propertiesviscosity

Viscosity is a measure of a fluid’s resistance to flow and to deformation under shear. It reflects internal friction between molecules as they move past one another. Viscosity is a key rheological property that influences how liquids and gases behave in pipes, channels, lubricants, and many industrial processes.

In Newtonian fluids, viscosity is constant over a range of shear rates; the shear stress is proportional to shear rate, described by tau = mu * (du/dy), where mu is the dynamic viscosity. Non-Newtonian fluids do not have a constant mu and can display shear-thinning, shear-thickening, or time-dependent (thixotropic) behavior, causing apparent viscosity to vary with shear conditions and time.

Commonly used terms include dynamic (or absolute) viscosity, denoted mu, with units of Pascal-seconds (Pa·s). The kinematic viscosity, nu, is mu divided by density (nu = mu / rho) and has units of square meters per second (m^2/s). Viscosity is often reported in centipoise (cP), where 1 cP = 0.001 Pa·s, and 1 Pa·s = 1000 cP.

Measuring viscosity employs viscometers and rheometers, including capillary viscometers (e.g., Ubbelohde), rotational viscometers (e.g., Brookfield), and falling-ball devices. Standards from ASTM and ISO describe procedures for liquids across temperatures and pressures. Temperature significantly influences viscosity: for most liquids, viscosity decreases with increasing temperature; for gases, viscosity generally increases with temperature. Pressure effects are typically smaller at ambient conditions.

Applications span lubrication, polymer processing, food science, pharmaceuticals, and chemical engineering. Understanding viscosity helps predict flow rates, pressure drops, heat transfer, and energy consumption, and supports formulation and quality control across industries.