viscosity basics - walking on custard?!
Posted by: woodchurchscience in ks3, ks4, miscellaneous, physics, questions, warrenwater is known as a newtonian fluid, because it continues to exemplify fluid properties no matter how fast it is stirred or mixed. i.e. its stress/strain curve is linear and passes through the origin. The constant of proportionality is known as the viscosity.
contrast this with a non-newtonian fluid, in which stirring can leave a “hole” behind, that gradually fills up over time - this behaviour is seen in materials such as rice pudding. non-newtonian fluids can also become thinner, the drop in viscosity causing it to flow more. this behaviour is often referred to as thixotropy. the opposite behaviour (getting thicker the more you shake it) is rarer and is referred to as rheopexy.
two classic examples: a thixotropic (sometimes called a pseudoplastic) liquid like tomato ketchup gets thinner the more you shake it, and a rheopectic (sometimes called a dilatant) liquid, such as non-drip paint, brushes on easily but becomes more viscous when on walls.
another example of a non-newtonian fluid can be made very easily by adding custard powder gradually to a cup of water. the so called “shear thickening” property of this non-newtonian fluid soon becomes apparent. the application of force - for example by stabbing the surface with a finger, or rapidly inverting the container holding it - leads to the fluid behaving like a solid rather than a liquid. More gentle treatment, such as slowly inserting a spoon, will leave it in its liquid state. trying to jerk the spoon back out again, however, will trigger the return of the temporary solid state. a person moving quickly and/or applying sufficient force with his feet can literally walk across such a liquid.
shear thickening fluids of this sort are being researched for bullet resistant body armour, useful for their ability to absorb the energy of a high velocity projectile impact but remain soft and flexible while worn.
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