Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Alfred Ehrenreich placoid scales, or dermal denticles; leather made with these scales still attached is referred to as Shagreen. However, Ehrenreich had to find a way to remove these scales from the hides in order for his product to reach a wider market. Initially he tried a process patented by a Danish chemist called Kristian Bendixon. Later the company switched to a system that involved the hides being treated by acid. Two chemists came up with similar ideas, Theodore H. Kohler and Dr Allen Rogers. Once treated in this way the skin could be split into layers, to both increase the yield