There are some kinds of sludge, like quicksand, that you sink into. There are some kinds of sludge, like custard, you can walk on. And there are some kinds of sludge that just get stronger the more you stand on them.

Are you about to step onto some suspicious sludge? I don't judge you for it. That's not my place. I would just like to point out that you need to ascertain what kind of sludge it is before embarking on your journey. It might just be garden-variety grossness, made of dirt and water and the feces of various kinds of animals. Then again, it might be a non-newtonian fluid.

Non-newtonian fluids act a little differently from how we expect them to act. Normally, when we double the force on something, we expect double the resistance. With water, or ordinary sludge, you'll probably get just that. On the other hand, there are things called thixotropic fluids. Put a little force on these - the kind that will agitate them - and they start to slide around. The more force you use, trying to extricate yourself from them or get over them quickly, the more they slide. Ketchup comes squirting out of the bottle all of a sudden because it's thixotropic. Snake venom stays secure inside the snake and then slides freely over the fangs and into your body because it's thixotropic. And quicksand starts out firm and goes to mush, burying you alive, because it's thixotropic.

Of less concern are dilatant fluids. Apply some force to these fluids and instead of giving way, they put up more resistance. This is the famous "oobleck" of cornstarch and water that dances on speakers and that various science shows mix up and send people walking across. If they walk quickly, striking the custard hard, they can walk right across it. If they stand, they sink in.

And then there is the rarest of all, the rheopectic fluid. This is a source of interest to all kinds of companies right now because it's hard to make, but very useful. Rheopectic gets hard when it is struck. So far, it's like a dilatant fluid. But the rheopectic fluid will increase in hardness the longer the force is kept up. Hitting it, or keeping it under constant stress for a longer period of time, just makes it stronger. This is one of the reasons why it can climb up that mixer blade. Unlike the thixotropic liquid, that gives way, rheopectic liquid just clings harder the longer it gets kicked around. People are trying to make something that's suitable for shoes and for body armor. A dilatant fluid is a good way of protecting a falling egg - the moment the fluid hits the ground it hardens evenly around the egg and spreads the shock. A rheopectic fluid wouldn't need just a quick impact. As long as it was kept under stress, it would stay tough, and as soon as it was released, it would dissolve back into liquid.

Via Science Learning, BBC.