Interstitial Water Enhances Sliding Friction

Motivated by the need to understand the role of interfacial processes like adhesion and friction between surfaces of different interfacial energies, we probe such an interface in this study. Here, we create surfaces having a range of surface energies and bring them in contact with a hydrophobic lens. Our results show that adhesion energy decreases monotonically with an increase in interfacial energy, whereas sliding friction maximizes when the interfacial energy is moderately hydrophobic, within the contact angle range of 60° - 70°. These results indicate that coefficient of friction measured underwater is related to adhesion hysteresis depending on the wetness of the contact area. Consequently, surface sensitive sum frequency generation spectroscopy is used to probe the interface during static contact and dynamic sliding to understand the nature of confined water. Although interstitial water is used to reduce the amount of friction between two surfaces, the spectroscopy results show that under certain cases of contact, the presence of water increases friction. The impact of our results provides guidance for tuning interfacial energy using a simple, scalable, and inexpensive method to mass-produce a variety of adhesive materials for underwater applications.