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In more technical language: some Non-Newtonian fluids show a change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.
Some fluids are anti-thixotropic: constant shear stress for a time causes an increase in viscosity or even solidification. Constant shear stress can be applied by shaking or mixing. They are much less common.
Some clays are thixotropic, and their behaviour is of great importance in structural and geotechnical engineering. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan disaster in Wales are evidence of this. Similarly, a lahar is a mass of earth liquefied by a volcanic event, which rapidly solidifies once coming to rest.
Some clay deposits found in the process of exploring caves exhibit thixotropism: an initially solid-seeming mudbank will turn soupy and yield up moisture when dug into or otherwise disturbed. These clays were deposited in the past by low-velocity streams which tend to deposit fine-grained sediment.
These properties are often used in commercial products which are applied to surfaces, such as paint or toothpaste. With paint, the fluidity vanishes quickly and the surface sets permanently as water (or oil) evaporates.
- Hendrickson T. 2003. Massage for orthopedic conditions. Lippincott Williams & Wilkins. p 9