Thixotropy is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed.
In more technical language:
some
non-NewtonianA non-Newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo...
pseudoplasticA Power-law fluid is a type of generalized Newtonian fluid for which the shear stress, τ, is given bywhere:*K is the flow consistency index ,...
fluids show a time-dependent change in
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...
; the longer the fluid undergoes
shear stressA shear stress, denoted , is defined as a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly.-General 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.
Thixotropy is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed.
In more technical language:
some
non-NewtonianA non-Newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo...
pseudoplasticA Power-law fluid is a type of generalized Newtonian fluid for which the shear stress, τ, is given bywhere:*K is the flow consistency index ,...
fluids show a time-dependent change in
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...
; the longer the fluid undergoes
shear stressA shear stress, denoted , is defined as a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly.-General 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
gelA gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute crosslinked system, which exhibits no flow when in the steady-state...
s 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 thinningShear thinning is an effect where viscosity decreases with increasing rate of shear stress. Materials that exhibit shear thinning are called pseudoplastic. This property is found in certain complex solutions, such as lava, ketchup, whipped cream, blood, paint, and nail polish. It is also a common...
fluid:
- A thixotropic fluid displays a decrease in viscosity over time at a constant shear rate.
- A shear thinning
Shear thinning is an effect where viscosity decreases with increasing rate of shear stress. Materials that exhibit shear thinning are called pseudoplastic. This property is found in certain complex solutions, such as lava, ketchup, whipped cream, blood, paint, and nail polish. It is also a common...
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
rheopecticRheopecty or rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shearing force, the higher its viscosity. Rheopectic fluids, such as some lubricants, thicken or solidify when shaken...
. They are much less common.
Natural examples
Some
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
s are thixotropic, with their behavior of great importance in
structuralStructural engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads. Structural engineering is usually considered a specialty within civil engineering, but it can also be studied in its own right....
and
geotechnical engineeringGeotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering includes investigating existing subsurface conditions and materials; determining their physical/mechanical and chemical properties that are relevant to...
. In
earthquakeAn earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph...
zones, clay-like ground can exhibit characteristics of
liquefactionSoil liquefaction describes the behavior of soils that, when loaded, suddenly suffer a transition from a solid state to a liquefied state, or having the consistency of a heavy liquid. Liquefaction is more likely to occur in loose to moderately saturated granular soils with poor drainage, such as...
under the shaking of a
tremorA tremor is an unintentional, somewhat rhythmic, muscle movement involving to-and-fro movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, head, face, vocal cords, trunk, and legs. Most tremors occur in the hands. In some people,...
, greatly affecting earth structures and buildings.
LandslideA landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
s, such as those common in the
cliffIn geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are...
s around
Lyme RegisLyme Regis is a coastal town in West Dorset, England, situated 25 miles west of Dorchester and east of Exeter. The town lies in Lyme Bay, on the English Channel coast at the Dorset-Devon border. It is nicknamed "The Pearl of Dorset." In the 13th century it developed into one of the major British...
,
DorsetDorset , is a county in South West England on the English Channel coast. The county town has been Dorchester since at least 1305, situated in the south of the county at . Between its extreme points Dorset measures from east to west and north to south, and has an area of...
and in the
AberfanAberfan is a village four miles south of Merthyr Tydfil in Wales. The village is chiefly known because of the catastrophic collapse of a colliery spoil-tip that occurred there in 1966, known as the Aberfan Disaster, which killed 144 people, including 116 children.-Aberfan disaster:For...
slag heapA slag heap is a pile built of accumulated tailings, which are either the by-products of mining or smelting...
disaster in
WalesWales is a country that is part of the United Kingdom, bordered by England to its east, and the Atlantic Ocean and Irish Sea to its west. It is also an elective region of the European Union...
are evidence of this phenomenon. Similarly, a
laharA lahar is a type of mudflow or landslide composed of pyroclastic material and water that flows down from a volcano, typically along a river valley. The term "lahar" originated in the Javanese language of Indonesia.-Description:...
is a mass of earth liquefied by a
volcanic3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
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 fluidSynovial fluid is a thick, stringy fluid found in the cavities of synovial joints. With its sperm-like consistency , synovial fluid reduces friction between the articular cartilage and other tissues in joints to lubricate and cushion them during movement.-Overview:The inner membrane of synovial...
found in joints between some bones. The
ground substanceGround substance is a term for the non-cellular components of extracellular matrix containing the fibers.It is usually not visible on slides, because it is removed during the preparation process....
in the human body is thixotropic, as is
semenSemen is an organic fluid, also known as seminal fluid, that usually contains spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...
.
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
NaplesNaples in Italy, is the capital of the region of Campania and of the province of Naples. The city is known for its rich history, art, culture, architecture, music and gastronomy, playing an important role throughout much of its existence; it is over 2,800 years old...
.
Semi-solid casting processes such as thixomoulding use the thixotropic property of some alloys (mostly
light metalLight metals are metals of low atomic weight. The cut off between light metals and heavy metals varies. Lithium, beryllium, sodium, magnesium and aluminium are almost always included. Additional metals up to nickel are often included as well. Metals heavier than nickel are usually called heavy...
s) (
bismuthBismuth is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all...
). 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 moldingInjection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity...
.
Solder pasteSolder paste is used for connecting the terminations of IC packages with land patterns on the PCB. The paste is applied to the lands by printing the solder using a stencil, while other methods like screening and dispensing are also used...
s used in electronics manufacturing printing processes are thixotropic materials.
Many kinds of inks—used in silkscreen
textile printingTextile printing is the process of applying colour to fabric in definite patterns or designs. In properly printed fabrics the colour is bonded with the fiber, so as to resist washing and friction...
—made from
plastisolPlastisol is a suspension of PVC particles in a plasticizer; it flows as a liquid and can be poured into a heated mold. When heated to around 177 degrees, the plastic and plasticizer mutually dissolve each other. On cooling the mold below 60 degrees C, a flexible, permanently plasticized product...
, 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 penThe Space Pen , marketed by Fisher Spacepen Co., is a pen that uses pressurized ink cartridges and is claimed to write in zero gravity, upside down, underwater, over wet and greasy paper, at any angle, and in extreme temperature ranges.The Fisher Space Pen was invented by American industrialist and...
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
ToothpasteToothpaste is a paste or gel dentifrice used with a toothbrush to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it can aid in the removal of dental plaque and food from the teeth, aid in the elimination and/or masking of halitosis and deliver...
,
ketchupKetchup , also known as tomato ketchup, tomato sauce, red sauce, Tommy sauce, Tommy K, or dead horse, is a condiment, usually made from tomatoes...
, and
paintPaint is any liquid, liquifiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film.-History:...
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.
Clutch-type automatic transmissions do not use
fluidsAutomatic transmission fluid is the fluid used in vehicles with self shifting or automatic transmissions. It is typically colored red or green to distinguish it from motor oil and other fluids in the vehicle...
with thixotropic properties. The oil in automatic transmissions is used solely for hydraulic actuation and there are no thixotropic properties utilized in the process of gear change or actuation of clutch packs.
Etymology
The word comes from
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
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
A dilatant material is one in which viscosity increases with the rate of shear. Such a shear thickening fluid, also known by the acronym STF, is an example of a non-Newtonian fluid....
(antonym)http://www.worsleyschool.net/science/files/thixotropic/materials.htmlhttp://www.newton.dep.anl.gov/askasci/gen99/gen99944.htm
- Kaye effect
The Kaye Effect is a property of complex liquids which was first described by the British engineer Alan Kaye in 1963.While pouring one viscous mixture of an organic liquid onto a surface, the surface suddenly spouted an upcoming jet of liquid which merged with the downgoing one.This phenomenon has...
- polymer
A polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
- Silly putty
Silly Putty is the Crayola owned trademark name for a class of silicone polymers...
- Shear thinning
Shear thinning is an effect where viscosity decreases with increasing rate of shear stress. Materials that exhibit shear thinning are called pseudoplastic. This property is found in certain complex solutions, such as lava, ketchup, whipped cream, blood, paint, and nail polish. It is also a common...
- viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...