Collaborative Research: Determining the Physical Mechanisms of Atomic Stick -Slip Friction by Closing the Gap between Experiments and Atomistic Simulations

该合作研究项目的目标是了解控制纳米级接触滑动的原子级摩擦机制，特别关注被称为“原子粘滑摩擦”的普遍现象。分子动力学（MD）模拟和原子力显微镜（AFM）实验尽可能地匹配摩擦学条件，将使两者的结果直接进行比较。比较AFM实验与动态原子模拟的能力，包括分子动力学（MD），目前是有限的。在原子力显微镜实验中，人们无法看到原子的位置和速度，这激发了原子力显微镜的研究。然而，原子力显微镜的结果不能直接与原子力显微镜的结果进行比较，因为传统的模拟必须以比原子力显微镜实验快几个数量级的速度运行。同样，许多MD和AFM研究在其他重要条件如材料、载荷和尖端尺寸上也存在差异。在这项工作中，速度将通过同时使用由主要研究人员专门为原子尺度摩擦研究合作开发的新方法来匹配。贵金属的使用将确保接口被良好定义并可靠地建模。其他关键参数，包括刚度，尖端尺寸和形状，环境，晶体表面和滑动方向取向，也进行了匹配。如果成功，这将使原子结构、力学和接触动力学与相应的摩擦力和能量耗散直接联系起来。具体而言，通过缩小模拟和实验之间的差距，只能在原子模拟中解决的详细结果和机制可以通过实验得到验证，实验观察到的现象可以通过模拟来解释，两者都可以形成可靠的预测模型描述纳米尺度摩擦滑动的基础。这将提供对单个磨粒摩擦的深入而可靠的理解，这是全面理解在更大规模接触中遇到的磨粒集合行为的重要一步。纳米摩擦学研究的长期目标。从技术的角度来看，这项工作可以为涉及接触、滑动表面的纳米机械器件的合理设计提供所需的知识基础。从教育的角度来看，两位主要研究人员的合作将产生重大影响。这包括基于AFM的多用途演示模块的开发，本科生和高中生的参与，为研究生组织和提供纳米摩擦学短期课程，以及积极参与国际协作网络社区。

The objective of this collaborative research project is to understand the atomic-level mechanisms of friction that control the sliding of nanoscale contacts, with a particular focus on the prevalent phenomenon known as "atomic stick-slip friction". Molecular dynamics (MD) simulations and atomic force microscopy (AFM) experiments that match tribological conditions as closely as possible will allow results from both to be directly compared. The ability to compare AFM experiments with dynamic atomistic simulations, including molecular dynamics (MD), is limited at present. One is unable see the positions and velocities of the atoms in AFM experiments, which motivates MD studies. However, MD results cannot be directly compared with those from AFM because conventional simulations must be run at speeds several orders of magnitude faster rates than AFM experiments. As well, many MD and AFM studies differ in other important conditions such as materials, load, and tip size. In this work, speeds will be matched through the concurrent use of new methods being developed collaboratively by the principal investigators specifically for atomic-scale friction studies. The use of noble metals will ensure that the interface is well-defined and reliably modeled. Other critical parameters, including stiffness, tip size and shape, environment, and crystal surface and sliding direction orientation, are also matched. If successful, this will enable the atomic structure, mechanics, and dynamics of the contact to be directly linked with the corresponding friction forces and energy dissipation. Specifically, by closing the gaps between simulation and experiment, the detailed results and mechanisms resolvable only in atomistic simulations can be validated by experiments, phenomena observed experimentally can be explained by reference to the simulations, and both can form the basis for reliable predictive models describing nanoscale frictional sliding. This will provide a deep and reliable understanding of single asperity friction, which is an important step toward fully understanding the behavior of collections of asperities that one encounters in larger-scale contacts ? a longstanding goal for nanotribology research. From the technological perspective, the work can contribute to the knowledge base needed for the rational design of nanomechanical devices that involve contacting, sliding surfaces. From an educational perspective, there will be significant impact through collaborative efforts between the two principal investigators. This includes development of a multi-purpose demonstration module based on AFM, involvement of undergraduates and high school students, organizing and delivering a short course on nanotribology to graduate students, and active participation in international collaborative cyber-network communities.