Viscosity

Lore & Background

The word 'viscosity' is derived from the Latin 'viscum' ('mistletoe'), which also referred to a viscous glue from mistletoe berries. In continuum mechanics, viscosity is defined scientifically as a force multiplied by a time divided by an area, giving SI units of pascal-seconds. For liquids, it corresponds to the informal concept of thickness; for example, syrup has a higher viscosity than water. In gases, viscosity results from molecular collisions, whereas in liquids it arises from cohesive molecular forces. Zero viscosity is observed only at very low temperatures in superfluids; otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. A fluid with zero viscosity is called ideal or inviscid. Viscosity generally depends on a fluid's state, such as temperature, pressure, and rate of deformation, though for Newtonian fluids the dependence on rate of deformation is negligible. For non-Newtonian fluids, there are pseudoplastic, plastic, and dilatant flows (time-independent) and thixotropic and rheopectic flows (time-dependent).

Reader's Guide

Viscosity is a fundamental material property in continuum mechanics, relating viscous stresses to the rate of change of deformation (strain rate). In a simple shearing flow like planar Couette flow, the force on a moving plate is proportional to the plate speed and area and inversely proportional to plate separation, with the proportionality factor being the dynamic viscosity. This relationship is expressed in Newton's law of viscosity: τ = μ ∂u/∂y. Viscosity is essential for understanding fluid flow in pipes, lubrication, and many engineering applications. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the center than near the walls, and a pressure difference is needed to overcome friction between fluid layers. The strength of the compensating force is proportional to the fluid's viscosity. The concept distinguishes elastic stresses (from deformation) from viscous stresses (from deformation rate). Viscosity is denoted by μ (common among engineers, mathematicians, and physicists) or η (used by chemists and IUPAC). It is sometimes called shear viscosity, though it can appear in non-shearing flows as well.

Did You Know?

• The word 'viscosity' derives from Latin 'viscum' meaning 'mistletoe', which also referred to a viscous glue from mistletoe berries.
• Zero viscosity is observed only at very low temperatures in superfluids; otherwise, all fluids have positive viscosity due to the second law of thermodynamics.
• In liquids, viscosity arises from cohesive molecular forces, while in gases it results from molecular collisions.
• For a Newtonian fluid, viscosity does not vary significantly with the rate of deformation.