Wetting at the Nanoscale: Molecular Mobility Induced by Contact Line Forces

In this paper, we study the interaction of a contact line with molecules physically adsorbed on a surface. We developed specific atomic force microscopy (AFM) experiments where a nanoneedle attached at the extremity of the cantilever is dipped in a liquid droplet. The motion of the contact line at the extremity of the meniscus formed depends on the presence of topographical and chemical defects at the surface of the nanoneedle. The analysis of the force measured by AFM based on a capillary model allows one to distinguish the effects of topographical and chemical defects and to monitor minute changes of surface properties. Using six different liquids and five tips, we show that the change of the surface properties of one nanoneedle results either from the adsorption of airborne molecules when the tip is left in the air or from their desorption by the moving contact line when the tip is repeatedly dipped in the liquid. The desorption rate is found to depend only on the number of dipping cycles and is not influenced by the velocity or the liquid properties. A model based on the estimation of capillary and adsorption energies confirms a capillary desorption mechanism in agreement with the experimental results. Finally, we demonstrate that three distinct desorption mechanisms may be at play. Interestingly, using a deliberate contamination with large hydrocarbon molecules, we show that the capillary desorption studied in this paper can be used to clean surfaces.