viscidity

Viscidity, more commonly spelled viscosity in scientific usage, is the property of fluids that describes their resistance to flow and to deformation by shear stress. It arises from intermolecular forces and molecular structure, causing liquids to behave as if they were thick or sticky. Viscidity varies with temperature, composition, and pressure, and it governs how substances move and mix.

Dynamic viscosity characterizes the shear stress required to produce a given rate of deformation. It is measured in pascal-seconds (Pa·s) and often expressed in the cP (centipoise) scale, where 1 Pa·s equals 1000 cP. Kinematic viscosity is the dynamic viscosity divided by density and has units of square meters per second (m^2/s), expressed in Stokes (St) for practical measurements.

In liquids, viscosity generally decreases as temperature rises; in gases, viscosity usually increases with temperature. Some fluids are Newtonian, with constant viscosity regardless of shear rate, while many liquids are non-Newtonian and exhibit shear-dependent, time-dependent, or thixotropic behavior.

Viscosity is measured with viscometers or rheometers. Capillary viscometers (e.g., Ubbelohde) are common for Newtonian fluids, while rotational viscometers and rheometers handle non-Newtonian materials and complex flows.

Applications span lubrication, paints and coatings, fuels, foods, polymers, and biomedical contexts. Viscidity affects energy use, mixing, heat transfer, and flow in pipes and devices. It is a standard parameter in design, quality control, and process engineering.