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by Gordon Wade '15 
Shark skin has so little drag that Speedo has made swimsuits with materials designed to imitate shark skin. [image via]
Evolutionary solutions to natural problems are often more effective than designed solutions. Biomimicry allows us to adapt these evolutionary concepts into great engineered designs. Sharks, for example, have evolved toward extremely high efficiency in traveling through water, meaning reductions in energy usage and increases in speed. Their physical skin design lowers drag forces associated with movement and has shown great potential for application in human transportation. Sharks are peculiar in that they live in an aquatic environment and never stop moving. Because of this, they must find effective ways to transport themselves through fluids. One adaptation lies in the small-scale structure of their skin. Under a microscope, the surface of shark skin is seen to comprise thousands of small teeth-like structures called denticles. These denticles manipulate fluid dynamics in order to decrease drag. The mechanism is based on the difference in resistance between laminar and turbulent flows. Laminar flow occurs when the movement of fluid past an object is contained in smooth, undisrupted layers. Turbulent flow is chaotic and occurs when these layers are disrupted. 
A representation of different flow patterns. [image via]
Intuitively, it appears that laminar flow would be more effective for quick movement, but it turns out that the opposite is true, due to the formation of a fluidic barrier. Thousands of denticles work together to form turbulent vortices just above the skin of the shark. These vortices form a boundary layer of turbulent flow that surrounds the shark. The shape of the denticles only allows the turbulent flow to contact the outermost tips of the structures. The turbulent flow exerts a high drag force, but it maintains contact with a low surface area at these tips. The greater surface area is present in valleys at the bottom of the denticles, and the vortices insulate these valleys from coming into contact with high-drag water. These valleys protect the majority of the surface area from contacting the laminar flow outside the turbulent vortices, leading to lower drag resistance. This decrease in resistance allows the shark to move through the water more efficiently, meaning it takes less energy to swim a given distance (1). Human applications often borrow sophisticated biological designs. The principles behind this shark skin have the potential to increase transportation efficiency. NASA began developing physical designs based on these properties over 20 years ago with the intention of improving high-speed civil transport. The surfaces used featured narrow groves, called riblets, modeled after denticles and demonstrated decreases in drag forces of up to 7% for airplanes made with these designs. The tests included planes flown at supersonic speeds and demonstrated the ability to significantly cut down fuel usage (2). Current 3D-printing technology, however, is revolutionizing our ability to effectively mimic the material. This has enabled us to make smaller denticle models, which have increased the speed of movement through water by 6.6% and energy efficiency by 5.9% (3,4). They are still ten times larger than the denticles they are based off (the 3D printed models are about 150 um), but as technology improves, so will our ability to more effectively manufacture these materials. Decreasing the size of the denticles to more closely mimic those found in nature will allow manufacturers to incorporate them into a wider range of materials, likely with better results. This could be great for developments in boats, airplanes, or any surface with the primary function of moving through fluids. 
A 3D model of a single denticle. [image via]
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