London, Nov 8 : Scientists have discovered that sharks can raise their scales to create tiny wells across the surface of their skin, just like the dimples on a golf ball, reducing drag to reach high speeds in the water.
According to a report in New Scientist, the finding was made by Amy Lang from the University of Alabama in Tuscaloosa and colleagues.
The minute scales, which are just 200 micrometers long, are made from tough enamel, such as that found on teeth, giving the skin a rough texture like sandpaper. Lying flat, they had previously been found to reduce drag as the shark swims.
Some reports had also suggested that sharks can bristle their scales, causing them to stand up on end.
So, Lang and her colleagues decided to investigate whether this too could help sharks travel at high speeds.
The team created artificial shark skin with a 16 x 24 array of synthetic scales, each 2 centimetres in length and angled at 90 degrees to the surface of the "skin".
They then placed the arrangement in a stream of water travelling at a steady 20 centimetres per second.
The water contained silver-coated nanospheres, which a laser illuminated to reveal the nature of the flow around the scales.
The experiments revealed that tiny vortices or whirlpools formed within the cavities between the scales.
These vortices form a kind of "buffer layer" between the skin's surface and the fast moving fluid, preventing a turbulent wake from forming behind the shark.
Since a wake has a lower pressure than the rest of the fluid, it exerts a backwards pull on an object, decreasing its speed and making it harder to change direction.
Eliminating this wake decreases the overall drag on the shark, allowing it to travel faster and move more easily without the thick, syrupy feeling humans get as they try to move through water.
The same principle explains the dimples on golf balls, which also create mini vortices to reduce drag in this way.
Ultimately, the team hope further investigations could be used to design torpedoes, underwater vehicles, and even aircraft inspired by shark skin that can move more quickly through water and change direction more easily.