Typically, liquids take on the shape of the container they are poured into. We call these ‘normal liquids’ Newtonian fluids. But some fluids don’t follow this rule. We call these ‘strange liquids’ non-Newtonian fluids.
In science, stress means that a force is applied to a body. The result of that stress is described as strain.Imagine hitting a metal with a hammer. The force that is applied on the metal causes stress to that particular area. The result of that stress is then described as strain – in this case, possibly a deformation of the metal. Newtonian fluids don’t resist much stress that is applied on them like solids would do, so they don’t show the signs of strain. If you hit water with a hammer, the liquid will not resist much to the stress applied and will also not show signs of strain.Thixotropic and rheopecticSome liquids behave differently with stress (application of force) over time. Rheopectic liquids increase in viscosity as stress over time increases. Thixotropic liquids decrease in viscosity as stress over time increases.Non-Newtonian fluids change their viscosity or flow behaviour under stress. If you apply a force to such fluids (say you hit, shake or jump on them), the sudden application of stress can cause them to get thicker and act like a solid, or in some cases it results in the opposite behaviour and they may get runnier than they were before. Remove the stress (let them sit still or only move them slowly) and they will return to their earlier state.Say you want to get some tomato sauce out of the bottle. You know there is some in there, but when you turn the bottle upside down, nothing comes out. So what do you do? You shake or hit the bottle. This causes the tomato sauce to become more liquid and you can easily squirt some out. In this case, the sauce’s viscosity decreases and it gets runnier with applied stress.Oobleck is a mixture of cornflour and water (similar to uncooked custard) named after a substance in a Dr Seuss book. This liquid is a runny goo until you apply stress to it, and then it suddenly acts like a solid. You can hit a bowlful with a hammer, and instead of splashing everywhere, the particles lock together. You can roll it into a solid ball in your hand, but if you stop moving it, it reverts to liquid and oozes out through your fingers. In this case, the oobleck’s viscosity or resistance to flow increases with applied stress.
Not all non-Newtonian Fluids behave in the same way when stress is applied – some become more solid, others more fluid. Some non-Newtonian fluids react as a result of the amount of stress applied, while others react as a result of the length of time that stress is applied.
The behaviour of non-Newtonian fluids has important implications:
