The hydrodynamic function of shark skin and two biomimetic applications

Abstract

SUMMARYIt has long been suspected that the denticles on shark skin reduce hydrodynamic drag during locomotion, and a number of manmade materials have been produced that purport to use shark-skin-like surface roughness to reduce drag during swimming. But no studies to date have tested these claims of drag reduction under dynamic and controlled conditions in which the swimming speed and hydrodynamics of shark skin and skin-like materials can be quantitatively compared with those of controls lacking surface ornamentation or with surfaces in different orientations. We use a flapping foil robotic device that allows accurate determination of the self-propelled swimming (SPS) speed of both rigid and flexible membrane-like foils made of shark skin and two biomimetic models of shark skin to measure locomotor performance. We studied the SPS speed of real shark skin, a silicone riblet material with evenly spaced ridges and a Speedo ® ʻshark skin-likeʼ swimsuit fabric attached to rigid flat-plate foils and when made into flexible membrane-like foils. We found no consistent increase in swimming speed with Speedo ® fabric, a 7.2% increase with riblet material, whereas shark skin membranes (but not rigid shark skin plates) showed a mean 12.3% increase in swimming speed compared with the same skin foils after removing the denticles. Deformation of the shark skin membrane is thus crucial to the drag-reducing effect of surface denticles. Digital particle image velocimetry (DPIV) of the flow field surrounding moving shark skin foils shows that skin denticles promote enhanced leading-edge suction, which might have contributed to the observed increase in swimming speed. Shark skin denticles might thus enhance thrust, as well as reduce drag.

Mechanical challenges to freshwater residency in sharks and rays

“…The fins' parasite drag coefficient likewise includes a similar friction factor. The coefficient K and exponent α (>0) parameterize the fluid's friction as the combined result of a shark's denticulated skin and swimming motions on the body (Oeffner and Lauder, 2012;Shelton et al, 2014). Recent studies of shark hydrodynamics make it clear that the interactions of the boundary layer generated by the skin's denticles interacts with the flows created by the tail's motions in ways that do not always minimize body drag, and moreover in ways that are difficult to quantify in simple formulas such as in Eqn 3 (Shelton et al, 2014).…”

Advances in Bio-inspired Tribology for Engineering Applications

“…But one important effect of the denticles is to enhance thrust, and not simply to reduce drag. It was later proved by Oeffner et al [86] that the mimicked shark denticles on the suit as seen in Fig. 13 had no beneficial loco-motor effect which should have helped propel the body, but in this case, it can only reduce the skin friction to a certain extent.…”

“…Such denticle structure is common on the body, fins, and tail, although denticles of this species on the head have a different morphology. Scale bar, 50 lm [86] plants. Wilke et al [88] in their attempt to diminish drag and resistance to flow of currents for these applications were inspired from the evolved scales of fast-swimming sharks.…”

Textiles: Fabrics of life

“…Biologically inspired materials might also lead to swifter swimmers. Swimwear companies such as Speedo have claimed to make shark-skin-like fabrics, but George Lauder, an ichthyologist at Harvard University, has demonstrated that the Speedo suits that were banned after the 2008 Olympics do not contain the microscopic shark scales, called denticles, that reduce drag 5 . Using a threedimensional (3D) printer, Lauder last year constructed true artificial skin of the shortfin mako shark (Isurus oxyrinchus) and showed that it not only limits drag but also increases thrust 6 .…”