Significance and importance of the project. The initiation of earthquakes on faults in the Earth?s crust is controlled, incredibly, by physical processes that occur at microscopic contacts between rough surfaces of rock that touch along the fault. Despite the success of empirical friction equations in describing the results of laboratory friction experiments and producing a variety of earthquake-related phenomena in computer models of earthquakes, these equations lack a physical basis. That is, the precise identity and nature of the physical mechanisms that occur at microscopic contacts, and give rise to the observed friction effects in experiments, remain unknown. The empirical nature of the descriptions reflects the difficulty of isolating and studying the physical processes at microscopic fault contacts. Without a sound physical understanding, we remain limited in our abilities to reliably apply the equations to earthquakes in nature, to obtain a better general understanding of the earthquake process, and to ultimately make reliable predictions of earthquakes. In this transformative study, we will make use of state-of-the-art materials science testing methods, namely atomic force microscopy and Nanoindentation, to provide a physical basis for friction observations at a coarser scale and thereby gain a much improved understanding of the earthquake process. This work may allow us to learn whether we are likely to be able to detect accelerating creep on faults just prior to an earthquake and thereby predict earthquakes days to hours before an earthquake, which would save many lives and mitigate damages to the built environment. From the perspective of the scientific fields of mechanics and materials science, these new insights, gained by identifying and connecting frictional behavior across many length scales, have potential application well beyond geophysics, for example, in many engineered systems, including silicon-based micromechanical devices.
overarching goals of the proposed studies are to isolate and identify the physical mechanisms that occur at asperity contacts at frictional interfaces. A more specific major goal of our study is to understand the origin of the friction state ?evolution? effect in rate and state friction, the simplest manifestation of which is an increase in ?static? friction with the time of quasi-stationary contact. To that end, we will conduct a coordinated, interdisciplinary collaboration that will employ laboratory experiments that investigate frictional phenomena over a wide range of length scales. One outcome will be to develop constitutive equations that will allow extrapolation of these mechanisms to the elevated temperatures and longer times relevant for earthquakes. We will perform macro-scale friction experiments on rocks at Brown University, micro-scale to nano-scale indentation creep, adhesion, and friction experiments in the Nanoindenters at Oak Ridge National Laboratory, and nano-scale adhesion and friction experiments in atomic force microscopes at the University of Pennsylvania. To make connections between the different types of experiments and to isolate different origins of the state evolution effect, we will vary the same environmental conditions in all three sets of experiments. These include tests as a function of humidity, pH (in liquid), and temperature. All three environmental factors have been demonstrated to influence the evolution effect in macroscopic rock friction experiments. Nanoindentation and AFM measurements should allow us to determine the processes on an asperity scale responsible for the macroscopic behavior.
The purpose of this project is to understand the reason that frictional resistance increases in strength in proportion to the logarithm of the time that the frictional interface is in stationary contact. The reason this is important is that this increase in frictional resistance with time, and the related decrease in frictional resistance with increases in sliding velocity, is responsible for unstable sliding when a frictional interface is loaded by elastic surroundings. A high-frequency manifestation of this in everyday life is the squeaking of an unlubricated door hinge. A much larger scale manifestation is the occurrence of earthquakes due to sliding on faults in the Earth’s crust. In order to better understand the occurrence of earthquakes and to model them accurately, we need to understand the processes that cause the time-dependent increase in static friction and to develop mathematical descriptions of the frictional processes. Presently empirical equations are used for such modeling that fit laboratory observations over limited ranges of time and sliding speeds, but until we understand the processes we cannot extrapolate these equations with confidence from the lab to the Earth. Two explanations have been offered as to why static friction should increase with stationary contact time. One is the possibility that the small contact points where the sliding resistance originates increase in area with time. The other possibility is that the strength of the chemical bonds at the small contact points increase with time. This project is designed to distinguish between these two explanations. We have used two distinct approaches. In the first we used an atomic force microscope (AFM) to discover whether at the atomistic scale increases in static friction increase with the log of the static hold time just as occurs for macroscopic frictional interfaces. The force perpendicular to the surface in the AFM is too small for increases in contact area to occur. We discovered that in the AFM static friction increases with the log of the static hold time (see attached Figure). Furthermore, this only occurred with chemically similar surfaces, indicating that increases in chemical bond strength is a likely explanation for the increase in friction. In our second approach, we have investigated the effect of pH on the plastic flow of small contact points, flow that could be responsible for the increase in contact area with time. Earlier we had predicted that if chemical bonding was responsible for the increase in static friction with hold time it would not occur for quartz at the point of zero charge, otherwise known as the isoelectric point, where the normally negatively charged quartz surfaces had no charge, due to protons embedding themselves in the surface at a pH of ~2.2. Experiments at a pH of 2.2 showed no increase in static friction with hold time. So this supports the idea that chemical reactions are responsible for the increase in static friction with time. However, it is possible that plastic flow could be inhibited at a pH of 2.2. To test this we did microindentation tests at a pH of 2.2 as well as at a neutral pH of 7.0. We find that plastic flow is slightly easier at a pH of 2.2 compared to 7.0. This means that if increases in contact area are responsible for increases in static friction with time, then those increases would be slightly larger at a pH of 2.2. Since increases in static friction did not occur at a pH of 2.2, but do at a pH of 7.0, this argues that increases in contact area are not responsible for time-dependent increases in static friction. In contrast, the explanation involving time dependence of chemical reactions is supported by the pH 2.2 friction experiments.
