viscosities

Viscosity is a measure of a fluid’s resistance to flow and to shear deformation between adjacent layers. It arises from cohesive forces and molecular interactions within the liquid. The dynamic (or absolute) viscosity, denoted mu, expresses this resistance as shear stress per unit rate of deformation. The related quantity, kinematic viscosity nu, equals mu divided by the fluid density (nu = mu / rho) and has units of area per time (m^2/s).

Fluids are often classified as Newtonian or non-Newtonian. Newtonian fluids have a constant viscosity that does not depend on the shear rate within typical conditions. Non-Newtonian fluids exhibit viscosity that changes with shear rate or time; examples include shear-thinning (pseudoplastic), shear-thickening (dilatant), and Bingham plastics that have a yield stress.

Viscosity depends strongly on temperature and, to a lesser extent, pressure. For liquids, viscosity generally decreases as temperature rises; for gases, it often increases with temperature. Many liquids follow Arrhenius-type behavior, where viscosity changes exponentially with temperature due to molecular activation processes.

Viscosity is measured with viscometers. Capillary viscometers determine mu from flow under gravity; rotational viscometers (cone-plate or parallel-plate) measure torque at controlled shear rates; falling-sphere viscometers infer mu from the descent of a sphere. Kinematic viscosity is obtained by dividing mu by density.

Applications span lubrication, oil and polymer processing, food and pharmaceutical industries, and many engineering calculations. In analyses of flow, viscosity is a key parameter in the Reynolds number, which helps predict flow regimes and related phenomena.