A non-Newtonian fluid is a fluid whose viscosity is variable based on applied stress. The most commonly known non-Newtonian fluid is cornstarch dissolved in water. Contrast with Newtonian fluids like water, whose behavior can be described exclusively by temperature and pressure, not the forces acting on it from second to second. Non-Newtonian fluids are fascinating substances that can be used to help us understand physics in more detail, in an exciting, hands-on way.

If you punch a bucket full of non-Newtonian fluid, the stress introduced by the incoming force causes the atoms in the fluid to rearrange such that it behaves like a solid. Your hand will not go through. However, if you shove your hand into the fluid slowly, it will penetrate successfully. If you pull your hand out abruptly, it will again behave like a solid, and you can literally pull a bucket of the fluid out of its container in this way. However, the effect doesn't last for long - if stress is not continuously applied, the non-Newtonian fluid turns back into a liquid, and will ooze right off your hand.

Non-Newtonian fluids help us understand the wide variety of fluids that exist in the physical world. Plastic solids, power-law fluids, viscoelastic fluids, and time-dependent viscosity fluids are others that exhibit complex and counterintuitive relationships between shear stress and viscosity/elasticity. However, non-Newtonian fluid is probably the most exciting to play with.

A search for non-Newtonian fluid on YouTube brings up some interesting results. On several game shows, hosts or contestants run across big vats of non-Newtonian fluid, able to traverse them unless they stop - in which case they sink immediately. When combined with a oscillating plate, non-Newtonian fluids demonstrate other unusual properties, like protruding "fingers" and holes that persist after creating them. An oscillating plate applies stress on a periodic basis, rapidly changing the viscosity of the fluid and putting it in an odd middle ground between a liquid and a solid.

A practical application for non-Newtonian fluids may be in body armor of the future. Since such fluids are usually flexible, they would allow soldiers to move freely when not under attack. But if confronted with a speeding bullet, they would quickly harder, performing like traditional armor. More research is necessary to see if non-Newtonian fluids are suitable for the military, but until then, it's sure fun to play with.