Measurement of Nanoscale Interfacial Contact Area Using Vibrational Spectroscopy

Understanding the extent of contact between objects is crucial for surface-driven phenomena in the natural world. While interactions are known to occur at subnanometer-length scales, the techniques needed to measure such small-length scales are lacking. We combined two spectroscopic techniques: attenuated total reflectance infrared (ATR-IR) spectroscopy coupled with an imaging detector and sum frequency generation (SFG) spectroscopy to characterize the real contact area between surfaces. Imaging ATR-IR provides the distribution of airgap separation between the surfaces at larger than molecular length scales as well as an estimation of the real contact area. Additionally, SFG directly probes the intermolecular interactions occurring at molecular scales. Through the combination of these techniques, we show how contact is achieved for samples of varying modulus and roughness across multiple length scales. Interestingly, we also show that the orientation of the molecules in contact is a critically important factor that influences interfacial interactions rather than just the number of molecular sites in contact.