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Friction is a topic of huge practical, technological and scientific interest that has challenged mankind for thousands of years. The basic laws of friction were introduced by Amontons and Coulomb around 1700. They found that the friction force is proportional to the normal force, and that the friction force required to start relative motion, the static friction force, is larger than the friction force needed to keep relative motion at a constant velocity - the dynamic friction force. The empirical laws of friction have later been reformulated into the rate-and-state friction law, which states that the friction force depends on the rate - on how fast the surfaces are moving relative to each other - and the state - how long the surfaces have been in contact and under what conditions. We have recently made a startling discovery: The static friction force may also strongly depend on the history of the frictional contact, on how the two surfaces stopped relative to each other. This indicates that the friction laws must be reformulated to include the history of the contact. In this project, we will determine under what circumstances history-dependent friction is important, develop a theory for history-dependent friction, test and apply this theory on atomic- and macroscopic scales and apply it to key problems in glaciology and geoscience. In 2020 and 2021 we have developed continuum models for slip pulses and a new model for the friction below glaciers taking into account the local water pressure. This opens for completely new possibilities to compare models and observations of glacier dynamics. In parallel, we have developed a machine-learning model to determine the parameters for a nano-scale model for friction in realistic materials from geoscience (silicates and water) and materials science (SiC). We have also developed methods to effectively determine the frictional behavior of a surface using machine-learning methods in order to rapidly find materials with specific frictional properties. This provides a foundation for developing predictive models for history-dependent friction in 2022.

Friction is a topic of huge practical, technological and scientific interest that has challenged mankind for thousands of years. However, it still remains poorly understood, probably due to the inherent multi-scale and multi-physics nature of processes at the frictional interface. The empirical laws of friction where introduced by Amontons and Coulomb, and later refined into the rate-and-state friction law, which is commonly used today. The rate-and-state friction law states that the coefficient of friction that depends on the rate - on how fast the surfaces are moving relative to each other - and the state - how long the surfaces have been in contact and under what conditions. However, we have recently made a startling discovery: The coefficient of friction may also strongly depend on the history of the frictional contact, on how the two surfaces stopped relative to each other, changing the research focus from detachment to reattachment. Our hypothesis is that the rate-and-state friction law must be reformulated to include the history of the contact - a history-dependent law of friction. The goal of this project is test this hypothesis; to understand under what circumstances history-dependent friction is important; to develop a consistent theory for history-dependent friction, to test and apply this theory on atomic-, meso- and macroscopic scales and apply it to key problems in glaciology and geoscience. This is a bold proposal with vast consequences if successful, since the laws of friction are widely used in many sciences. The project will be carried out at the Physics of Geological Processes, an established cross-disciplinary collaboration between physics and geology lasting more than 20 years with a strong track record in producing research of fundamental importance with practical applications.The center is therefore in a unique position to address the proposed cross-disciplinary project.