Influence of ultrathin water layer on the van der Waals/Casimir force between gold surfaces

In this paper we investigate the influence of ultrathin water layer (∼1–1.5 nm) on the van der Waals/Casimir force between gold surfaces. Adsorbed water is inevitably present on gold surfaces at ambient conditions as jump-up-to contact during adhesion experiments demonstrate. Calculations based on the Lifshitz theory give very good agreement with the experiment in the absence of any water layer for surface separations d≳10 nm. However, a layer of thickness h≲1.5 nm is allowed by the error margin in force measurements. At shorter separations, d≲10 nm, the water layer can have a strong influence as calculations show for flat surfaces. Nonetheless, in reality the influence of surface roughness must also be considered, and it can overshadow any water layer influence at separations comparable to the total sphere-plate rms roughness wshp+w.