Non-Newtonian fluid

From Academic Kids

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A non-Newtonian fluid is a fluid in which the viscosity changes with the applied shear force. As a result, Non-Newtonian fluids may not have a well-defined viscosity.

Although the concept of viscosity is commonly used to characterize a material, it can be inadequate to describe the mechanical behavior of a substance, particularly non newtonian fluids. They are best studied through several other rheological properties which relate the relations between the stress and strain tensors under many different flow conditions, such as oscillatory shear, or extensional flow which are measured using different devices or rheometers. The rheological properties are better studied using tensor-valued constitutive equations, which are common in the field of continuum mechanics.

An inexpensive, non-toxic sample of a non-Newtonian fluid can be made quite easily. Just add corn starch to a cup of water. Add the starch in small portions and stir in slowly. When the suspension nears the critical concentration, the so called "shear thickening" property of this non-Newtonian fluid becomes apparent. The application of force from the spoon, your fingers etc. cause the fluid to behave in a more solid like fashion. If left at rest it will recover its liquid like behavior. Shear thickening fluids of this sort are being researched for bullet resistant body armor, useful for their ability to absorb the energy of a high velocity projectile impact but remain soft and flexible when struck at low velocities.

Template:Clear Principal types of non-Newtonian fluid include:

Type of fluid Behaviour Characteristics Examples
Plastic solids Perfectly plastic Strain does not result in opposing stress Ductile metals
Bingham plastic Linear relationship between shear stress and rate of strain once threshold shear stress exceeded Mud, some colloids
Yield pseudo-plastic Pseudo-plastic above some threshold shear stress
Yield dilatent Dilatent above some threshold shear stress
Power-law fluids Pseudo-plastic or "shear thinning" Apparent viscosity reducing with rate of shear Some colloids, clay, milk, gelatine, blood and liquid cement
Dilatant or "shear thickening" Apparent viscosity increasing with rate of shear Concentrated solution of sugar in water, suspensions of rice starch or corn starch
Viscoelastic - having both viscous and elastic properties Maxwell material "Series" linear combination of elastic and viscous effects metals, composite materials
Oldroyd-B fluid Linear combination of Maxwell and Newtonian behaviour Bitumen, dough, nylon, and Silly Putty
Kelvin material "Parallel" linear combination of elastic and viscous effects
Anelastic Material returns to a well-defined "rest shape"
Time-dependent viscosity Rheopectic Apparent viscosity increases with duration of stress Some lubricants
Thixotropic Apparent viscosity decreases with duration of stress Non-drip paints and tomato ketchup
Generalized Newtonian fluids Stress depends on normal and shear strain rates Blood, Custard

See also

Jargon Dictionary definition of ooblick: Fluid it:Fluido non newtoniano pt:Fluido não newtoniano


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