Measuring Real Contact Area and Its Consequences on Adhesion and Friction in Dry and Wet Conditions

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PART 1: NON-TECHNICAL SUMMARYAdhesives and friction play important roles in engineering and biology, and the global market for adhesives is valued in the tens of billions of dollars. Many research efforts, such as improving the fuel efficiency of tires, require an understanding of friction and adhesion, which have important consequences in terms of energy savings. There is also a strong need for improving adhesion in wet environments for biomedical applications such as bandages, tissue adhesives, and health monitoring devices. The main challenge in these applications is understanding how roughness can affect adhesion and friction. This research project is focused on measuring the actual area of contact between a soft polymer that has elastic properties (such as rubber) and a rough surface, as this measure is critical for gaining insight on the magnitude of the forces of friction and adhesion. The project will involve the use of sophisticated optical instruments and the further development of techniques to directly probe the contact area between the elastomer and the surface during sliding under both dry conditions and under water. These direct measurements of the contact area will help in developing a fundamental understanding of the velocity dependence of friction and adhesion and will be useful in testing the current theoretical models in this field. Two graduate students will be trained during this project, and research experiences will be provided to undergraduate and high school students as part of the outreach activities. The project will also leverage the Biomimicry Center (BRIC) at the University of Akron to improve the diversity of students engaged in science projects and increase participation of students from inner-city schools. PART 2: TECHNICAL SUMMARYIt was first highlighted by David Tabor and colleagues that both adhesion and friction are related to the real contact area and the strength of interfacial adhesion. In the last two decades, new analytical models have been developed to predict how the real contact area is influenced by roughness, elastic modulus, and the sliding velocity. Recently, it has been shown that these models are inadequate for predicting changes in friction due to roughness or changes in the sliding velocity for soft elastomers. Resolving the discrepancy between theory and experiments will require a direct measurement of the real contact area, particularly during sliding. The proposed research will leverage recent developments in surface-sensitive sum frequency generation spectroscopy (SFG) to directly measure the shift in the surface -OH peak, which can be related to the strength of the interfacial interactions. SFG will be coupled with an oscillatory friction cell (with a velocity in the range of 10-7 to 0.2 m/s) to directly measure the contact area during sliding. These measurements will be conducted in both dry conditions and underwater to understand the influence of hydrodynamics in the boundary lubrication regime. The direct measurement of the contact area will help in developing a fundamental understanding of the velocity dependence of friction and adhesion and will be useful in testing the current theoretical models in this field..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.