Cure Terms Glossary

Dynamic Viscosity

The viscosity of a fluid is a measure of its resistance to flow, and the dynamic viscosity is the (coefficient of) viscosity at a particular point in space or time within a fluid where the rate of shear is not constant. For example, in steady flow, where the shear rate varies across the moving the fluid, the dynamic viscosity may be obtained from the equation,

τ = ηdν/dh

where τ is the shear stress and ν the laminar velocity at height h in the flowing liquid.

In oscillating flow, where the velocity is varying with time, then the dynamic viscosity is obtained from the loss component of the dynamic modulus according to the equation,

η' = G''/ω

where η' is the dynamic viscosity, G'' is the loss modulus and ω is the angular frequency.

In this latter case, the notation η' is used because this is the real part of a complex viscosity.

At very low frequencies η' approaches η (the steady-flow viscosity). With increasing frequency, η' falls rapidly in relation to η, so that finite values of η' occur even at gelation.