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 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. 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.

The distinction between a thixotropic fluid and a shear thinning fluid:

• A thixotropic fluid displays a decrease in viscosity over time at a constant shear rate.
• A shear thinning fluid displays decreasing viscosity with increasing shear rate.

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. Fluids which exhibit this property are usually called rheopectic. They are much less common.

Contents 1 Natural examples 2 Applications 3 Misconceptions 4 Etymology 5 See also 6 References

[edit] 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, as is semen. ^[1]

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.

[edit] Applications

The ink developed for the Fisher space pen is thixotropic so that the ink flows only when the roller ball is pressed on paper.^[2]

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.

Thixotropy has been proposed as a scientific explanation of blood liquification "miracles" such as that of Saint Januarius in Naples.^[3]

Semi-solid casting process such as thixomoulding uses thixotropic property of some alloys (mostly light metals). At certain temperature range, with an appropriate preparation, an alloy can be put into a semi-solid state, which can be injected with less shrinkage and better overall property than by normal injection moulding.

Solder pastes used in electronics manufacturing printing processes are thixotropic materials.

Many kinds of inks—used in silkscreen textile printing—made from plastisol, exhibit thixotropic qualities. Some, such as those used in CMYK-type process printing, are designed to quickly regain viscosity once they are applied to protect the structure of the dots for accurate color reproduction. This is a sort of reverse thixotropy.

[edit] Misconceptions

Toothpaste, ketchup, and paint are frequently mis-labeled as being thixotropic materials. In reality, these are often pseudoplastic or "shear thinning". The viscosity of these materials decrease under increasing shear rate, not increasing time. When squeezed out of a tube, toothpaste flows easily but will set-up on the toothbrush. When shaken or squeezed out of a bottle, ketchup will thin and flow readily but will retain its shape on a burger or plate. When modern paints are applied the shear created by the brush or roller will allow them to thin and wet out the surface evenly. Once applied the paints regain their higher viscosity which avoids drips and runs.

[edit] 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.

[edit] See also

• Pseudoplastic
• Aberfan disaster
• Rheopexy (antonym)
• Kaye effect
• polymer
• Silly putty
• Shear thinning
• viscosity

[edit] References

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