Comparison of surface forces in air and water

Abstract

We measured the surface force between a PDMS (polydimethylsiloxane) coated sphere and a PEEK (polyetheretherketone) or Si plane sample using a surface force apparatus that can measure the surface force with ultra-high accuracy. In particular, we focused on the differences in surface force characteristics between air and water environments. The measurement consisted of 4 processes: approach, lift-up, stabilization, and withdrawal processes. Significant differences in probe displacement between air and water were observed in each process. Furthermore, we compared the surface force characteristics between PEEK and Si plane samples. The results revealed that a comparison of the maximum surface forces (or adhesion forces) in air and water showed that they depend on the combination of materials. We deduced that the wettability of the material affected the adhesion forces. Specifically, the low affinity of the two surfaces to water resulted in hydrophobic attraction, while a hydration repulsion acts when one surface becomes hydrophilic.