In solid mechanics, stress and strain are often discussed; however, in fluid mechanics, shear stress and shear strain rate are investigated. Shear stress is the stress due to a shear forces on the fluid and is equal to the shear force divided by the area that the force acts on. Shear strain rate is the rate at which the fluid deforms due to shear forces on the fluid. As with solid mechanics, the shear stress and shear strain rates are proportional to each other; the functional form of this proportionality determines which of the following types each fluid belongs to.
Newtonian Fluids - have a linear relationship between shear stress and shear strain rate; viscosity is the proportionality constant in this relationship
Non-Newtonian Fluids - have a non-linear relationship between shear stress and shear strain rate; there are three subcategories (see below)
shear thinning fluids - viscosity decreases with increasing shear rate; also known as pseudoplastics (ex: hair gel)
shear thickening fluids - viscosity increases with increasing shear rate; also known as dilatants (ex: cornstarch and water mixture)
Bingham Plastics - neither fully solid nor fully fluids, but have properties of each; sitting still they behave as a solid, but they flow like a fluid as soon as a shear force is applied (ex: toothpaste, may)
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