Frictional motion plays a central role in diverse systems and phenomena that span vast ranges of scales, from the nanometer contacts inherent in micro- and nanomachines and biological molecular motors to the geophysical scales characteristic for earthquakes. Despite the practical and fundamental importance of friction and the growing efforts in the field, many key aspects of dynamics of friction are still not well understood. One of the main difficulties in understanding and predicting frictional response is the complexity of highly non-equilibrium processes going on in any tribological contact which include detachment and re-attachment of multiple microscopic contacts (bonds) between the surfaces in relative motion while still in contact. In this lecture I will discuss microscopic models which establish relationships between the dynamics of formation and rupture of individual contacts and frictional phenomena. First, I will focus on dynamics of nanoscale friction studied in friction force microscopy experiments. In this case we found that experimentally observed velocity and temperature dependencies of friction can be rationalized by explicitly considering the influence of temperature on the formation and rupturing of microscopic contacts. Then, dynamics of cracklike processes that occur at the interface between two macroscopic blocks prior to the onset of frictional motion will be discussed. In agreement with experimental observations, the proposed model demonstrates that the onset of sliding is preceded by well-defined detachment fronts initiated at the slider trailing edge and extended across the slider over limited lengths smaller than the overall length of the slider. We found that three different types of detachment fronts may play a role in the onset of sliding: (i) Rayleigh (surface sound) fronts, (ii) slow detachment fronts, and (iii) fast fronts. The important consequence of the precursor dynamics is that before the transition to overall sliding occurs, the initially uniform, unstressed slider is already transformed into a highly nonuniform, stressed state. Our model allows to explain experimental observations and predicts the effect of material properties on dynamics of transition to sliding.
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