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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?
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.
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.
