recently, there is such a news: an embarrassing car accident occurred in Oregon, USA. A truck carrying blind eels suddenly overturned on the highway, spilling blind eels all over the ground and causing successive N cars to crash together.

the scene is like this:

there is also this:

fortunately, several drivers only suffered minor injuries. However, the hearts of them and the local traffic police were devastated, because the blind eel was accompanied by a huge amount of mucus, which was very troublesome to clean up. People even use bulldozers and water cannons.

this news has also seen some accounts reported, but it seems that onlookers are still not fully aware of what these guys scattered on the ground have done. So today we're going to talk about how powerful the mucus of blind eels is.

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first of all, let's be clear: blind eels are called eels, but they are not eels. It's just that their English common name of "slime eel" may cause confusion. It also looks weirder than eels:

jelly-like mucus is a way for blind eels to protect themselves. When threatened by predators, they secrete ingredients that form a large pile of jelly-like mucus around them when exposed to water, which can paste the predator's mouth and block their gills.

therefore, it is understandable that frightened blind eels covered the road with mucus in the event of a car accident.... This is just the instinctive reaction of the poor blind eel when he is scared to pee, but the blind eel itself is not bothered by mucus (except in a car accident? ), it only needs to do a "knot" action, that is, to make a circle, and drill through it, it can easily get rid of the slime.

this is a blind eel with mucus:

this is a moving picture of a blind eel knotting:

so, why on earth do you tie a knot? Some researchers believe that this can be explained by the unique hydrodynamic properties of blind eel mucus.

the water content of these mucus is very high, but the texture is quite sticky, thanks to the ultra-long filamentous protein structure. These mucus are viscoelastic non-Newtonian fluids (we have previously introduced the properties of viscoelastic fluids: motion picture appreciation: what kind of liquid can climb up and flow down? Its viscosity is obviously different in two different flow states: one is "stretching" and the other is "shearing". If the mucus as a whole is stretched in one direction, it becomes thicker; and if each layer of liquid within the mucus flows at different speeds, causing more friction inside (this is called shear in hydrodynamics), then the viscosity will decrease. The decrease in viscosity is mainly due to the entanglement of those protein macromolecules in the mucus, which leads to the destruction of the internal structure of the jelly-like mucus.

the following figure compares the different effects of predators and blind eels on mucus:

you can see that making a circle through the middle is conducive to friction inside the fluid, so at this time, the local viscosity decreases, which is really beneficial for blind eels to break away from the mucus.

but the fish that want to eat the blind eel will usually "suck" it, which will produce the effect of stretching, so the mucus will appear more sticky. Therefore, the purpose of defending predators is achieved. There are many examples of non-Newtonian fluids in life, but blind eels are also very good at using non-Newtonian fluids skillfully.

Lukas B ö ni, Peter Fischer, Lukas B ö cker, et al. Hagfish slime and mucin flow properties and their implications for defense. Scientific Reports 6, Article number: 30371 (2016) doi:10.1038/srep30371

finally posted two cute cartoon blind eels _ (: cute "∠) _