Two studies conducted at the University of London show that the roughness of fish scales does not act like the typical roughness of other materials. Although the shape and size vary between species, fish scales are essentially a series of overlapping, shell-like protrusions. The peaks of the uneven scales create a slower flow of water behind them. The overlapping sides of the scales create valleys through which most of the water flows. As the fish swims, it creates alternating bands of “low flow”; behind the peaks of the fish scales, “high flow” follows the valleys.

This striated pattern prevents the scales from behaving like a typical rough surface, keeping the flow over the fish smooth, like the gentle flow of a stream. Physicists call this “laminar” flow, which means the fluid flows in parallel streams, or laminations, rather than swirling and eddying. Its opposite—turbulent flow—is like a rushing river, foaming around rocks, swirling, and mixing. It makes sense that fish would swim more easily against the flow of a stream than against a rushing river. But this study isn’t about how laminar or turbulent a body can be in water. It’s about the flow right next to the skin of a swimming fish, in an area called the “boundary layer.”

Bruecker of the University of London explains that for fish swimming in the ocean, water within 30.48 m is not a hindrance, and that distant water does not contribute to the friction that the fish must overcome to move forward. Even a few centimeters of water does not have a significant impact on drag. The drag is affected by the critical millimeter of water volume located right next to the fish's skin.

If the flow in this boundary layer becomes turbulent, Bruecker says, friction increases by almost fivefold. Fish scales, with their lower drag, maintain a steady, laminar flow along the fish's body.

In one experiment, researchers used a dye to visualize flow over a flat, smooth plate and compared it to a scaly surface. Over the smooth plate, the flow quickly “degraded” into turbulent swirling and mixing because there were no structures to smooth it out. In contrast, the red lines on the scale showed that the laminar profile was maintained for a much longer distance from the smooth plate. Of course, as the fish swims faster and faster, the flow will eventually become turbulent. The fish’s scales delay the transition.

Scientists have found that boundary layer flow is determined not only by the shape of fish scales, but also by their size. If the scales were too wide, they would act like a traditional rough surface, mixing the water and increasing drag. So faster swimmers, which have thinner boundary layers, need smaller scales. The researchers think this could explain why some fast swimmers, such as tuna, have smaller scales than slower swimmers, such as carp.

One of the key findings of the study was that delaying the transition to turbulent flow over scaly surfaces can reduce drag by up to 27 %, compared to smooth surfaces. Why is this important for fish? “It’s all about saving kinetic energy,” says Bruecker. More efficient swimming allows fish to travel longer distances with less food. 

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Incorporating fish scales into material designs could also help people use less fuel. Coated coatings in pipes could reduce friction losses along pipelines. Etching the scales onto the surfaces of airplanes, submarines, and cars could also improve fuel efficiency and reduce greenhouse gas emissions.

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Summarized by AskNature.org