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A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. (Source)

I'm trying to make an artificial species that has tiny, subdermal sacs of non-Newtonian fluid that are designed to reduce injuries due to minor physical trauma; something like a punch or a baseball comes in, hits the skin, and these sacs suddenly change from liquid to solid to absorb a good chunk of the energy.

What are some good naturally generatable non-Newtonian fluids that I can use for this purpose?

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    $\begingroup$ This is answerable as-is, but I'm curious, how big is this creature in relation to a baseball or a fist? $\endgroup$ Jul 26 '21 at 5:47
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    $\begingroup$ What about sinovial fluid? (The liquid that's inside of the joint pads, it does this exact function for our bones) $\endgroup$ Jul 26 '21 at 5:53
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    $\begingroup$ Can you explain what you mean by "biologically-generatable?" Oobleck, a substance named after a goo described by Dr. Suess, is a mixture of water and corn starch. That's a "biologically-generatable" mixture that does what you want, but I'm 97% sure it's not what you're looking for. Are you specifically looking for a fluid that is or could be created by natural physiological processes (no intelligent or, perhaps more accurately, human interference to create the fluid)? $\endgroup$ Jul 26 '21 at 6:15
  • $\begingroup$ @ARogueAnt. Human-sized. $\endgroup$
    – KEY_ABRADE
    Jul 26 '21 at 16:06
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    $\begingroup$ @JulianaKarasawaSouza That might work, but do you know how quickly the human body can regenerate that? $\endgroup$
    – KEY_ABRADE
    Jul 26 '21 at 16:07
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What the body already uses.

We as mammals already make and use a non-Newtonian fluid for a similar application, called synovial fluid.

It's used in synovial joints such as the ankles and elbows as a lubricant and shock absorber for the hyaline cartilage.

One caveat: I wasn't able to find anything on how quickly it's produced by the body. Per @JulianaKarasawaSouza, there are at least some species where it can take a prohibitively long time.

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The classic non-Newtonian fluid is a starch solution sometimes called "oobleck". Starch is a biological substance (though not one generally produced by animals), and everything biological runs on water -- and that's all there is to oobleck.

Oobleck is non-Newtonian because the molecules of starch (oligosaccharides) are large enough and of a shape to interact significantly in a strong solution in water, and there's nothing that says a protein couldn't have a very similar structure -- which would put it more in the realm of something an animal might naturally produce. Existing animal body fluids that are close might include the synovial fluid that lubricates joints, or the vitreous humor inside the mammalian eyeball. Expanding the animal domain a little, spider silk has some of the same characteristics.

Regeneration rate is almost irrelevant with non-Newtonian fluids, because they aren't used up when they stiffen in response to a high stress -- oobleck returns to its "normal" low-rate viscosity instantly; you can run across a pool filled with it, and immediately open a drain and it'll pour out. Like the synovial fluid in the joints, the body makes it as the joint grows, and only needs to replace it at the rate it breaks down and is removed by bodily maintenance functions.

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