Thixotropy
Natural examples
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.
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
Thixotropy has been proposed as a scientific explanation of blood liquification "miracles" such as that of Saint Januarius in Naples.
Semi-solid casting processes such as thixomoulding use the thixotropic property of some alloys (mostly light metals) (bismuth). Within certain temperature ranges, with appropriate preparation, an alloy can be put into a semi-solid state, which can be injected with less shrinkage and better overall properties than by normal injection molding.
Solder pastes used in electronics manufacturing printing processes are thixotropic materials.
Many kinds of inkssed in silkscreen textile printingade 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.
The ink developed for the Fisher space pen is a visco-elastic, thixotropic ink, with a consistency similar to that of very thick rubber cement, flows as a result of the shearing action of the rolling ball in its socket. This shearing action liquefies the solid gel thixotropic ink, allowing the pen to write smoothly and dependably on most surfaces and even underwater.
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.[dubious discuss] 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.
Clutch-type automatic transmissions do not use fluids with thixotropic properties. The oil in automatic transmissions is used solely for hydraulic actuation and there are no thixotropic properties used in the process of gear change or actuation of clutch packs.
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.
See also
Pseudoplastic
Aberfan disaster
Rheopexy (antonym)
dilatant (antonym)
Kaye effect
polymer
Silly putty
Shear thinning
viscosity
References
^ Hendrickson, T: "Massage for Orthopedic Conditions", page 9. Lippincott Williams & Wilkins, 2003.
^ Garlaschelli, Ramaccini, Della Sala, "The Blood of St. Januarius", Chemistry in Britain 30.2, (1994:123)
^
Reiner, M., and Scott Blair, Rheology terminology, in Rheology, Vol. 4 pp. 461, (New York: Achedemic Press, 1967)
Dam break wave of thixotropic fluid in Journal Hydraulic Engineering, Vol. 132, No. 3, pp. 280-293
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Topics in geotechnical engineering
Soils
Clay Silt Sand Gravel Peat Loam
Soil properties
Hydraulic conductivity Water content Void ratio Bulk density Thixotropy Reynolds' dilatancy Angle of repose Cohesion Porosity Permeability Specific storage
Soil mechanics
Effective stress Pore water pressure Shear strength Overburden pressure Consolidation Soil compaction Soil classification Shear wave Lateral earth pressure
Geotechnical investigation
Cone penetration test Standard penetration test Exploration geophysics Monitoring well Borehole
Laboratory tests
Atterberg limits California bearing ratio Direct shear test Hydrometer Proctor compaction test R-value Sieve analysis Triaxial shear test Hydraulic conductivity tests Water content tests
Field tests
Crosshole sonic logging Nuclear Densometer Test
Foundations
Bearing capacity Shallow foundation Deep foundation Dynamic load testing Wave equation analysis
Retaining walls
Mechanically stabilized earth Soil nailing Tieback Gabion Slurry wall
Slope stability
Mass wasting Landslide Slope stability analysis
Earthquakes
Soil liquefaction Response spectrum Seismic hazard Ground-structure interaction
Geosynthetics
Geotextile Geomembranes Geosynthetic clay liner Cellular confinement
Instrumentation for Stability Monitoring
Deformation monitoring Automated Deformation Monitoring
Categories: Continuum mechanics | Fluid dynamics | Non-Newtonian fluids | Soil mechanics | Soil physicsHidden categories: All accuracy disputes | Articles with disputed statements from January 2010
by: gaga
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