Non-Newtonian fluid

A non-Newtonian fluid is a thixotropic fluid which changes its viscosity when the forces on it change.

Paint is a non-Newtonian fluid. In non-Newtonian fluids, viscosity varies with the stress they are exposed to. Paint that is under low stress will behave differently from paint that is under high stress. When a surface with paint turned from horizontal to vertical orientation, it will be exposed to stress from gravitation. Therefore, instead of slipping along the surface, it forms very large drops, that are very dense and almost solid. This creates the great effect that can be seen in the figure.

Other flow properties may also be affected. Most commonly viscosity of such fluids is not independent of the shear rate or the shear rate history. Examples of such fluids are blood, quicksand and certain components of cement. Many lubricants, ketchup, and oobleck. Also certain forms of dough and pudding also have this property.

The other kind of fluids that do not show this behavior is called Newtonian fluids.

It is very simple to make a non-Newtonian fluid at home. All one needs is water and cornstarch. Mix these two ingredients together at a rate of one part cornstarch and one and a half parts water. That makes a non-Newtonian fluid called oobleck.

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