 NonNewtonian fluid

Continuum mechanics LawsScientistsA nonNewtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity (resistance to deformation or other forces) of nonNewtonian fluids is not independent of shear rate or shear rate history. However, there are some nonNewtonian fluids with shearindependent viscosity, that nonetheless exhibit normal stressdifferences or other nonNewtonian behaviour. Many salt solutions and molten polymers are nonNewtonian 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 nonNewtonian fluid, the relation between the shear stress and the shear rate is different, and can even be timedependent. Therefore a constant coefficient of viscosity cannot be defined.
Therefore, 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 nonNewtonian fluids. They are best studied through several other rheological properties which 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 tensorvalued constitutive equations, which are common in the field of continuum mechanics.
Contents
Types of nonNewtonian behaviour
Summary
Comparison of nonNewtonian, Newtonian, and viscoelastic properties Viscoelastic Kelvin material "Parallel" linearstic combination of elastic and viscous effects Anelastic Material returns to a welldefined "rest shape" Timedependent viscosity Rheopectic Apparent viscosity increases with duration of stress^{[1]} Some lubricants, whipped cream Thixotropic Apparent viscosity decreases with duration of stress^{[2]} Some clays, some drilling mud, many paints, synovial fluid Timeindependent viscosity Shear thickening (dilatant) Apparent viscosity increases with increased stress^{[3]} Suspensions of corn starch or sand in water Shear thinning (pseudoplastic) Apparent viscosity decreases with increased stress^{[4]}^{[5]} Paper pulp in water, latex paint, ice, blood, syrup, molasses Generalized Newtonian fluids Viscosity is constant
Stress depends on normal and shear strain rates and also the pressure applied on itBlood plasma, custard Shear thickening fluids
Shear thickening fluids are also used in all wheel drive systems utilising a viscous coupling unit for power transmission.
Shear thinning fluid
A familiar example of the opposite, a shear thinning fluid, or pseudoplastic fluid, is paint: one wants the paint to flow readily off the brush when it is being applied to the surface being painted, but not to drip excessively.
Bingham plastic
There are fluids which have a linear shear stress/shear strain relationship which 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.
Rheopectic
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).
Examples
Oobleck
An inexpensive, nontoxic example of a nonNewtonian 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).^{[7]}^{[8]} Uncooked imitation custard, being a suspension of primarily cornflour, has the same properties. The name "oobleck" is derived from the children's book Bartholomew and the Oobleck.
Flubber
Main article: Flubber (material)Flubber is a nonNewtonian fluid, easily made from polyvinyl alcohol based glues and borax, that flows under low stresses, but breaks under higher stresses and pressures. This combination of fluidlike and solidlike properties makes it a Maxwell solid. Its behaviour can also be described as being viscoplastic or gelatinous.^{[9]}
Chilled caramel topping
Another example of this is chilled caramel ice cream topping. The sudden application of force—for example by stabbing the surface with a finger, or rapidly inverting the container holding it—leads to the fluid behaving like a solid rather than a liquid. This is the "shear thickening" property of this nonNewtonian 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. A person moving quickly and applying sufficient force with their feet can literally walk across such a liquid.
Silly Putty
Main article: Silly PuttySilly Putty is a silicone polymer based suspension which will flow, bounce, or break depending on strain rate.
Ketchup
Ketchup is a shear thinning fluid.^{[3]} 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.
See also
 Bingham plastic
 Complex fluid
 Dissipative particle dynamics
 Newtonian fluid
 Herschel–Bulkley fluid
 Navier–Stokes equations
 Pseudoplastic
 Dilatant
 Quicksand
 Rheology
 Superfluids
References
 ^ Springer handbook of experimental fluid mechanics, Cameron Tropea, Alexander L. Yarin, John F. Foss, Publisher: Springer, 9 October 2007, ISBN 3540251413, ISBN 9783540251415, p.676, Google books
 ^ Springer handbook of experimental fluid mechanics, Cameron Tropea, Alexander L. Yarin, John F. Foss, Publisher: Springer, 9 October 2007, ISBN 3540251413, ISBN 9783540251415, p.661, Google books
 ^ ^{a} ^{b} Pump Application Desk Book, 3rd edition, Paul N. Garay, Prentice Hall, August 1996, ISBN 0881732311, ISBN 9780881732313, p.358, Google books
 ^ Rheology of Fluid and Semisolid Foods: Principles and Applications, M. A. Rao, Publisher: Springer, 2nd edition, 28 August 2007, ISBN 0387709290, ISBN 9780387709291, p.8, Google books
 ^ Emulsions, Foams, and Suspensions: Fundamentals and Applications, Laurier L. Schramm, Publisher: Wiley VCH, 26 July 2005, ISBN 3527307435, ISBN 9783527307432p.173, Google books
 ^ 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.
External links
 Classical experiments with NonNewtonian fluids by the National Committee for Fluid Mechanics on YouTube
Categories: Continuum mechanics
 Fluid dynamics
 NonNewtonian fluids
 Viscosity
 Polymers
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