Hybrid Molecular Simulation Studies of Nano-Scale Adhesion and Friction: Chemical Termination and Solvent Effects

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Adhesion and friction are critical to the performance of micro- or nanoelectromechanical systems (MEMS/NEMS). It is of great importance to consider surface chemical terminations for applications in dry or liquid environment (e.g., BioMEMS/NEMS). However, there is little understanding of the general laws that govern adhesion and frictional properties at this scale. The dependence of adhesion and friction on interfacial and solvent properties does not seem to have general rules to follow based on several recent experimental results. Thus, the first objective of this work is to study how interfacial and solvent properties affect nano-scale adhesion and friction important for bioMEMS/NEMS.Chemical force microscopy (CFM) provides a method of probing molecular interactions with chemical sensitivity. By covalently modifying atomic force microscopic (AFM) tips and substrates with self-assembled monolayers (SAMs) that terminate in distinct functional groups, one is able to apply this technique to measure adhesion and frictional forces between various probe tips and substrates with specific surface chemistry. Due to the complexity of the system, it is hard to interpret CFM results and to examine interfaces buried in CFM experiments. Furthermore, the difference in time scale between conventional molecular dynamics (MD) simulations and AFM (or CFM) experiments is six orders of magnitude or larger. Thus, the second objective of this work is to simulate CFM with various interfaces and solvents at the experimental time scale. In this work, the PI will apply the temporally hybrid molecular simulation technique that the PI's group developed recently to simulate adhesion and friction at a wide range of time scales from fast (in MEMS/NEMS devices) to slow (in CFM experiments) either between two surfaces or between alkanethiol SAM-modified AFM tips and surfaces immerged in solvents. Interfacial properties of nano-scale contacts will be hydrophobic/hydrophobic, hydrophilic/hydrophilic, and hydrophobic/hydrophilic while various solvents from polar to non-polar will be considered. For confined fluids between two surfaces, simulations will be performed in a new ensemble, in which confined fluids are in contact with the bulk. This ensemble is closer to bioMEMS/NEMS and surface force apparatus (SFA) experiments. Simulations will then be extended to examine the effect of SAM defects and surface asperities on adhesion and friction, and the nanotribology of the technologically important alkyl monolayers on silicon.The success of this work will provide a fundamental understanding of how surface chemical terminations and solvent polarities affect nano-scale adhesion and friction. Simulation results will be used to guide the design of bioMEMS/NEMS and to interpret CFM and SFA experiments. It will have significant impacts on BioMEMS/NEMS technology, CFM technique, biomolecular recognition, and biomechanics. Graduate students and undergraduate students will participate in this interdisciplinary research project. New simulation capacities generated from this work will be beneficial to students in special courses (e.g., ENG 100 for freshmen) and the course on computational nanotechnology offered by the PI. With this support, the PI's group will continue to serve many research groups on and off campus interested in molecular simulation and modeling.