Greens function molecular dynamics simulation of sliding, adhesive contacts

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This proposal evolves around the question: When does adhesion between solid bodies cause friction? From every day experience, we might be tempted to expect a large degree of correlation between these two properties. However, this is scarcely understandable from a microscopic view, since roughness (in the more precise jargon of physics: broken translational invariance) on both solids is a basic requirement for friction to occur. Roughness, however, counteracts adhesion and every surface is microscopically rough. The central goal of this proposal is to identify simple rules for a large class of surfaces that allow us to predict from a few (mainly, but not exclusively microscopic) parameters, if and how strongly (macroscopic) surfaces attract and what amount of adhesion-induced friction between the surfaces is unavoidable.Computer simulations shall be used to answer the just raised questions. They allow one to define all relevant quantities, such as the surface topographies, the elastic properties of the solids and their interactions, with mathematical precision. Since virtually all surfaces bear multi-scale roughness (in fact, they can be considered self-similar or fractal over several decades), the simulations must reflect a multitude of scales as well, which, in turn, necessitates efficient methods. Owing to significant progress in the recent past, criteria could be identified allowing one to predict with relatively simple rules when surfaces are sticky. However, in order to address the questions of 'how sticky?' and 'how much friction does adhesion induce?', the methods have to be much further advanced. Likewise, we see it as critical for a successful execution of the project to improve our understanding of the effect of temperature, waiting times (thermal aging), and the influence of the thickness of solid bodies on adhesion.A purely experimental analysis of the just raised question does not appear sensible at the moment, because it is technically not yet possible or unjustifiable expensive to measure sufficiently large surface topographies with the needed precision. In addition, only simulations allow one to vary all relevant parameters systematically and continuously. Experiments yet remain without any doubt indispensible for the validation of simulations. Colleagues outside the framework of this proposal agreed to conduct them.One goal of this proposal is to apply the newly gained insights and newly developed methods to optimize the geometry of adhesive systems (pick-up devices) of technological relevance. In this context, it might be worth mentioning that -- once all features are coded up -- exploring the effect of a single parameter on the device performance necessitates only one hour of manpower plus one day supercomputer time.