Fluid You Can Walk On

In fluid mechanics we learn about the viscosity of a fluid. In an intuitive way we know that viscosity has to do with the rate at which a fluid will flow. For instance water will flow much more easily and quickly out of a cup than something like oil or honey. Therefore we would say that honey is more viscous. In engineering terms, we define viscosity (more precisely, absolute or dynamic viscosity) as

the constant of proportionality between the shearing stress and the rate of shearing strain.

With solids we talk about the shear modulus of elasticity, G,  as being the slope or constant of proportionality between the shear stress and shear strain. Since fluids will theoretically infinitely shear strain under a constant shear stress (which is the definition of a fluid) we must compare shear strain rate instead of shear strain. And much like with solids, the slope of the graph is often a straight line and thus a constant. This slope or constant is the absolute viscosity. Fluids with this linear type of behavior are called Newtonian Fluids.

Now this is where the interesting part comes in. Not all fluids act in this nice neat fashion. This other group of fluids are called Non-Newtonian Fluids and there are two categories. The slope of the shear stress / shear strain rate graph for Non-Newtonian Fluids is called the apparent viscosity and it changes at every point along the graph. The first group of Non-Newtonian Fluids are called shear thinning fluids or pseudoplastic fluids. With shear thinning fluids, the apparent viscosity decreases as you increase the rate of shear strain. In other words, the faster you push through it, the easier it will be. Latex paint is an example of this type of fluid. The second group is called shear thickening fluids or dilatants. These are the opposite of the other group in that the faster you push through the fluid, the harder it is to push. This results in the odd but fascinating phenomenon you can see in the following video clips.

Walking on Fluid

http://youtu.be/f2XQ97XHjVw

Make Your Own Non-Newtonian Fluid at Home

http://youtu.be/hvJikar9Vqk?t=5s