[chris (OP)]

Great white shark skin is covered in tiny tooth-like scales called denticles:


denticle.jpg (284.69 kB . 1379x1014 - viewed 6272 times)

Scientists have proved that they make the fish move faster and, if the design is copied onto a swimsuit, it can make a swimmer move faster.

But how? Why should make a surface rougher mean that it feels less water resistance?

(Image credit - Pascal Deynat/Odontobase - Wikipedia)

[mrsmith2211]

It reduces turbulence, like dimples on a golf ball or a rough surface in a windsurfer,  A smooth golf ball hit by a professional golfer would travel only about half as far as a golf ball with dimples does.  https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/

[chris (OP)]

It reduces turbulence, like dimples on a golf ball or a rough surface in a windsurfer...

I think you've got that wrong: from the reference you supplied above:

"Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball's surface..."

So, actually, the role of the rough surface - either of the golf ball or the shark skin denticles - is to CREATE turbulence at the surface-fluid boundary, which reduces the lower-pressure wake behind the moving body and thereby reduces drag forces...

[evan_au]

There is a major difference between denticles and dimples in a golf ball - the golf ball is not alive.

I read many years ago that each shark denticle is coupled to an underlying muscle, making it seem possible that the sharks are able to control the buildup of turbulence by active control of their surface. But modern research is not so clear on this finding.

Dolphins have a slightly grooved skin, which develops large, moving dimples when they swim quickly. This definitely reduces turbulence, and it is definitely controlled by muscles.

The following article in Nature is a bit technical, but it illustrates that both shark skin and dolphin skin significantly reduce friction, by controlling turbulence.
See: https://www.nature.com/articles/srep06650

[RD]

... large ... dimples ... definitely reduces turbulence ...

[evan_au]

(denticles) make the fish move faster

Today's podcast described how mantis shrimp are able to strike their prey with extreme speed underwater.
But attempts to reproduce this effect in a man-made model are not able to achieve the same speeds, and lose lots of energy in forming bubbles underwater.

Perhaps your guest should next study mantis shrimp skin?
..or maybe it is the mantis shrimp slime that does the trick? A shear-thinning liquid might assist rapid movement...

[chris (OP)]

Indeed - perhaps that's their trick...