The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling. Because of its enormous practical and technological importance, the friction problem has stimulated progress over the centuries. Everyday operations on a broad range of scales, from nanometer and up, depend upon the smooth and satisfactory functioning of countless tribological systems. Friction is intimately related to both adhesion and wear, and all three require an understanding of highly nonequilibrium processes occurring at the molecular level to determine what happens at the macroscopic level. Despite its practical and fundamental importance, and the growing efforts in the field, many key aspects of the dynamics of friction are not yet well understood.
Three quiet revolutions of broad nature are radically changing this state of affairs. First, progress in the general area of complexity provided new tools to tackle nonequilibrium disordered systems with interacting degrees of freedom. Second, and crucially, the developments in nanotechnology extended the study of friction and permitted its analysis on well-characterized materials and surfaces at the nanoscale and microscale. Notably the invention of scanning tip instruments of the atomic force microscope (AFM) family has opened nanofriction as a brand new avenue; the use of the surface force apparatus (SFA) led to the systematic studies of confined mesoscopic systems under shear; while instruments such as the quartz crystal microbalance (QCM) measure the inertial sliding friction of adsorbate submonolayers. Thanks to these methods, a mass of fresh data and information on well-defined systems, surfaces, materials, and physical conditions has accumulated in the last two decades. Third, computer simulations have had a strong boost, also allowed by the fantastic growth of computer power. The numerical study of frictional models on the one hand, and direct atomistic molecular dynamics simulations, on the other hand, are jointly advancing our theoretical understanding.
In this workshop we will focus on different methods to simulate interfacial dynamical processes responsible for energy dissipation. The aim of the proposed workshop is to discuss recent scientific results on different aspects of nano- up to meso-scale friction, and to formulate important unresolved problems. Due to the intrinsic interdisciplinarity of the subject, at the boundary between physics and engineering, computer and materials science a combination of complementary theoretical, computational and experimental efforts and skills is an essential ”ingredient” in order to form a predictive understanding of the distinct, but interrelated, dynamical processes involved over such a wide range of length scales. It is thus beneficial to bring together different communities and researchers working on various aspects of atomic-scale friction, colloidal systems and granular materials and using different theoretical and computational approaches. Moreover, in order to compare more effectively theoretical predictions with experimental data and methodologies, this event envisages strategically the simultaneous presence of scientific renowned experimentalists.
Experiments in tribology have long suffered from the inability to directly observe what takes place at a sliding interface. Now the field of friction can benefit from the opportunities offered by trapping and handling nanoparticles with potentials artificially created by interfering lasers; a technique originally applied to cold atoms. Colloidal friction provides an unprecedented real-time insight into the basic dynamical mechanisms at play. Unlike atomic force microscopy, surface force apparatuses and quartz crystal microbalances, which provide averaged frictional data, such as the overall static and kinetic friction, mean velocities and slip times - the colloidal experiments observe the true motion of every individual particle with time during sliding. This has so far been restricted to the ideal world of molecular dynamics simulations.
Over the past few decades, computer simulations complemented experiments by revealing atomic and molecular mechanisms in dynamic contact interfaces. Simulations allowed for a first glimpse on the different paths of energy dissipation induced by tribological loading, which could not be analyzed in situ. Today simulations have become an indispensable tool to better understand the nature of tribological contacts and interfacial dynamics at fairly large scales. However, there still remains a large gap between the time; and length;scales considered by computer simulations and those in experiments.
Different models and computational techniques are employed to mimic and finally understand interfacial processes from nano up to the macroscale:
1) We can consider simple, “minimalistic” models, which are based on simplified interaction potentials and focus only on the most relevant degrees of freedom of the system, trying to retain the most important features. Despite their simplicity, these models can explain phenomena of high complexity and have contributed to unravel some important concepts of the mechanisms of friction (e.g., the role of commensurability of the contacting surfaces, the transition from intermittent stick-slip to smooth sliding, the onset of sliding motion, the mechanisms of detachment fronts preceding overall macroscopic sliding, the scaling laws describing statistics of earthquakes, some aspects of the creeping-to-sliding transition in fault like mechanics).
2) A different kind of simplified approach which allows us to connect microscopic and meso-macroscopic scales consists in the inclusion of suitable noise terms in otherwise homogeneous equations for the meso-scale. Recent results show that this approach is successful in effectively describing intermittency and fluctuations often observed in experiments as the theory predicts non-equilibrium phase transitions (pinning/depinning is one such). An important issue is how to couple this approach with microscopic details.
3) Another possible approach is via extensive molecular dynamics simulations with more realistic interaction potentials and geometries. This route is usually taken to understand intricate processes such as, for example, wear and plastic deformations in the contact of sliding solid surfaces. The time and length scales for a truly realistic approach are still beyond reach, and the computer simulations still rather heavy. An important issue, therefore, is how to reduce the large-scale, many-parameter MD simulations to simpler descriptions with only a few equations of motion.
4) An additional understanding of friction phenomena come from phenomenological Rate-State (RS) models that have been used to describe a wide range of observed frictional behaviors, such as the dilation of a liquid under shear and the transition between stick–slip (regular or chaotic) and smooth sliding friction. However, most “state variables” in RS models cannot yet be quantitatively related to physical system properties, and the main challenge is to define these variables starting from a microscopic description. It is also important to develop the connection to models of the type 2) from above, which is until now lacking.
Each of these methods has its strengths and weaknesses. Large-scale simulations allow to reproduce quite accurately some experimental features, but are not really well suited to extract general information at a fundamental level; Rate-State models capture many experimental features quantitatively, but the physical nature of the state variable is unspecified, not allowing to fully rationalize the results of observations; minimalistic models have the advantage of being computationally cheap and simple enough to enable us to work out the general mechanisms at play of the problem, but, obviously, they are not system specific.