Influence of surface topography on adhesive and long-range capillary forces between hydrophobic surfaces in water.

We report on the interactions between a hydrophobic probe particle and surfaces with nanoscopic surface features. These surfaces have been prepared by spin-coating of nanoparticles and by polishing. The surface topography was characterized by AFM, using the methods of high-resolution imaging, low-resolution imaging using the probe particle, and by the rolling ball method. The spin-coated surfaces can be characterized as nanostructured due to the high density of nanoparticles that on a short length scale provides a regular pattern of crevices and hills. On these surfaces a larger waviness is also distinguished. In contrast, the polished surfaces display sharp nanoscopic peaks and hardly any crevices. In all cases the dominant force at short separations was found to be a capillary attraction due to the formation of an air/vapor condensate. Our data show that the large-scale waviness of the surface does not significantly influence the range and magnitude of the capillary attraction, but large local variations in these quantities are found. The large variation in adhesion force corresponds to a small variation in local contact angle of the capillary condensate at the surfaces. The report discusses how the nature of the surface topographical features influences the capillary attraction by influencing the local contact angle and by pinning of the three-phase contact line. The effect is clearly dependent on whether the surface features exist in the form of crevices or as extending ridges.