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
Ostrich leather Ostrich leather Ostrich leather is the result of tanning skins taken from African ostriches farmed for their feathers, skin and meat. The leather is distinctive for its pattern of bumps or vacant quill follicles, ranged across a smooth field in varying densities. It requires an intricate, specialised and expensive production process making its aesthetic value costly. Although the first commercial farming began in South Africa in 1850, the industry collapsed after World War I and the drop in demand for the feathers for fashionable hats and military uniforms. Other products were marketed, with each success battered by world events and