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.

Some clays are also thixotropic, with their behavior of great importance to structural engineering, especially in earthquake zones. Clay-like ground can practically liquefy under the shaking of a tremor, greatly increasing the effect on buildings. 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. 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.

One example of a thixotropic fluid is Laponite, which is a mixture of water and synthetic clay. Examples of applications for thixotropic fluids are the thickening of food stuffs and medical products. For example toothpaste is thixotropic, which allows it to be squeezed out of the tube, yet remain a solid shape on the brush. 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.

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.