pascalseconds

Pascalseconds, written as pascal-second (Pa·s), is the SI unit of dynamic viscosity, the measure of a fluid’s resistance to shear or gradual deformation by an applied stress. It is defined as the pressure unit (pascal) multiplied by time, and equivalently equals 1 newton-second per square meter (1 Pa·s = 1 N·s/m^2).

Dynamic viscosity describes how much shear stress is needed to produce a given rate of shear strain in a fluid. For a Newtonian fluid, the relationship is linear: τ = μ γ̇, where τ is the shear stress, μ is the dynamic viscosity, and γ̇ is the shear rate. The unit μ = Pa·s reflects both the internal friction within the fluid and its resistance to flow.

Common values illustrate its range: water at around 20°C has μ ≈ 0.001 Pa·s (1 mPa·s or 1 cP). Air at room temperature has μ ≈ 1.8×10^-5 Pa·s. Glycerol is much more viscous, with μ on the order of 1–2 Pa·s. In terms of conversions, 1 Pa·s equals 1000 mPa·s, and equals 10 Poise (since 1 Poise = 0.1 Pa·s). It also relates to kinematic viscosity ν through μ = ρ ν, where ρ is density; ν has units of m^2/s and is measured in Stokes (1 St = 1 cm^2/s).

Applications span fluid mechanics, lubrication, polymer processing, and biomedical flows. Viscosity measurements are performed with viscometers and rheometers to characterize Newtonian and non-Newtonian fluids, informing design and analysis across science and engineering.