This article is about extensional viscosity. For other uses, see Rheology.
Extensional viscosity (also known as elongational viscosity) is a viscosity coefficient when the applied stress is extensional stress.[1] It is often used for characterizing polymer solutions.
Extensional viscosity can be measured using rheometers that apply extensional stress. Acoustic rheometer is one example of such devices.
Extensional viscosity is defined as the ratio of the normal stress difference to the rate of strain. For uniaxial extension along direction :[2]
where
is the extensional viscosity or elongational viscosity