Thixotropy

Source: Wikipedia, the free encyclopedia.
Mānuka honey is an example of a thixotropic material.

Thixotropy is a time-dependent

flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed (time-dependent viscosity). They then take a fixed time to return to a more viscous state.[1]
Some
colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated. Thixotropy arises because particles or structured solutes require time to organize.[2]

Some fluids are anti-thixotropic: constant shear stress for a time causes an increase in viscosity or even solidification. Fluids which exhibit this property are sometimes called rheopectic. Anti-thixotropic fluids are less well documented than thixotropic fluids.[2]

Natural examples

Quicksand on the shore of the River Thames. Quicksand demonstrates thixotropy in the form of shear thinning, in that it is solid when at rest, but quickly liquefies when agitated.

Some clays are thixotropic, with their behavior of great importance in structural and geotechnical engineering. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan spoil tip 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 rest.

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

Both cytoplasm and the ground substance in the human body are thixotropic, as is semen.[3]

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.

A thixotropic fluid is best visualised by an oar blade embedded in mud. Pressure on the oar often results in a highly viscous (more solid) thixotropic mud on the high pressure side of the blade, and low viscosity (very fluid) thixotropic mud on the low pressure side of the oar blade. Flow from the high pressure side to the low pressure side of the oar blade is non-Newtonian. (i.e., fluid velocity is not linearly proportional to the square root of the pressure differential over the oar blade).

Applications

Many kinds of paints and inks—e.g.,

CMYK
-type process printing, are designed to regain viscosity even faster, once they are applied, in order to protect the structure of the dots for accurate color reproduction.

Thixotropic ink (along with a gas pressurized cartridge and special shearing ball design) is a key feature of the

Fisher Space Pen
, used for writing during zero gravity space flights by the US and Russian space programs.

Solder pastes used in electronics manufacturing printing processes are thixotropic.

Thread-locking fluid is a thixotropic adhesive that cures anaerobically.

Thixotropy has been proposed as a scientific explanation of blood liquefaction

Saint Januarius in Naples.[5]

Semi-solid casting processes such as

injection molding
.

Fumed silica is commonly used as a rheology agent to make otherwise low-viscous fluids thixotropic. Examples range from foods to epoxy resin in structural bonding applications like fillet joints.

Etymology

The word comes from

sol-gel transformation.[6]

See also

References

  1. ISBN 978-0471238966
    .
  2. ^
    PMID 19012872
    .
  3. ^ Hendrickson, T: "Massage for Orthopedic Conditions", page 9. Lippincott Williams & Wilkins, 2003.
  4. ISBN 978-3527306732
    .
  5. ^ Garlaschelli, L; Ramaccini, F; Della Scala, S (1994). "The Blood of St. Januarius". Chemistry in Britain. 30 (2): 123.
  6. ^ Reiner, M; Scott Blair, G W (1967) in Eich, F. R., (ed) Rheology, Theory and Applications Vol 4 p 465 (Academic Press, NY)

External links

  • The dictionary definition of thixotropy at Wiktionary
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