Evolution behavior of adhesion force with continually changed relative humidity revealed on AFM

Abstract

ABSTRACT Adhesion forces on various surfaces with different hydrophilicity were measured by atomic force microscopy in the force-volume mode and at a location to study the relative humidity (RH) dependence. With an increasing-stable-decreasing trend in RH, the force in the force-volume mode usually exhibits a constant-increase-constant-decrease-constant variation. Surfaces with partial or complete hydrophobicity exhibited a much larger increase compared to a hydrophilic one. However, the behavior on some surfaces was distorted due to a decrease at high stable RHs. Contrastively, the behavior at a location could also display the same behavior. Both gradual and sudden changes were observed at a single location as RH changed. At high stable RHs, the force displayed complex behaviors, indicating unique characteristics at each location. Therefore, the measurement at a location is not suitable to study the RH dependence. Specially, the force measured on silica in the force-volume mode has the mentioned behavior. However, the force at a location can remain stable, gradually/sharply increase or decrease, and suddenly jump or drop with RH. These observations are attributed to the evolution of a water bridge, which is largely influenced by thin-film flow. The findings provide deeper insights into adhesion mechanisms at different RHs.