Non-newtonian Fluids

"It is within the realm of possibility that you could make a non-Newtonian fluid that is primarily plasma or gas? I've only ever heard of non-Newtonian fluids that are liquid-based." Nick L.

Interesting question. A non-newtonian fluid goes through a reversible phase transition on the application of pressure. That's the definition I'm guessing you are asking about. Really, a non-newtonian fluid is any fluid that doesn't follow Ohm's law of pressure/ resistance to flow = flow rate. Gasses under moderately high vacuum are non-newtonian, because they are in transition from laminar (newtonian) flow into ballistic flow. Plasmas are funky, because they are made of ions, which interact at larger distances. I hadn't really thought about it before, but I suspect that plasma physics does not meet a strict newtonian definition for fluid dynamics.

The classic non-newtonian fluid is a mixture of cornstarch and water. Some people call it ooblek, a name I despise. If you are looking for a gas that goes solid when you step on it, I think you are out of luck. It's much easier to go from a liquid (molecules held together with low energy bonds = "hydrogen" bonds = Van der Waals forces, where the molecules are in causal contact but free to rearrange their positions, to a solid, where the bonds are strong enough to stop the free movement. You can make this happen by cooling the fluid down past the phase transition, or by introducing a magnetic field into a ferrofluid, or by adding pressure to a mixture that allows the liquid portion to move out from between the solids so they jam together.

The trick to running on the cornstarch-water mix is that the cornstarch molecules are complicated, so you can force the water molecules into their structure and out of the solution, reversibly. For something like quicksand, the bed of sand is fluidized by rising water, which you can fall into if you struggle, but float on if you lay flat like floating in plain water.