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

非技术：两个固体表面相对移动总是与摩擦和磨损有关。润滑对于减少损坏和使发动机或机器的任何运动部件可靠运行是必要的。从根本上说，摩擦、磨损和润滑现象涉及分子尺度上发生的机制，因此，在这个尺度上很好地理解润滑剂的行为对于设计更高效、更环保的润滑剂至关重要。所涉及的经济价值是巨大的。在发达国家，根据大多数估计，由于加强对摩擦和磨损的注意而节省的资金将相当于国民生产总值的1- 2%。这项研究将通过在分子水平上研究受固体表面限制的液体的性质，从而更好地理解摩擦。它将进行纳米尺度力测量和时间相关荧光光谱实验，以研究纳米尺度流体系统的动力学。力测量将告诉我们有多少分子在共同作用时的行为，而光谱学可以跟踪流体层中的单个分子。参与该项目的学生和博士后研究人员将学习到尖端的激光光谱学和原子力显微镜技术，以及材料科学和纳米技术领域的研究技能，这些领域具有巨大的发展潜力。他们将为未来在大底特律地区重要的技术领域的职业生涯做好充分准备，包括汽车工业，其中润滑，磨损和薄膜问题发挥着重要作用。技术：该项目的目标是通过结合单分子敏感荧光相关光谱（FCS）和原子力显微镜（AFM），对纳米厚受限流体膜内的分子弛缓过程进行直接测量。提出的研究将确定单分子弛豫过程（如扩散）与AFM实验中测量的力学性能（如刚度和阻尼系数）之间的关系。这将有助于更好地理解最近观察到的这些系统在增加接近速度时的非平衡行为，并在分子水平上验证一个假设，即薄膜中从静止（静摩擦）到滑动（动摩擦）的转变源于类似于固体熔化转变的相变。这项研究意义重大，因为它弥补了封闭流体的单分子响应和整体平均响应之间的差距。这些结果也将与凝聚态物理的许多当代思想有关，例如界面上液体的顺序，极端条件下的润湿现象和系统，特别是分子尺度的限制。这项基础研究的进展可以对许多技术应用产生重要影响。例如，在纳米机电系统中，该研究可能为摩擦耗散的管理提供见解。在更快的接近速率下，系统表现出弹性，对这一观察结果的更好理解可能会导致设计利用受限润滑剂作为“智能液体”来控制小型设备的接近速率。该跨学科研究项目将培养尖端激光光谱学和原子力显微镜技术的学生和博士后研究人员。他们将为未来在大底特律地区重要的技术领域的职业生涯做好充分准备，包括汽车工业，其中润滑，磨损和薄膜问题发挥着重要作用。

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.