= Lubrication resistance
{title2=$F\sim\mu a^2 U/h$}
<Lubrication theory> predicts large viscous resistance when fluid must be squeezed through a narrow gap. For normal approach of smooth curved surfaces with curvature scale $a$ and minimum separation $h\ll a$, the resisting <force> scales as $\mu a^2U/h$. Under a bounded normal <force> this gives $|\dot h|=O(h)$, so ideal smooth surfaces do not make contact in finite time. Tangential and rolling motions have different resistance laws; the normal squeeze law should not be applied to them.
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