Molecular Tribology of Thin Films

University Of Washington, Seattle WA

Investigators

Abstract

9988745 Jiang The PI proposes to interpret scanning force microscopy (SFM) and surface force apparatus (SFA) experiments rigorously by molecular simulations. For simulations of SFM experiments on self-assembled monolayers (SAMs), tip-based molecular simulations will be carried out where an SFM tip scans at a constant velocity over alkanethiols/Au(111) in either constant height or constant load mode. The tip-cantilever system (TCS) will be explicitly described by representing it with three orthogonal springs connecting a rigid holder to the center of mass of the tip. A temporal-hybrid molecular simulation technique will be developed to extend the simulation time scale to that of SFM experiments. This method combines a dynamic element model for the TCS and a molecular dynamics relaxation approach for SAMs. Experimental values for the inertial mass of the TCS and spring constants of the cantilever determined by the PI's group will be used in the simulations. The hybrid technique will be used to study the dependence of friction on the chain length and terminal group of SAMs, scanning velocity and direction, tip size, and temperature. Simulation results will be compared with quantitative nano-scale frictional properties measured by the PI's group using SFM. Mechanical properties of SAMs (e.g., lateral stiffness and elastic modules) and various energy dissipation mechanisms will be examined. The new hybrid simulation technique is of general significance. It can be applied to validate not only simulations of SFM and SFA experiments, but also simulations of nano-indentation and many other important problems involving different time scales. For simulations of SFA experiments, non-equilibrium molecular dynamics simulations will be carried out to study rheological properties of long-chain molecules in a grand canonical ensemble where confined fluids are in equilibrium with bulk fluids. This is an improved model for SFA experiments. In addition, novel lubricating strategies for applications in microelectromechanical systems (MEMS) will be explored. ***