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I just had a weird thought... We now have a kind of technology that cushions impacts called non Newtonian fluids. Is there a reason we are not yet making judo mats using these materials?

They have been used for helmets and military body armour so why not judo mats?

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    Well, it would certainly feel pretty interesting to fall on them. I suspect one of the reasons might be that non-Newtonian fluids only act in the desirable way you would want a mat to act if they are hit with low-impact. High impact causes them to compress and become solid and stiff, so it would be like falling on a rock. This is good for bike helmets where the goal is to REDUCE the amount of injury done to you, but mats should not be giving you an injury at all. Just a guess, though.
    – LemmyX
    Mar 24 at 14:15
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    Back in the early stone age when I practiced Judo, the mats were there to dampen the impact from the fall. You seem to be advocating the opposite, to want a harder than normal impact when falling. Did I misinterpret the point of the judo mats?
    – pipe
    Mar 24 at 22:57
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    @LemmyX There are also non-Newtonian fluids that behave the opposite way: a high impact causes them to become much less viscous ("thinner" or "runnier") than they are at rest. Mar 24 at 23:51
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    @DmitryGrigoryev How is that opposite? Both shear thickening and shear thinning are non-Newtonian regimes. Newtonian means linear shear stress dependence on the strain. Both concave and convex non-linear are non-Newtonian. And then there is thixotropic and stuff.. None is the opposite of non-Newtonian.
    – Vladimir F
    Mar 25 at 12:03
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There are non-Newtonian fluids that get harder when stressed (like cornflour and water), and there are others that get softer when stressed (like tomato ketchup, that acts solid until you apply stress by shaking it when it turns liquid and runny). There may be potential in filling a judo mat with the latter type of fluid, in that it will be a hard and solid surface when walked on, but will soften if hit with a hard impact. The big problem with fluids is that they can be slow to return to their original shape, and they're messy if they leak.

A better option is nonlinear elasticity in solids. A linear solid follows Hooke's law and stretches or compresses in proportion to the force applied, but then returns to its original shape when the force is removed. A nonlinear spring can be either hardening (it gets stiffer the more it is stretched or compressed) or softening (it gets less stiff the more it is stretched or compressed). A mat made of softening springs could be hard when walked on, but would soften when hit by a large force. And when the force was removed, it would spring back into shape.

They can be constructed using various combinations of rigid and elastic materials. For example, if you consider a rigid rod and push inwards along its length, it is stiff, but if you rotate it and push inwards, it rotates further and is increasingly easy to push. Connect it with elastic to top and bottom surfaces, and you can see that at the start a small downward compression stretches the elastic a lot, but as the rigid rod approaches horizontal, the elastic hardly changes in length any more, and doesn't require much extra force. Theoretically possible, but I'd guess is probably more complicated and expensive to manufacture than it is worth.

Soft spring mechanism

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    There would also be the question at which force you would want to start softening. Sure, a fall has a lot of energy, but if properly executed, it's spread out over a large area, so locally the stress is not that high. After all, that is the main idea of the exercise. On the other hand, during some judo throws, there is a moment where the weight of two people mostly rests on a really small area, possibly even just the part of one foot resulting in a relatively large stress. And this would be precisely the worst moment for a mat to suddenly get all soft and wobbly...
    – mlk
    Mar 25 at 12:27
  • @mlk Yeah, I think a key aspect here too is that if you're trained in falling as part of martial arts, you probably didn't practice that skill on a non-Newtonian surface. I could definitely see people actually getting hurt worse on these mats when they try to apply skills meant for ordinary mats. Mar 26 at 4:46
  • Main problem here is that the optimal points would be dependent on weight, so you'd basically need weight-class optimised mats and/or training rooms. Great thing to have, but indeed rather impractical. I've seen tire-wood-mat installations (bottom to top, obviously) which where heavenly to train on. You didn't learn to fall properly cause it was too forgiving, though. Apr 1 at 10:28
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Those of you who aren't familiar with the concept of Non-Newtonian Fluids, the thing to realize is that they are fluid or solid depending on the stress exerted on them. If you fill a swimming pool with a non-Newtonian fluid, you'll be able to walk right across it. So long as you remain in motion, it will support your weight. As soon as you stop, however, you'll sink. It's a bizarre property of matter that's usually outside of our own everyday experience.

And so a non-Newtonian fluid will act solid or liquid depending on the forces exerted on it.

You can see a good demonstration in the video below:

https://www.youtube.com/watch?v=RIUEZ3AhrVE

Could you use it as a cushion in the same way foam rubber sheeting is used in Judo mats? I don't think it has ever been done, but I can guess as to why that may be. If you think about it, you don't want something that feels solid when you're impacting on it. You want to feel some give. A non-Newtonian fluid would react to a falling judoka by feeling as hard as a brick at first. Then a few seconds later, it will feel squishy and will displace around the judoka.

So for padding impacts, it fails.

Also, if you're just walking across a hypothetical mat filled with a non-Newtoninan fluid, it's going to be fine. You'll feel like there's solid ground under you. But as soon as you stop, your feet will squish in to the mat, leaving your feet in a 2 inch depression. That's a tripping hazard and a good way to stub your toe.

Lastly, foam rubber is dry. If you puncture or rip that judo mat, the worst that happens is you expose some of the foam rubber material inside. There are vinyl repair kits that easily seal that hole. But if it's filled with a non-Newtonian fluid, it will just leak right out with the first tiny puncture.

So to answer the question, no, I don't believe this could be used in judo mats very effectively. You could try to do a kind of hybrid material of some sort which combines a foam scaffolding with some kind of non-Newtonian fluid. But it's hard to see how that would be an improvement over what already exists. And you would still have problems with leaks.

Hope that helps.

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    You've described only shear-thickening non-newtonian fluids in your answer. Mar 24 at 23:21

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