Nanoscale Dynamics of Confined Fluids by Time-Correlated Fluorescence Spectroscopy within an Atomic Force Microscope

Wayne State University, Detroit MI

Investigators

Abstract

Non-technical: Moving two solid surfaces with respect to each other is always associated with friction and wear. Lubrication is necessary to reduce damage and to enable reliable operation of any moving part of an engine or machine. Fundamentally, the phenomena of friction, wear and lubrication involve mechanisms occurring on a molecular scale, and a good understanding of lubricant behavior on this scale is thus of primary importance to design more efficient and environment friendly lubricants. The economic value involved is enormous. In developed countries, financial savings resulting from improved attention to friction and wear would, by most estimates, amount to 1-2 percent of gross national product. The research will lead to better understanding of friction by investigating at the molecular level the properties of liquids confined by solid surfaces. It will perform both nanoscale force measurement and time-correlated fluorescence spectroscopy experiments to study the dynamics of nanoscale fluid systems. The force measurements will tell us about how many molecules behave when they act together, while the spectroscopy can track single molecules in the fluid layer. Students and postdoctoral researchers working in this project will learn cutting-edge laser spectroscopy and atomic force microscopy techniques, as well as research skills in areas of materials sciences and nanotechnology, which have huge growth potential. They will be well prepared for future careers in technological fields important for the greater Detroit area, including the auto industry, where questions of lubrication, wear and thin films play a significant role. Technical: The goal of this project is to perform direct measurements of molecular relaxation processes within nanometer thick confined fluid films by incorporating single-molecule sensitive fluorescence correlation spectroscopy (FCS) with atomic force microscopy (AFM). The proposed research will identify the relation between single-molecule relaxation processes, such as diffusion, and the mechanical properties measured in AFM experiments, such as stiffnesses and damping coefficients. This will lead to better understanding of the recently observed non-equilibrium behavior of these systems at increased approach speeds and test at the molecular level the hypothesis that the transition from rest (static friction) to sliding (kinetic friction) in thin confined films springs from a phase transition analogous to melting transition of a solid. The research is significant because it bridges the gap between the single-molecule and the ensemble-averaged response of confined fluids. The results will also be relevant to many contemporary ideas of condensed matter physics, such as order of liquids at interfaces, wetting phenomena and systems under extreme conditions, in particular molecular-scale confinement. The progress in this fundamental research can have important consequences for many technological applications. For example, in nano-electromechanical systems, the research may provide insight for the management of frictional dissipation. An improved understanding of the observation that under faster approach rates the system behaves elastically may lead to designs that exploit the confined lubricant as a 'smart liquid' to control approach rates in small devices. The interdisciplinary research program will train students and postdoctoral researchers in cutting-edge laser spectroscopy and atomic force microscopy techniques. They will be well prepared for future careers in technological fields important for the greater Detroit area, including the auto industry, where questions of lubrication, wear and thin films play a significant role.