Johannes L. Hörmann
In this thesis, I orchestrate molecular dynamics simulations to explore adsorption film friction in high-dimensional parametric spaces. Thereby, I look for friction trends controlled by adsorbant concentration, adsorbant film morphology, sliding direction, electrostatic potential bias, and pressure. Simulations of a sliding spherical asperity on anionic surfactant monolayers and hemi-cylindrical aggregates physisorbed on gold reveal two friction regimes: At high loads, the friction force is independent of load as long as no direct solid-solid contact occurs. At low loads, the films show Amontons’ friction with a friction force that rises linearly with normal load. Here, the friction coefficient reaches a minimum at intermediate surface concentrations. I attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing. Shear simulations on saline solution confined in a nano gap between gold electrodes apply a fixed electrostatic potential across the lubricant. The lubricant’s viscosity is governed strongly by pressure, but behaves independently of the polarization of the electrodes. The electrostatic bias, however, introduces a pronounced asymmetry in wall slip and the interfacial friction coefficient at anode and cathode. The interfacial friction coefficient depends linearly on pressure and approximately quadratically on electrostatic bias. Such complex parametric studies are made feasible by the right balance of rigidity and flexibility in data documentation offered by dtool, a lean, decentralised data management framework. dserver, our own contribution to this ecosystem, adds aspects of centrality by making collections of dtool datasets searchable. Together, dtool and dserver facilitate the annotation, localization, and evaluation of datasets in a high-dimensional, dynamically evolving parametric space without much administrative overhead. Conveniently, working within this permissive, transparent framework contributes directly to the traceability and reproducibility of our research in the spirit of FAIR data and Open Science – without the need for top-down dictated standardization.