Thixotropy is the property of some non-newtonian pseudoplastic fluids to show a time dependant change in viscosity; the longer the fluid undergoes shear, the lower its viscosity. However, this is not a universal definition1, and the term is sometimes applied to pseudoplastic fluids without a viscosity time component.
Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated. Some clays are also thixotropic, with their behavior of great importance to structural engineering in earthquake zones. Clayey ground can practically liquefy under the shaking of a temblor, greatly increasing the effect on buildings.
The classic example of a thixotropic fluid is ketchup, where waiting for it to pour can be more effective than pounding on the bottom of the bottle. Thixotropic compounds are important paint additives, allowing a thick, smooth application that doesn't run.
The opposite property, in which shaking for a time causes solidification, is called Rheopectic and is much less common.
See also
References
- Reiner, M., and Scott Blair, Rheology terminology, in Rheology, Vol. 4 pp. 461, (New York: Achedemic Press, 1967)