Friction and adhesion affect almost all aspects of technology and everyday life. Despite their importance, our fundamental understanding of their origins and our ability to predict them in practical applications has remained limited. In large part, this is because of the wide range of length scales that are important. Adhesion and friction forces between surfaces result from interactions in regions where atoms are separated by less than a nanometer, but the geometry of these contact regions and the forces within them are determined by subsurface deformations on micrometer and longer scales. Over the last decade, advances in computational algorithms and hardware have begun to allow us to capture all of these length scales for the first time. The results have revealed qualitative shortcomings of traditional engineering models and resulted in simple equations that predict the area and stiffness of the contacts between rough surfaces. The proposed research will extend these studies to calculate friction and adhesive forces. The goal is to provide new understanding of fundamental processes that will allow predictive models for the relation between surface roughness and macroscopic friction and adhesion to be developed. New computational tools will be developed and shared as open source software.
Contact, adhesion and friction are complex problems that involve a wide range of length scales. Atomic-scale interfacial interactions determine adhesive and frictional forces, but the area where these forces operate is determined by surface roughness and substrate deformation on nanometer to micrometer or longer scales. Traditional models of contact, adhesion and friction are based on macroscopic continuum equations with phenomenological constitutive relations for the interfacial interactions and macroscopic deformations. By their very nature, continuum equations do not include effects associated with discrete atoms, but most of the adhesive energy comes from changes in separation by a few angstroms and lateral frictional forces on a crystal are periodic with displacements by a lattice constant. The proposed research will address fundamental questions associated with the gaps between atomistic and continuum descriptions. These include the relation between atomic structure and detachment forces, the origins of friction and its connection to roughness, plasticity and disorder, and the extent of correlations in the dynamics at the interface. The goal is to provide new understanding of the physical processes at different scales and develop macroscopic models that incorporate atomic level processes. Efficient multiscale methods that retain atomistic detail at the interface and capture the long-range response of the substrate will be extended, ported to open source simulation packages, and used to simulate contact and friction. The simulations will examine contact of planes and spheres with self-affine or experimentally measured surface roughness. The response of crystals, polymer glasses and elastomers will be studied in elastic and plastic limits with different atomic-scale roughness and adhesive interactions. Studies of contact and adhesion will determine how the contact area and detachment force are related to interfacial interactions, material properties and surface roughness. Studies of friction will address how interactions within these contacts lead to static and kinetic friction, how roughness and plasticity contribute to friction, and examine precursors to steady sliding where only part of the surface slips forward.
