Thixotropy
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Thixotropy is the property of some non-newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear, the lower its viscosity. However, this is not a universal definition; 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 tremor, greatly increasing the effect on buildings. Ketchup is frequently thixotropic. Many clutch-type automatic transmissions use fluids with thixotropic properties to engage the different clutch plates inside the transmission housing at specific pressures which then changes the gearset.
Fluids which exhibit the opposite property, in which shaking for a time causes solidification, are called rheopectic and are much less common.
Etymology: Greek thixis, touch (from thinganein, to touch) + -tropy, -tropous, from Greek -tropos, of turning, from tropos, changeable, from trepein, to turn.
See also
References
- Reiner, M., and Scott Blair, Rheology terminology, in Rheology, Vol. 4 pp. 461, (New York: Achedemic Press, 1967)