Direct Measurement of Molecular Contact Area Using\nSum Frequency Generation Spectroscopy

The molecular interactions between\ntwo solid surfaces at a subnanometer-length\nscale can control phenomena such as adhesion, friction, and wetting\nproperties. These interactions, which comprise the actual contact\narea at the molecular scale between two materials, have historically\nbeen a challenge to measure. Here, we show the use of surface-sensitive\nsum frequency generation spectroscopy to probe the molecular contact\nbetween two solid surfaces that vary in roughness and modulus. The\nmolecular interactions are measured by a shift in the vibrational\nenergy of surface hydroxyl groups (OH) on sapphire substrates after\ninteracting with rough poly(methyl methacrylate) sheets. We relate\nthe shift of the OH peak to the contact area and find a dramatic drop\nin the contact area with increases in roughness and modulus. In contrast,\nsofter sheets were able to deform and create a complete mechanical\ncontact. Interestingly, we find that complete mechanical contact\nalone was insufficient to maximize the molecular contact. Instead,\nthe molecular rearrangement of interfacial molecules was necessary\nto enhance the interaction energy.