Thixotropy

Thixotropy is the property of certain gels or fluids that are viscous (thick) under normal conditions, but flow (become thin, less viscous) when shaken, agitated, or otherwise stressed.

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.

Natural examples
It is a common experience walking on sand that in places it liquifies as one walks on it. This is a thixotropic response to pressure. In extremes, a quicksand may be danger to people and animals.

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. 

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

Another example of a thixotropic fluid is the synovial fluid found in joints between some bones.Hendrickson T. 2003. Massage for orthopedic conditions. Lippincott Williams & Wilkins. p 9

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.

 References 

Category:Fluid mechanics
Category:Materials science