designed and made by humans. Tetrapods have their shape for a very good reason. The prolonged legs make it very difficult for a pile of those blocks to flow. Contrary to unusual boulders, they stay of their place, and because of this they do what they were intended to do: prevent the coastline from changing.
Easy compression, easy flow
The instance of tetrapods shows that it is comparatively easy to make a granular system which is nearly incompressible and flows far worse than sand. On the opposite end of the spectrum, it has turned out to be very difficult to create a cloth which is simple to compress and flows higher than sand. On account of the work of the researchers from Amsterdam and Santiago, published within the Proceedings of the National Academy of Sciences this week, this example has now modified — opening up very interesting opportunities.
Daan Haver, first creator of the publication, explains: “In the sphere of metamaterials, we construct the geometry of a cloth in such a way that the fabric has a desired response. For instance, we normally expect an elastic band to turn out to be thinner after we stretch it. In previous work, researchers have shown ways to make materials which turn out to be thinner not when stretched but when compressed, purely based on the geometry of the fabric. This instance shows that it is feasible to tune the properties of your material. We were wondering if we could use this concept to also tune granular materials.”
Within the lab, the researchers made grains that shrink radially when there may be an external pressure. That implies that when a packing of those grains is compressed, the quantity of free space between the grains stays roughly the identical, and because of this the flowing behaviour of the grains stays just like that of a liquid. Haver: “The forces contained in the medium remain low. Due to this fact, the packing shouldn’t be only very compressible but also can flow higher. We put the grains in a funnel. Normally, grains will form an obstructing arc. Nonetheless, when the grains shrink with respect to the opening size, the grains will eventually flow. It was all the time thought that granular materials could be difficult to compress and that changes to grains would deteriorate flowing properties. With our recent grains, we opened up a direction where we are able to create totally different packings which might be easy to compress and which still flow easily.”
Shock dampening
The brand new results could have great potential in shock dampening. The researchers showed that a metal disk, when dropped right into a packing of the brand new grains, slows down over an extended time period and barely bounces back. The energy of the disk’s motion is thus transferred more consistently and homogeneously to the packing. Haver: “Imagine, as an alternative of a metal disk, someone falling in a speed skating race. The impact on the skater when hitting the wall could be small if the brand new grains are used contained in the cushion. As an enormous bonus, the person wouldn’t be bounced back on the track, making the situation safer for everybody involved.”
