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. Modern alkyd and latex paint varieties are often thixotropic and will not run off the painter's brush, but will still spread easily and evenly, since the gel-like paint "liquefies" when brushed out. 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.

Some clays are also thixotropic, with their behavior of great importance to structural and geotechnical engineers. In earthquake zones, clay-like ground can exhibit characteristics of liquefaction under the shaking of a tremor, greatly effecting earth structures and buildings. Drilling muds used in geotechnical applications can be thixotropic. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan, Wales slag heap disaster are evidence of this phenomenon.

One example of a thixotropic fluid is Laponite, which is a mixture of water and synthetic clay. An interesting application of Laponite is it's use in the lubrication of robotic snails/slugs (gastropods), which rely on Laponite's thixotropic property for locomotion.

Examples of applications for thixotropic fluids are the thickening of food stuffs and medical products. Toothpaste is thixotropic, which allows it to be squeezed out of the tube, yet retain a solid shape on the brush.

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.