= Extensional flow
An <extensional flow> stretches material along one or more directions while contracting it along others. A local ideal model of planar extension has <velocity> $\mathbf u=(\dot\epsilon x,-\dot\epsilon y,0)$: its <rate-of-strain tensor> is $\operatorname{diag}(\dot\epsilon,-\dot\epsilon,0)$ and it is an <incompressible flow>. <Uniaxial extensional flow> instead contracts both transverse directions. These geometries probe <extensional viscosity> and can stretch <polymers>.
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