Viscosity

Viscosity is a measure of a fluid’s resistance to gradual deformation by shear or tensile stresses. In liquids, it reflects internal friction that acts to slow flow. The dynamic (or absolute) viscosity, denoted μ, relates shear stress τ to shear rate γ̇ by τ = μ γ̇. The kinematic viscosity ν is μ divided by density ρ (ν = μ/ρ) and has units of square meters per second.

The SI unit of viscosity is the pascal-second (Pa·s). Commonly, viscosity is expressed in centipoise (cP), where 1 cP = 0.001 Pa·s and 1 Pa·s = 1000 cP. For reference, water at room temperature has a dynamic viscosity of about 1.0 mPa·s (0.001 Pa·s). Viscosity also relates to flow through dimensionless numbers used in engineering, and it influences pressure drops and energy requirements in piping.

Temperature and pressure affect viscosity. For liquids, viscosity decreases as temperature rises; for gases, viscosity increases with temperature. Pressure effects on liquids are typically small at ordinary pressures but become relevant at high pressures or with highly viscous liquids.

Newtonian versus non-Newtonian fluids. Newtonian fluids have a constant μ independent of shear rate. Most simple liquids are Newtonian. Non-Newtonian fluids show shear-rate dependence: shear-thinning (viscosity decreases with increasing shear), shear-thickening (viscosity increases with shear), or more complex viscoelastic behavior. Some materials exhibit a yield stress, as in Bingham plastics.

Measurement methods include capillary viscometers (e.g., Ostwald) and rotational viscometers (e.g., Brookfield). Viscosity is a fundamental property for applications in lubrication, polymer processing, food and cosmetics, paints, and many industrial processes, influencing pump design, mixing, and product performance.