This man can walk on water!
Okay, I’m kidding. It’s not the man that’s the cause, it’s the water. And it’s not really water. It’s a mix of cornstarch and water. Here’s the video.
The fluid is supporting the guy’s weight. But how is that possible? The Stokes equation for the shear-stress in a Newtonian fluid is this:
Which means, basically, that the shear-rate is proportional to the applied stress. The constant of proportionality, the viscosity, can be measured to be quite small. You wouldn’t be able to stand on water, for example; the viscosity isn’t nearly enough to support your weight. So what gives (or holds!)? How is the fellow walking on water?
The answer is to note that the guy isn’t ‘walking’ on water: he’s running. And when he walks slowly or stands still, he sinks. The reason our model can’t predict this is because, as is usually the case, the model we’re working with is incomplete. The fluid is non-Newtonian. In this particular instance, it’s called a dilatant or a shear-thickening fluid. A better model for this fluid is this:
The first term on the LHS is the same as that for a Newtonian fluid. The second term is what you would expect with a regular solid. Because this mixture has properties of both liquids and solids, it’s called a visco-elastic material. The model itself was proposed by Maxwell, and is named after him.
Now we see what happens. Because the rate of increase in the stress is much greater than the stress itself (this is when the guy is running), the fluid behaves like a solid, and can therefore support his weight. If, on the other hand, the guy stands still, the rate of increase of the stress is nil (or small), and the mixture starts to behave like a (fairly viscous, as you can see) liquid.
Good. Now that you know of the Maxwell model, you can explain why it is that people can break bones if they land the wrong way on water. To wit: divers are taught to go in arms (or legs) first, so that the rate of increase of stress isn’t too high when the torso gets to the water’s surface. Alternatively, you could fall flat on your back onto the water… you know, if you want a raw back that hurts like it’s been whipped!