Thixotropy

Thixotropy is the property of some non-Newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. 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.

It is important to note the distinction between a thixotropic fluid and a shear thinning fluid. The former displays a decrease in viscosity over time at a constant shear rate, while the latter displays decreasing viscosity with increasing shear rate. Fluids which exhibit the opposite property, in which constant shear stress for a time, such as shaking or mixing, causes an increase in viscosity or even solidification, are called rheopectic, sometimes called anti-thixotropic, and are much less common.

Natural examples
Some clays are thixotropic, with their behavior of great importance in structural and geotechnical engineering. In earthquake zones, clay-like ground can exhibit characteristics of liquefaction under the shaking of a tremor, greatly affecting earth structures and buildings. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan slag heap disaster in Wales are evidence of this phenomenon. Similarly, a lahar is a mass of earth liquefied by a volcanic event, which rapidly solidifies once coming to a rest.

Drilling muds used in geotechnical applications can be thixotropic. Honey from honey bees may also exhibit this property under certain conditions.

Another example of a thixotropic fluid is the synovial fluid found in joints between some bones. The ground substance in the human body is thixotropic.

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.

Applications
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. The ink developed for the Fisher space pen is thixotropic so that the ink flows only when the roller ball is pressed on paper. Ketchup is frequently thixotropic.

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. 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.

Etymology
The word comes from Greek thixis, touch (from thinganein, to touch) + -tropy, -tropous, from Greek -tropos, of turning, from tropos, changeable, from trepein, to turn.