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A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity (the measure of a fluid's ability to resist gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history. Some non-Newtonian fluids with shear-independent viscosity, however, still exhibit normal stress-differences or other non-Newtonian behavior. Many salt solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, custard, toothpaste, starch suspensions, paint, blood, and shampoo. In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, the constant of proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the shear rate is different and can even be time-dependent. Therefore, a constant coefficient of viscosity cannot be defined.
Although the concept of viscosity is commonly used in fluid mechanics to characterize the shear properties of a fluid, it can be inadequate to describe non-Newtonian fluids. They are best studied through several other rheological properties that relate stress and strain rate tensors under many different flow conditions--such as oscillatory shear or extensional flow--which are measured using different devices or rheometers. The properties are better studied using tensor-valued constitutive equations, which are common in the field of continuum mechanics.
Types of non-Newtonian behaviour
|Viscoelastic||Kelvin material||"Parallel" linearstic combination of elastic and viscous effects||Some lubricants, whipped cream|
|Rheopecty||Apparent viscosity increases with duration of stress||printer ink, gypsum paste|
|Thixotropic||Apparent viscosity decreases with duration of stress||Yogurt, xanthan gum solutions, aqueous iron oxide gels, gelatin gels, pectin gels, synovial fluid, hydrogenated castor oil, some clays (including bentonite, and montmorillonite), carbon black suspension in molten tire rubber, some drilling muds, many paints, many floc suspensions, many colloidal suspensions|
|Time-independent viscosity||Shear thickening (dilatant)||Apparent viscosity increases with increased stress||Suspensions of corn starch in water, sand in water, Silly Putty|
|Shear thinning (pseudoplastic)||Apparent viscosity decreases with increased stress||Nail polish, whipped cream, ketchup, molasses, syrups, paper pulp in water, latex paint, ice, blood, some silicone oils, some silicone coatings|
|Generalized Newtonian fluids||Viscosity is constant
Stress depends on normal and shear strain rates and also the pressure applied on it
|Blood plasma, custard, water|
Shear thinning fluid
A familiar example of the opposite, a shear thinning fluid, or pseudoplastic fluid, is wall paint: The paint should flow readily off the brush when it is being applied to a surface but not drip excessively. Note that all thixotropic fluids are extremely shear thinning, but they are significantly time dependent, whereas the colloidal "shear thinning" fluids respond instantaneously to changes in shear rate. Thus, to avoid confusion, the latter classification is more clearly termed pseudoplastic .
Fluids that have a linear shear stress/shear strain relationship require a finite yield stress before they begin to flow (the plot of shear stress against shear strain does not pass through the origin). These fluids are called Bingham plastics. Several examples are clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and mustard. The surface of a Bingham plastic can hold peaks when it is still. By contrast Newtonian fluids have flat featureless surfaces when still.
There are also fluids whose strain rate is a function of time. Fluids that require a gradually increasing shear stress to maintain a constant strain rate are referred to as rheopectic. An opposite case of this is a fluid that thins out with time and requires a decreasing stress to maintain a constant strain rate (thixotropic).
Many common substances exhibit non-Newtonian flows. These include:
- Soap solutions and cosmetics
- Food such as butter, cheese, jam, ketchup, mayonnaise, soup, and yogurt
- Natural substances such as magma, lava, gums, and extracts such as vanilla extract
- Biological fluids such as blood, saliva, semen, and synovial fluid
- Slurries such as cement slurry, emulsions such as mayonnaise, and some kinds of dispersions
An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of starch (e.g. cornstarch) in water, sometimes called "oobleck" or "ooze" (1 part of water to 1.5–2 parts of corn starch].  Uncooked imitation custard, a suspension of primarily cornflour, has the same properties. The name "oobleck" is derived from the Dr. Seuss book Bartholomew and the Oobleck.
Flubber is a non-Newtonian fluid, easily made from polyvinyl alcohol–based glues and borax. It flows under low stresses but breaks under higher stresses and pressures. This combination of fluid-like and solid-like properties makes it a Maxwell solid. Its behaviour can also be described as being viscoplastic or gelatinous.
Chilled caramel topping
Another example of this is chilled caramel ice cream topping (so long as it incorporates hydrocolloids such as carrageenan and gellan gum). The sudden application of force—by stabbing the surface with a finger, for example, or rapidly inverting the container holding it—causes the fluid to behave like a solid rather than a liquid. This is the "shear thickening" property of this non-Newtonian fluid. More gentle treatment, such as slowly inserting a spoon, will leave it in its liquid state. Trying to jerk the spoon back out again, however, will trigger the return of the temporary solid state.
Silly Putty is a silicone polymer based suspension which will flow, bounce, or break depending on strain rate.
Plant resin is a viscoelastic, solid polymer. When left in a container, it will flow slowly as a liquid to conform to the contours of its container. If struck with greater force, however, it will shatter as a solid.
Ketchup is a shear thinning fluid. Shear thinning means that the fluid viscosity decreases with increasing shear stress. In other words, fluid motion is initially difficult at slow rates of deformation, but will flow more freely at high rates.
Tricks with non-Newtonian fluids
A person moving quickly and applying sufficient force with his feet can walk across certain types of non-Newtonian fluid, such as oobleck. People can also put oobleck on a subwoofer to make it thicken and bounce.
- Bingham plastic
- Complex fluid
- Dissipative particle dynamics
- Newtonian fluid
- Generalized Newtonian fluid
- Herschel–Bulkley fluid
- Navier–Stokes equations
- Tropea, Alexander L. Yarin, John F. Foss, Publisher: Springer, 9 October 2007, ISBN 3-540-25141-3, ISBN 978-3-540-25141-5, p.676, Google books
- Springer handbook of experimental fluid mechanics, Cameron Tropea, Alexander L. Yarin, John F. Foss, Publisher: Springer, 9 October 2007, ISBN 3-540-25141-3, ISBN 978-3-540-25141-5, p.661, Google books
- Pump Application Desk Book, 3rd edition, Paul N. Garay, Prentice Hall, August 1996, ISBN 0-88173-231-1, ISBN 978-0-88173-231-3, p.358, Google books
- Rheology of Fluid and Semisolid Foods: Principles and Applications, M. A. Rao, Publisher: Springer, 2nd edition, 28 August 2007, ISBN 0-387-70929-0, ISBN 978-0-387-70929-1, p.8, Google books
- Emulsions, Foams, and Suspensions: Fundamentals and Applications, Laurier L. Schramm, Publisher: Wiley VCH, 26 July 2005, ISBN 3-527-30743-5, ISBN 978-3-527-30743-2p.173, Google books
- Chhabra, R.P. (2006). Bubbles, Drops, and Particles In Non-Newtonian Fluids. (2nd ed. ed.). Hoboken: Taylor & Francis Ltd. pp. 9–10. ISBN 1420015389.
- This demonstration of oobleck is a popular subject for YouTube videos, such as this.
- Oobleck: The Dr. Seuss Science Experiment
- Outrageous Ooze
- Glurch Meets Oobleck. Iowa State University Extension.
- The Rheology of Caramel, Giuseppina Barra, Ph.D. thesis, 2004.
- Microscopy reveals why ketchup squirts, Chemistry World, Sep 2 2011, Royal Society of Chemistry
- Classical experiments with Non-Newtonian fluids by the National Committee for Fluid Mechanics on YouTube