Our research group studies the nanomechanical properties of materials. In particular, we aim to understand microscopic mechanisms of friction and wear through dedicated experiments and to contribute to the design of new tribological materials. High-resolution force microscopy (AFM) is our most important method. In AFM, an extremely fine tip is scanned over the material surface. The microscope is sensitive enough to detect the friction force of each single molecular bond broken. Furthermore, we develop novel experimental methods in order to contribute to the understanding of the complex world of friction. Materials under investigation include metals and metallic glasses, micro-structured and fiber reinforced polymers, macromolecular surface functionalization, and novel lubricants. PhD students and physics undergraduate students of Saarland University collaborate in most of our projects.
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Nanoscale friction on metallic surfaces depends critically on the atomic structure of the surfaces and on adsorbates. We investigate friction on atomically flat and clean surfaces of gold, platinum, and copper and of metallic glasses in ultra-high vacuum. The experiments aim to clarify the role of structure and self-diffusion in friction and explore the microscopic mechanisms of friction on ultra-thin oxide and graphene films.
Liquid lubricants do not exhibit any long-range order. However, when confined to a nanometer gap, the density correlations lead to a molecular order which manifests itself as force oscillations in force microscopy experiments. The order also changes the shear viscosity, as we reveal using a novel mode of dynamic shear force microscopy. For ionic liquids, we explore the influence of the electrochemical potential on these phenomena.
Supramolecular reversible guest-host interactions are being suggested for controlling adhesion and friction. Our experiments investigate the functionalization of surface with supramolecular layers, the role of molecular cooperativity for friction and adhesion, and the control of these effects through external stimuli such as light or potential. The projects also include bio-inspired hydrogels.
The perception of materials in contact with skin is the subject of haptics research. We contribute to this emerging field of science through measurement of friction between the fingertip and various materials, correlated with simultaneously recorded EEG signals. We are looking for characteristic waves in the brain activity, which follow on frictional stimuli.
Connecting biological objects with electronics requires soft electrical contacts. To that end, we explore the fabrication of micro-fibrillar adhesion devices from electrically conductive materials. Detailed characterization of these devices reveals the relationship between adhesion properties and electrical resistivity. Their application as electrically tunable devices is also explored.