= Normal-stress smallness in lubrication
{title2=$\max_i\frac{|\psi_i|U}{\mu\ell}\ll1$}
For gap $h\ll\ell$ and speed $U$, shear <stress> scales as $\mu U/h$ and viscoelastic <normal stresses> as $\psi_iU^2/h^2$. Their force contributions scale respectively as $\mu U/h^2$ and $\psi_iU^2/(h^2\ell)$. Thus the displayed criterion permits neglecting the normal-stress corrections in the leading <lubrication approximation>. It is weaker than requiring the normal stresses themselves to be smaller than the shear stress. Rapid time dependence can impose an additional material-time criterion.
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