OK, so maybe it isn't quite the same as really walking on water. But this is an interesting demonstration of the properties of a "non-newtonian fluid."
A non-newtonian fluid is one in which the viscosity changes depending on the force applied. Viscosity is a measure of the resistance of a fluid to being deformed. You can think of it as the fluid's internal resistance to flow, or its internal friction. For example, water has a lower viscosity than vegetable oil. Both, however, are newtonian fluids which maintain their viscosity whether you stir them slowly or quickly.
This demonstration uses a mixture of cornstarch and water to create a non-newtonian fluid. When applying a quick or sharp force (such as running across the surface) the viscosity of the fluid increases, and it acts more like a solid. When applying a slow, steady force (such as standing still), the viscosity decreases and it acts more like a normal fluid.
Quick Science Project - the cornstarch experiment
You can make your own non-toxic non-newtonian fluid, sometimes known as oobleck, by adding corn starch to a cup of water. Add the cornstarch in small amounts and stir it in slowly. When the suspension reaches the critical concentration, you can observe the so called "shear thickening" property. The application of force - for example by quickly poking the surface with your finger - causes the fluid to behave like a solid. Gentle stirring with a spoon will leave it in its liquid state. Trying to jerk the spoon back out quickly, however, will trigger the return of the temporary solid state.
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