The spreading of an incompressible viscous liquid over an isotropichomogeneous unsaturated porous substrate is considered. It is shown that,unlike the dynamic wetting of an impermeable solid substrate, where the dynamiccontact angle has to be specified as a boundary condition in terms of thewetting velocity and other flow characteristics, the `effective' dynamiccontact angle on an unsaturated porous substrate is completely determined bythe requirement of existence of a solution, i.e. the absence of a nonintegrablesingularity in the spreading fluid's pressure at the `effective' contact line.The obtained velocity dependence of the `effective' contact angle determinesthe critical point at which a transition to a different flow regime takesplace, where the fluid above the substrate stops spreading whereas the wettingfront inside it continues to propagate.
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