Molecular desorption by a moving contact line*

The surface properties at the molecular scale influence the contact angle of a macroscopic sessile drops [1, 2]. Therefore, the contact line is very sensitive to these properties which are at the origin of the hysteresis. We address the fundamental questionof a possible action of the contact line on the surface, at molecular scale by desorbing molecule from the solid surface.

To answer this question, we use atomic force microscopy (AFM) as used before with good results [3]. Thanks to this tools, nanoneedles (with a radius between 60 and 500 nm) are repeatedly dipped in droplets of 6 liquids and the capillary force which appears with the formation of a meniscus is measured.

As the contact line sweeps the nanoneedle surface, the capillary force increases by yielding to a more "hydrophilic" surface. We interpret this effect by the desorption atmospheric contaminants on the solid surface and show that the capillary force at the contact line is at the origin of the mecanism. The dynamic does not change with the liquid or the solid surface but the magnitude is very sensitive with the surface tension of the liquid. Our results open new outlooks on the comprehension of molecular effects at the contact line [4].