Wetting on heterogeneous surfaces: Imperfect Hele-Shaw cell

We study the influence of the surface wettability defects on the behavior of a liquid front advancing in a thin Hele-Shaw cell. Defects are made by spraying ink or by deposition of a photosensitive resin on a glass surface, giving a well-characterized system with controlled defects. We have done an experimental and theoretical study of the interface motion past a single defect. The main result is the existence of a pinning transition: the contact line cannot move over defects that are too large and an air bubble remains trapped on the defect. Smaller defects do hold the contact line for a while, but they are eventually wetted by the advancing liquid. We determine the critical defect size for this transition and calculate the equilibrium shapes of the interface in a two-dimensional geometry. We also investigate the motion of an interface moving over multiple defects. We analyze the relation between the statistical properties of the wetting front as function of the geometrical properties of the defect distribution and gravity effects.