Pascalsecond

The pascal-second (Pa·s) is the SI unit of dynamic viscosity, the measure of a fluid's resistance to shear. It is defined as one newton-second per square meter, equivalently kg/(m·s). The unit is derived within the International System of Units and is named after Blaise Pascal.

Dynamic viscosity expresses how a liquid deforms under applied shear stress. Fluids with higher viscosity flow more slowly and resist motion more than low-viscosity fluids. Viscosity is strongly temperature dependent, typically decreasing as temperature rises. The concept is central to rheology, lubrication, polymer processing, and fluid dynamics, where it helps describe laminar flow, drag, and energy dissipation.

In related units, 1 Pa·s equals 10 Poise and 1 Pa·s equals 1000 centipoise (cP). Since a pascal is N/m^2, Pa·s can also be written as N·s/m^2. By contrast, kinematic viscosity has units of m^2/s (the Stokes, equal to 0.01 m^2/s, is a common subunit in the CGS system).

Examples of viscosity magnitudes: at room temperature, water has about 0.001 Pa·s (1 mPa·s). More viscous liquids, such as glycerol or honey, typically range from around 0.1 to several Pa·s, depending on composition and temperature. The pascal-second remains a fundamental measure for predicting and modeling fluid flow in engineering and science.