Shark skin: three-dimensional structure and hydrodynamic function

The skin of sharks consists of numerous tooth-like scales (denticles) that form a rough surface covering the body. Imaging this surface has mostly been accomplished using scanning electron microscopy. But in order to understand the hydrodynamic function of shark skin, it is critical to quantify surface roughness in three dimensions and measure fluid flow on both engineered models and living animals. We provide an overview of our recent work on three-dimensional surface imaging on a diversity of shark species and body locations using gel-based stereo profilometry, and summarize ongoing experiments on (1) denticle models mounted on airfoils to quantify their effect on lift and drag, (2) the propulsion of 3D-printed shark skin flexible foils, and (3) flow over the denticle surface in living sharks. Surface roughness in smooth dogfish varied from 9 to 42 µm and particularly interesting transitions in denticle shape and roughness were observed on the skin over the gills and on the fins. Denticles mounted on a NACA 0012 airfoil increased the lift-to-drag ratio by up to 323%. Experiments on the propulsion of flexible shark skin membranes show that denticle-containing surfaces are capable of both increasing self-propelled speed and reducing the cost of transport.