Study of the flow of materials that behave in an unusual manner is known as Rheology.
Types of Fluid-
(1) Newtonian fluid
(2) Non-Newtonian fluid
Newtonian flow of fluid
Shear stress is given by, τ = Force/Area
Shear Rate, γ = Small change in velocity (dv)/Small height (dh)
Newton’s law of flow: Shear stress is directly proportional to the shear rate.
By equation, τ α γ or τ = ηγ
e.g. water, Benzene, Mineral oil, sugar solution etc.
Non-Newtonian systems: Fluids which do not obey Newton’s law of flow.
a) Pseudoplastic behaviour:viscosity is inversely proportional to the shear rate. In other words, as the rate of shear is increased viscosity of these fluid decreases. Expressions for these fluid is; τ = Kγn, where n is less than 1.
e.g. Suspensions, Dispersion, Creams, Lotions, Gels etc.
b) Dilatant: viscosity is directly proportional to the shear rate. In other words, as the rate of shear is increased viscosity of these fluid decreases. Expressions for these fluid is; τ = Kγn, where n is greater than 1.
e.g. Surfactant solution, Corn starch dispersion, Wet sand etc.
c) Plastic fluid (Bingham bodies): These types of fluid do not show any shear rate initially after applying shear stress. After a certain value of shear stress they start behaving like Newtonian fluid. e.g. Toothpaste
d) Thixotropy: viscosity of the fluid is decreased with shear stress, after removal of the stress fluid returns to the normal. Such systems are also known as Gel-Sol-Gel transformation.
e.g. Paints, Ketchup, Pastes etc.
e) Rheopexy: This is totally opposite to the thixotropy and therefore also known as negative thixotropy. In this type of behavior, viscosity of the fluid is increased with shear stress, after removal of the stress fluid returns to the normal. Such systems are also known as Sol-Gel-Sol transformation.
