Normal stress
= Normal stress
{title2=$\sigma_{nn}=n\cdot\sigma n$}
The component of traction normal to a surface with unit normal $n$. For an incompressible <Newtonian fluid>, $\sigma=-pI+2\mu e$, so $\sigma_{nn}=-p+2\mu n\cdot e n$. A free-interface normal-stress jump includes the curvature force from <surface tension>.