Influence of water adsorbed on gold on van der Waals/Casimir forces

TL;DR

This study examines how ultra-thin water layers on gold surfaces affect van der Waals and Casimir forces, revealing that water can influence forces at separations below 10 nm, but surface roughness often dominates.

Contribution

The paper provides experimental and theoretical analysis of water layer effects on van der Waals/Casimir forces, highlighting the significance of surface roughness at small separations.

Findings

Water layers up to 1.5 nm are consistent with force measurements at separations >10 nm.

Water influences become significant at separations <10 nm.

Surface roughness effects can overshadow water layer influences at very small distances.

Abstract

In this paper we investigate the influence of ultra thin 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 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 w_{shp}+w.