Non-Newtonian fluids have varying viscosity based on applied stress, unlike Newtonian fluids. They can behave like solids or liquids depending on stress and are used to understand complex fluid relationships. They have practical applications, such as flexible armor. Non-Newtonian fluids are also fun to play with, as seen on YouTube.
A non-Newtonian fluid is a fluid whose viscosity varies according to the applied stress. The most commonly known fluid of this type is corn starch dissolved in water. Contrast this with Newtonian fluids such as water, whose behavior can be described solely by temperature and pressure, not by the forces acting on it from second to second. These fluids are fascinating substances that can be used to help us understand physics in more detail, in an exciting and practical way.
If you punch a bucket full of a shear-thickened non-Newtonian fluid, the stress introduced by the incoming force causes the atoms in the fluid to rearrange themselves so that it behaves like a solid. Your hand won’t fold. If you slowly stick your hand into the fluid, however, it will penetrate successfully. If you pull your hand out sharply, it will behave like a solid again and that way you can literally pull a bucket of liquid out of its container.
A non-Newtonian fluid that thins shear behaves in the opposite way. In this type, the fluid becomes thinner, rather than thicker, when stress is applied. Also called a pseudoplastic, examples of this type of fluid include ketchup, toothpaste, and paint. The effect usually doesn’t last long in either type, continuing only as long as stress is applied.
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 fluids with time-dependent viscosity are others that exhibit complex and counterintuitive relationships between shear stress and viscosity/elasticity. However, the non-Newtonian fluid is probably the most exciting to play with.
A search for non-Newtonian fluids on YouTube yields some interesting results. At several game shows, the presenters or contestants come across large vats of shear-thickening non-Newtonian fluid, capable of flowing through them unless they stop, in which case they immediately sink. When combined with a swash plate, these fluids exhibit other unusual properties, such as protruding “fingers” and holes that persist after they are created. A swashplate applies stress on a periodic basis, rapidly changing the viscosity of the fluid and placing it somewhere between a liquid and a solid.
A practical application for shear-thickening non-Newtonian fluids could be the armor of the future. Because such fluids are usually flexible, they would allow soldiers to move freely when not under attack. But when confronted with a speeding projectile, they would quickly get tougher, behaving like traditional armor. More research is needed to see if non-Newtonian fluids are suitable for the military, but until then it sure is fun to play with.
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