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Sir Isaac Newton, the same physicist who -- at least according to legend -- figured out gravity because of an apple, also described how liquids behave. Today, we call this description Newton's Law of Viscosity, and it describes how fluids move in response to stress. Take a straw in a glass of water as an example. Suck on the straw, and you pull water from the glass. Suck harder, and the water moves faster. Its viscosity, or resistance to flow, doesn't change: Everything's in proportion. Fluids that follow the law of viscosity are Newtonian fluids, and the ones that break the rules -- the non-Newtonian fluids -- don't act the way you might expect them to. Yet you see examples around you every day. There's gravy, which gets thicker as you stir it. Ketchup is a lot more willing to coat your french fries once you smack the bottle to get it moving. An example you might not see every day is a mixture of cornstarch and water. At rest, it acts like a liquid you can stir and pour. But smack the surface of such a mixture, and, for a brief moment, it acts like a solid. You can run or hop across a pool of it, but hold still, and you'll sink like a stone -- it changes from a swimming pool to a sidewalk and back again. Spread a thin layer of cornstarch and water on a speaker, and its vibrations will cause the liquid to bunch up in eerily lifelike, moving shapes. The secret to this particular non-Newtonian fluid is that it's a colloid of cornstarch particles floating in water. At rest, these particles keep their distance from each other, so everything moves freely. But if you hit the surface of the liquid, or apply shearing stress, the particles bunch together, creating a temporary solid. As soon as the force goes away, the particles spread out again, and the fluid is back to being a liquid. For this reason, a mix of cornstarch and water is known as a shear-thickening fluid. |
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