TABLE OF CONTENTS These are fluids that have the ability to store and recover shear energy and, hence, show some properties of a solid and some properties of a liquid. There are two easily observable examples of viscoelastic behavior:
The soup bowl effect: Suppose we gently stir a bowl of soup with a spoon so that the soup revolves around the bowl. If the spoon is lifted out of the soup (i.e., the energy source is removed), the soup will gradually come to rest (the inertial circulation dies out due to the effects of viscous forces), and then if the soup is viscoelastic (as many canned soups are), it may seem to rotate in the opposite direction or unwind a little.
Die swell: The jet formed by a viscoelastic fluid emerging from a nozzle or die may be of larger diameter than the aperture through which it flows. This is due to the removal of a constraining force on the fluid.
Other examples of viscoelastic materials are synovial fluid, molten polymers (with thread forming properties used in fiber spinning or film blowing), and bouncing putty or nutty putty, which will flow if stretched slowly, but bounces if struck hard against a hard surface.
