Impact of humidity on adhesion between rough surfaces

The surface forces apparatus was used to study the variation with relative humidity of adhesion between mica and rough gold films coated with either a hydrophobic or hydrophilic self-assembled monolayer (SAM). Pull-off forces, as a function of relative humidity, were measured between mica and gold coated with either n-octadecanethiol or 1-hydroxylunicosanethiol. For the alcohol-terminated SAM, pull-off forces increase from 370 mN/m (dry conditions) to 845 mN/m (saturated conditions). For the methyl-terminated SAM, pull-off forces increase from 160 mN/m (dry conditions) to 643 mN/m (saturated conditions). A quartz crystal microbalance was used to obtain adsorption isotherms on the two different SAMs. Angstrom-level changes in adsorbed layer thickness are associated with marked changes in measured adhesion forces.