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rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...
, shear thinning is the non-Newtonian behavior of fluids whose
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
decreases under shear strain. It is sometimes considered synonymous for pseudo-
plastic Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...
behaviour, and is usually defined as excluding time-dependent effects, such as thixotropy. Shear thinning is the most common type of non-Newtonian behavior of fluids and is seen in many industrial and everyday applications. Although shear thinning is generally not observed in pure liquids with low
molecular mass The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...
or ideal solutions of small molecules like
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...
or
sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
, it is often observed in
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
solutions and molten polymers, as well as complex fluids and suspensions like
ketchup Ketchup or catsup is a table condiment with a sweet and sour flavor. "Ketchup" now typically refers to tomato ketchup, although early recipes for different varieties contained mushrooms, oysters, mussels, egg whites, grapes, or walnuts, amon ...
,
whipped cream Whipped cream, also known as Chantilly cream or (), is high-fat dairy cream that has been aerated by whisking until it becomes light, fluffy, and capable of holding its shape. This process incorporates air into the cream, creating a semi-soli ...
,
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...
,
paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...
, and
nail polish Nail polish (also known as nail varnish in British English or nail enamel) is a lacquer that can be applied to the human Nail (anatomy), fingernails or toenails to decorate and protect the nail plates. The formula has been revised repeatedly t ...
.


Theories behind shear-thinning behaviour

Though the exact cause of shear thinning is not fully understood, it is widely regarded to be the effect of small structural changes within the fluid, such that microscale geometries within the fluid rearrange to facilitate
shearing Sheep shearing is the process by which the woollen fleece of a sheep is cut off. The person who removes the sheep's wool is called a '' shearer''. Typically each adult sheep is shorn once each year (depending upon dialect, a sheep may be sai ...
. In colloid systems,
phase separation Phase separation is the creation of two distinct Phase (matter), phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. This type of phase separation is kn ...
during flow leads to shear thinning. In polymer systems such as polymer melts and solutions, shear thinning is caused by the disentanglement of polymer chains during flow. At rest, high molecular weight polymers are entangled and randomly oriented. However, when undergoing agitation at a high enough rate, these highly
anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
polymer chains start to disentangle and align along the direction of the shear force. This leads to less molecular/particle interaction and a larger amount of free space, decreasing the viscosity.


Power law model

At both sufficiently high and very low shear rates,
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
of a polymer system is independent of the shear rate. At high shear rates, polymers are entirely disentangled and the viscosity value of the system plateaus at ''η'', or the infinite shear viscosity plateau. At low shear rates, the shear is too low to be impeded by entanglements and the viscosity value of the system is ''η0'', or the zero shear rate viscosity. The value of ''η'' represents the lowest viscosity attainable and may be orders of magnitude lower than ''η0'', depending on the degree of shear thinning. Viscosity is plotted against shear rate in a log(η) vs. log(\dot\gamma) plot, where the linear region is the shear-thinning regime and can be expressed using the Ostwald and de Waele power law equation: : \tau = K(T)\left(\right)^n = K(T)^n The Ostwald and de Waele equation can be written in a logarithmic form: : \log(\tau) = \log(K) + n\log\left(\dot\right) The apparent viscosity is defined as \eta= , and this may be plugged into the Ostwald equation to yield a second power-law equation for apparent viscosity: : \eta = K(T)^ This expression can also be used to describe
dilatant A dilatant (, ) (also termed shear thickening) material is one in which viscosity increases with the rate of shear strain. Such a ''shear thickening fluid'', also known by the initialism ''STF'', is an example of a non-Newtonian fluid. This be ...
(shear thickening) behaviour, where the value of n is greater than 1.


Herschel–Bulkley model

Bingham plastics require a critical shear stress to be exceeded in order to start flowing. This behaviour is usually seen in polymer/silica micro- and nanocomposites, where the formation of a silica network in the material provides a solid-like response at low shear stress. The shear-thinning behavior of plastic fluids can be described with the Herschel-Bulkley model, which adds a threshold shear stress component to the Ostwald equation: : \tau = \tau_y + K(T)^n


Relationship with thixotropy

Some authors consider shear thinning to be a special case of thixotropic behaviour, because the recovery of the microstructure of the liquid to its initial state will always require a non-zero time. When the recovery of viscosity after disturbance is very rapid however, the observed behaviour is classic shear thinning or pseudoplasticity, because as soon as the shear is removed, the viscosity returns to normal. When it takes a measurable time for the viscosity to recover, thixotropic behaviour is observed. When describing the viscosity of liquids, however, it is therefore useful to distinguish shear-thinning (pseudoplastic) behaviour from thixotropic behaviour, where the viscosity at all shear rates is decreased for some duration after agitation: both of these effects can often be seen separately in the same liquid.


Everyday examples

Wall
paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...
is a pseudoplastic material. When modern wall paint is applied, the shear created by the brush or roller will allow it to thin and wet out the surface evenly. Once applied, the paint regains its higher viscosity, which avoids drips and runs.
Ketchup Ketchup or catsup is a table condiment with a sweet and sour flavor. "Ketchup" now typically refers to tomato ketchup, although early recipes for different varieties contained mushrooms, oysters, mussels, egg whites, grapes, or walnuts, amon ...
is a shear-thinning material, viscous when at rest, but flowing at speed when agitated by squeezing, shaking, or striking the bottle.
Whipped cream Whipped cream, also known as Chantilly cream or (), is high-fat dairy cream that has been aerated by whisking until it becomes light, fluffy, and capable of holding its shape. This process incorporates air into the cream, creating a semi-soli ...
is also a shear-thinning material. When whipped cream is sprayed out of its canister, it flows out smoothly from the nozzle due to the low viscosity at high flow rate. However, after whipped cream is sprayed into a spoon, it does not flow and its increased viscosity allows it to be rigid.


See also

* * * * * * Time-dependent viscosity * Rheopecty: The longer the fluid is subjected to a shear strain, the higher the viscosity. Time-dependent shear thickening behavior. * Thixotropy: The longer a fluid is subjected to a shear strain, the lower its viscosity. It is a time-dependent shear thinning behavior. * Shear thickening: Similar to rheopecty, but independent of the passage of time. * Thickening agent *
Paint thinner Paint thinners are diluents — solvents used to adjust the viscosity of paint, as well as similar preparations such as varnish and lacquer, by dilution. For water-based paints, such as acrylic paints, water is the primary solvent, but notably, ...


External links


The Great Ketchup Mystery


References

{{reflist Continuum mechanics Rheology Non-Newtonian fluids Smart materials Tribology