Contact Angle of Glycerol Nanodroplets Under van der Waals Force

We have studied the contact angle of glycerol nanodroplets on a partial wetting substrate using the tapping mode of an atomic force microscope. Our experiment shows strong contact angle hysteresis of nanodroplets (radius <100 nm) on mica substrate with atomic-level roughness. The theoretical analysis reveals and our experiment confirms that the van der Waals (vdW) force becomes a critical factor for the mechanical balance of droplets as the drop size reduces to nanometers. The vdW force introduces a dispersion force that significantly lowers the force threshold for producing contact angle hysteresis.