viscometry

Viscometry is a branch of rheology focused on measuring the viscosity of liquids, the resistance to flow under an applied stress. Dynamic viscosity (μ) quantifies this resistance, while kinematic viscosity (ν) equals μ divided by the fluid density (ν = μ/ρ). Viscosity depends on temperature and, for many liquids, on shear rate, leading to Newtonian (constant μ with respect to shear) and non-Newtonian behavior (μ varies with shear).

Common viscometry methods include capillary viscometry, rotational viscometry, and falling-ball viscometry. Capillary viscometers measure the time a liquid takes to flow through a narrow tube under gravity; the viscosity is proportional to the flow time with a geometry-dependent constant. Rotational viscometers determine viscosity from the torque required to rotate a spindle or a cone between parallel plates at a controlled speed. Falling-ball viscometry relates the terminal velocity of a ball falling through a liquid to viscosity via Stokes' law. Ultrasonic and microviscometry techniques are used for small samples or special applications.

Units and terminology: the dynamic viscosity is measured in pascal-seconds (Pa·s) or poise (P), with 1 P = 0.1 Pa·s; commonly 1 centipoise (cP) = 0.001 Pa·s. Kinematic viscosity is reported in stokes (St) or centistokes (cSt), where 1 cSt ≈ 1 mm^2/s at approximately 20 °C for water-like liquids. Temperature control and calibration are essential, and industry standards (for example, ASTM or ISO methods) guide routine viscometry.

Applications span polymers, paints, foods, cosmetics, fuels, and pharmaceuticals, where viscosity informs formulation, quality control, and process design. Viscometry helps distinguish Newtonian from non-Newtonian fluids and characterizes shear-thinning or shear-thickening behavior.