Glossary term
Viscosity
A fluid property describing resistance to shear deformation and relative motion between layers, strongly affected by temperature and rheology.
Definition
quantityViscosity is a measure of a fluid's resistance to shear deformation and relative motion between adjacent layers.
Viscosity controls pressure drop, Reynolds number, lubrication, mixing, heat transfer, sedimentation, pumping power, boundary layers, and many process calculations. For Newtonian fluids it is a material property at a given temperature and pressure; for non-Newtonian fluids it depends on shear rate, time history, temperature, and microstructure.
For a Newtonian fluid in simple shear, viscosity relates shear stress to velocity gradient:
where \tau is shear stress and \mu is dynamic viscosity. Kinematic viscosity is:
where \rho is density. Dynamic viscosity controls shear stress and pressure drop directly, while kinematic viscosity is often used in Reynolds-number and momentum-diffusion calculations.
Engineering use
Viscosity strongly depends on temperature. Liquids usually become less viscous when heated; gases generally become more viscous when heated. This affects pump sizing, valve selection, lubrication film thickness, heat-exchanger pressure drop, flowmeter calibration, hydraulic response, and mixing energy.
Many fluids are non-Newtonian: apparent viscosity may change with shear rate, time, solids content, or previous mixing. Paints, slurries, polymer melts, blood, food products, drilling fluids, and gels require rheological data rather than one viscosity number.
Measurement and specification
Viscosity can be measured with capillary, rotational, falling-ball, vibrational, or process viscometers, and each method imposes its own shear field and assumptions. Engineering specifications should state whether the value is dynamic or kinematic, the temperature, pressure if relevant, shear rate or instrument geometry, sample preparation, and allowed tolerance. For process control, repeatability under the actual operating condition may matter more than agreement with a handbook value.
Common mistakes
A common mistake is quoting viscosity without temperature. Another is using a single Newtonian viscosity for a shear-thinning or yield-stress material. A strong viscosity review states dynamic or kinematic basis, units, temperature, pressure if relevant, shear rate, measurement method, fluid composition, and whether the fluid is Newtonian, time-dependent, or multiphase.