Non Newtonian fluids
Non Newtonian fluid are perplexing, as they are fluids which really don’t behave how fluids classically do. The examples in the GIFs above are of a cornflour/cornstarch and water mix, the suspension of which is so bizarre that it can allow a person to walk on it. You can literally walk on water (well… a liquid) just by mixing up some cornflour and water at home, pouring it into a shallow tray and quickly (and carefully) walking across it!
A non-Newtonian fluid is a fluid whose viscosity depends on the force applied (and sometimes time as well) and therefore has very interesting mechanical properties.
When you mix cornstarch and water, you create a colloidal suspension, which means that on the microscopic level you have a bunch of solid particles floating around and suspended in a liquid. Because a cornstarch/water mix is very thick, the particles of cornstarch floating around in the water are packed very close to each other (so that they are actually touching), but they are still able to slip past each other. This is what happens when you move the mixture slowly — the suspended particles have time to move and slowly slip past each other, so the mixture acts like a liquid that can easily flow. However, when you suddenly put pressure on the mixture (like poking it), the particles do not have enough time to move out of the way (they are fairly massive), so they stay where they are and the mixture feels like a solid. This is a somewhat simplified picture of what is actually happening, but is hopefully a useful way to think about it.
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