Collaborative Research: What Processes Cause State Evolution in Rate and State Friction?

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Friction is the resistance to sliding between two surfaces in contact, whether it be on an earthquake fault, the parts of a door hinge, or pieces in an automobile engine. People have observed that if two surfaces are in contact for longer times with little sliding, greater forces are required to really get them sliding again. Although this has been known for nearly 50 years, no one is sure why it is true. Frictional surfaces actually only touch each other in many small contact spots. The most popular idea for why longer contact times make friction higher is that being in contact longer increases the total area or size of the many contact spots. However, recent results by the team planning this research project suggest that changing the strength or “quality” of those small contact spots is more important than changing their area. The team of experimentalists and theoreticians will conduct and analyze experiments in order to better understand which explanation is correct. It is important to understand this because the variations in frictional strength influence many processes of practical importance. These include whether two surfaces slide steadily or undergo alternating sticking and slipping motion. Such alternating motions occur, for example, during earthquakes or when a bow is pulled across a violin string. A better understanding of friction has implications for several economically-important industries, including manufacturing and transportation. If the results are as revolutionary as anticipated, this project will alter the research directions of scientists trying to understand jerky sliding in many disciplines. It could influence the research directions of people who are pursuing the possibility that changes in contact area are responsible for changes in friction. It would show that it is important to understand the chemical bonding at frictional contact spots. It would result in new theoretical investigations of the appropriate equations to use for applying lab results to earthquake faults. The project will also increase the skills, the knowledge, and the networks of an undergraduate student and a graduate student, as well as of the two early-career scientists involved in the theoretical parts of this project.The behavior described by rate and state constitutive equations for friction has been recognized for nearly 50 years and is widely accepted as being important in the nucleation of earthquake slip. Nevertheless, there is still uncertainty regarding the micromechanical meaning of what is represented by a “state” variable in this formulation; in other words, what physical or chemical changes control state, dictating its evolution. This is unsatisfactory from a fundamental scientific point of view. It is also unsatisfactory because if the processes involved in the evolution of state are not understood and characterized by process-based equations, then extrapolation of laboratory results to understanding earthquake mechanics does not rest on a firm foundation. It is believed by many in the community that the evolution process involves time-dependent increase in the size of contacts across a frictionally sliding interface, namely evolution of contact quantity. However, evolution of contact quantity alone cannot explain recent experimental observations of friction phenomenology following normal stress steps. In contrast, the quality of the contact interface, in other words the contact shear strength per unit area, dominantly controls strength evolution following normal stress steps, a transformative observation. The team brings together experimentalists and theoreticians. They will conduct a large suite of experiments on a wide variety of geological and other materials, together with intensive theoretical modeling and inversion of the experimental results, to determine the relative contributions of changes in contact quantity and quality to evolution of state. The theoretical modeling will include discrete element modeling of granular gouge which has been recently shown to reproduce a variety of experimental findings.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.