Effect of surface properties on adhesion and friction of particle systems

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Granular matter in form of powders has widespread application in process engineering and various adjacent fields. The macroscopic properties of the particle systems are determined by the microscopic properties of the single particles. Fundamental mechanisms of particle interactions are still not fully understood. In particular, many physical and chemical mechanisms at small particle distances (of a few nm) and their effect on the contact mechanics are still unknown. The project focuses on an improved understanding of mechanisms in the contact area of particulate systems. The mechanical, tribological and adhesive properties of single particles are studied with specially developed in-situ techniques giving unique insight into the contact mechanics between particles. The elastic-plastic deformation behavior of single micro- and nanoparticles under mechanical stress as well as their adhesive and tribological behavior as a function of material and system parameters are investigated. The results allow the control of adhesion and friction forces during particle processing.Within the previous funding periods unique in-situ techniques have been developed and used for the characterization of the micro-mechanical behavior of microparticles (with high statistical significance) and self-assembled monolayers under mechanical stress. Novel insights into the adhesive and tribological mechanisms of particles in contact already improved the fundamental understanding and allowed the quantification of micro-mechanical particle properties.In the final funding period we will focus mainly on studies in the liquid phase. The systems investigated in the previous funding periods will be transferred to the liquid phase in order to study variations in tribological and adhesive properties as well as the behavior of liquid electrolytes in the contact area. We will explore the possibilities of extremely low friction (superlubricity) by two different approaches: On the one hand the effect of repulsive van der Waals forces will be studied by careful control of the dielectric properties of the contact partners, on the other hand the effect of adsorbate layers on the surfaces of the two contacting bodies will be investigated. This will enable us to identify superlubricating particle systems and reveal basic mechanisms of superlubricity by application of our novel in-situ techniques.