A 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. In a Newtonian fluid, the relation between the shear stress and

the strain rate is linear (and if one were to plot this relationship, it would pass 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

strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined.

A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more

useful for fluids without time-dependent behavior.

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 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.

An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of corn starch (corn flour) in water,

sometimes called oobleck (uncooked imitation custard, being a suspension of primarily corn flour, has the same

properties). The 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 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. A person moving quickly and applying

sufficient force with their feet can literally walk across such a liquid.

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 while worn. Some shear thickening fluids are

also used in all wheel drive systems utilising a viscous coupling unit for power transmission.

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.

There are fluids which have a linear shear stress, shear strain relationship, that requires a finite yield stress before they

begin to flow. That is the shear stress, shear strain curve doesn't pass through the origin. These fluids are called

Bingham plastics. Several examples are clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and

mustard. The classic case is ketchup which will not come out of the bottle until you stress it by shaking.

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).

Source: WikipediA

polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as

ketchup, starch suspensions, paint, blood and shampoo. In a Newtonian fluid, the relation between the shear stress and

the strain rate is linear (and if one were to plot this relationship, it would pass 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

strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined.

A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more

useful for fluids without time-dependent behavior.

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 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.

An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of corn starch (corn flour) in water,

sometimes called oobleck (uncooked imitation custard, being a suspension of primarily corn flour, has the same

properties). The 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 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. A person moving quickly and applying

sufficient force with their feet can literally walk across such a liquid.

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 while worn. Some shear thickening fluids are

also used in all wheel drive systems utilising a viscous coupling unit for power transmission.

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.

There are fluids which have a linear shear stress, shear strain relationship, that requires a finite yield stress before they

begin to flow. That is the shear stress, shear strain curve doesn't pass through the origin. These fluids are called

Bingham plastics. Several examples are clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and

mustard. The classic case is ketchup which will not come out of the bottle until you stress it by shaking.

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).

Source: WikipediA

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