viscous

Viscous is a property of fluids that describes their resistance to gradual deformation by shear or tensile stress. It reflects internal friction between adjacent layers as they slide past one another. A highly viscous fluid flows slowly and yields a thick, sticky consistency; a low-viscosity fluid, such as water, flows readily.

Viscosity is the measure of that resistance. Dynamic viscosity, denoted mu, quotes the shear stress required to produce a given rate of shear. Newtonian fluids have a constant mu independent of shear rate, while non-Newtonian fluids change mu with shear rate or time. Viscosity is expressed in units of pascal-seconds (Pa·s) or poise (P), with 1 P = 0.1 Pa·s.

Viscosity depends on temperature and composition. For most liquids, increasing temperature lowers viscosity. For gases, viscosity tends to rise with temperature. Impurities, molecular structure, and phase state also affect viscosity.

Kinematic viscosity nu = mu / rho, with units of square meters per second, relates viscous effects to density. It is commonly used in characterizing flow in pipes and channels. Practical measurement uses viscometers or rheometers.

In physics, viscosity appears in the Navier-Stokes equations as a viscous stress term, governing dissipation and damping of motion. In engineering and industry, viscous fluids influence lubrication, hydraulics, ink and paint spreading, food texture, and biomedical contexts involving mucus or synovial fluid.