Like a fish in water - Saving energy with slime

The natural model ...

The flow resistance of water is much higher than that of air. Fish therefore need more energy than land animals when they move around. But evolution has found ways to keep the energy expenditure as low as possible when swimming through water. One way is to dampen turbulence by using mucus substances from the class of glycoproteins. But how does the mucus layer reduce the fish's resistance in the water?

... and how it works

When water molecules flow along a surface and touch it, friction slows them down to a standstill. In a thin boundary layer, the velocity of the flowing particles increases to the value of the external flow. Particles of different velocities can mix in vortices. This disorder of the boundary layer is called "turbulent flow".

The fish slime lowers the resistance in the micro- and nanometer range. The vortices of the turbulent flow get caught in the long-chain molecules and lose energy in the process: the turbulence is damped. Possible technical uses include firefighting.

Captions

Pike slime is transferred to a test specimen.

Artificial fish slime made of polyethylene glycol under a scanning electron microscope.

Vortex damping in a liquid jet by polyethylene glycol.

Text of the information board in the Botanical Garden