Extensional viscosity (also known as elongational viscosity) is a viscosity coefficient when applied stress is extensional stress.[1]

This parameter 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.

\[\eta_e = \frac{\sigma_n}{\dot{\varepsilon}}\,\!\]

where

\[\eta_e\,\!\] is the extensional viscosity or elongational viscosity \[\sigma_n\,\!\] is the normal stress\[\sigma_n = \frac{F}{A}\,\!\] \[\dot{\varepsilon}\,\!\] is the rate of strain\[\dot{\varepsilon} = \frac{1}{L}\frac{dL}{dt}\,\!\]

For a Newtonian Fluid, the elongational viscosity is three times the shear viscosity which can be derived using the continuity equation for incompressible fluids.

\[\eta_e = 3\eta\,\!\]

References

  1. Barnes, H.A. "A handbook of elementary rheology", Institute of non-Newtonian Fluid Mechanics, 2000

See also


de:Dehnviskosität

fr:Viscosité élongationnelle