Demo #1: Rheology

Purpose: To demonstrate the properties of a rheid, which represents essentially all Earth materials.

Supplies: Silly Putty

Background and Demonstration:

Essentially all Earth materials behave as a rheid. That is, they behave elastically over short time scales and plastically over long time scales. Silly putty is an excellent demonstration of this. Roll the silly putty into a ball and bounce it on the table. This bouncing applies a stress (force on the area of the ball) over a very short time scale, the ball momentarily deforms, but quickly bounces back into its original shape with no net deformation (it also bounces into the air). On the other hand, if you slowly stretch the silly putty (applying a long time-scale stress), it becomes elongated (even stringy) and has obvious permanent deformation.

In elastic behavior, the deformation (strain) is proportional to the applied stress (or force), so that when the stress is removed, there is no remaining strain. The proportionality constant is the inverse of the elastic modulus (the rigidity or incompressibility, depending on the type of stress applied). On the other hand, in plastic behavior, the rate of deformation is proportional to the applied stress. If you pull gently, the silly putty deforms slowly, but if you pull harder, it stretches more quickly. In this case, the proportionality constant is the inverse of the viscosity of the material (the more viscous a material, the slower it deforms for the same applied stress).

In a rheid, the change from elastic to plastic behavior depends on the time scale of the applied stress. However, this changeover is also temperature and pressure dependent. The temperature dependence is easily demonstrated. If you chill the silly putty (in the refrigerator), it will seem much more elastic and will require a longer period of pulling before it stretches plastically. On the other hand, if you heat the silly putty (hold it in your hand or keep it in your pocket), it will be much easier to deform plastically (in fact, it may not bounce well at all).

Finally, silly putty can be used to demonstrate the yield strength of a material. Materials behave elastically only as long as the applied stress is less than the "strength" of the material. If a greater stress is applied, the material breaks (this is why earthquakes occur). To demonstrate this, give a rapid, strong pull on the silly putty; it will break. Even more impressive is to hit the silly putty with a hammer, but be careful since it shatters and pieces can go flying (you might want to wrap the silly putty in paper first).

Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.

Questions or comments: jbarker@binghamton.edu

Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)