Extensional viscosity

Extensional viscosity is a measure of a fluid's resistance to deformation under extensional (stretching) flow conditions. Unlike shear viscosity, which describes a fluid's resistance to flow when layers slide past one another, extensional viscosity relates to how the fluid behaves when it is being stretched or elongated. In extensional flow, a fluid element experiences forces that cause it to be pulled apart, creating a change in shape without necessarily changing volume.