Hydrodynamic drainage force in a highly confined geometry: role of surface roughness on different length scales

We measured the hydrodynamic drainage force of an aqueous, Newtonian liquid squeezed between two hydrophobic or two hydrophilic surfaces by means of the colloidal probe technique. We controlled the wettability, the roughness, the topology, and also the approaching velocity of the surfaces. We found that asperities on the surfaces caused an artificial decrease of the measured drainage force that must be considered by the interpretation of the force curves. Even considering the effect of asperities, our experimental results could be interpreted only with the aid of a partial slip model. Or else, interpreted assuming that the viscosity close to the surfaces is different from bulk. On patterned hydrophilic surfaces, we demonstrated that the drainage force depends not only on the overall surface roughness or micro structuring but also on the specific length scale of the surface nanostructures.