viskozity

Viskozity, often written viscosity in English, is a property of fluids that describes their resistance to flow. It arises from internal friction between molecules as they move past one another. The dynamic viscosity, denoted mu, measures the shear stress required to produce a given rate of deformation, with common SI units of pascal seconds (Pa·s). The kinematic viscosity, denoted nu, is mu divided by the fluid density and has units of square metres per second (m^2/s); it is often reported in centistokes (cSt) for liquids.

Fluids can be classified by their response to shear. Newtonian fluids have a constant mu independent of shear rate; their shear stress tau is proportional to shear rate gamma_dot (tau = mu * gamma_dot). Non-Newtonian fluids exhibit viscosity that changes with gamma_dot or time, including shear-thinning (pseudoplastic), shear-thickening (dilatant), and Bingham plastics that require a yield stress before flow.

Temperature and composition strongly influence viscosity. In liquids, viscosity generally decreases as temperature rises; in gases, viscosity tends to increase with temperature. Pressure and composition (impurities, polymers) also alter mu.

Viscosity is measured with specialized instruments. Capillary viscometers (e.g., Ubbelohde, Ostwald) relate flow time to mu; rotational viscometers measure torque for a shearing element; falling-ball viscometers assess rate of a ball through a liquid. Kinematic viscosity is common in lubricants and does not require density measurement when reported as cSt at a specified temperature.

Applications span engineering, lubrication, paint and coating, food and pharmaceutical processing, and polymer manufacture. Viscosity helps determine laminar versus turbulent flow via the Reynolds number and affects heat transfer and energy efficiency.