Rheometry · Rheometer
A non-newtonian fluid is a thixotropic fluid which changes its viscosity when the forces on it change.
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, certain forms of dough and pudding also have this property.
The other kind of fluids that do not show this behaviour are called Newtonian fluids.
It is very fun and easy to make your own Non-Newtonian fluid. There are many websites that show different recipes, such as cornflour mixed with water.
Images for kids
Demonstration of a non-Newtonian fluid at Universum in Mexico City
Oobleck on a subwoofer. Applying force to oobleck, by sound waves in this case, makes the non-Newtonian fluid thicken.