= Precursor film
{title2=$h\to h_*>0$}
A thin liquid layer extending ahead of a thicker drop can replace a sharp zero-height <contact line> in a continuum film model. Keeping the height positive avoids the zero-mobility singularity of a purely no-slip $h^3$ <lubrication theory>[lubrication law] at the apparent contact edge. Additional microscopic surface interactions usually determine its thickness and matching to the macroscopic apparent <contact angle>. It is one possible contact-line regularization, rather than a universal assumption for every substrate.
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