Measurement of Real Contact Area and Its Consequences on Adhesion and Friction

The nature of the contact interface has a profound influence on electrical conductivity, adhesion, friction, and wetting. This problem has remained unresolved due to the complexity of probing the contact separation with sub-nm resolution. In addition, these interfaces are buried within two materials (solids or liquids). Here, we present the use of surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) to study the shift in OH frequency to probe the real contact area between two solid surfaces. The position of the OH peak in the SFG spectra is sensitive to sub-nm changes in separation distance (acid-base interactions) and provides a spectroscopic ruler to study the contact interface in static and dynamic conditions that are encountered during adhesion and friction. The influence of this contact as a function of chemistry, roughness, and the presence of confined fluids (water) will be discussed.