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

这一合作研究项目的目标是了解控制纳米级接触滑动的原子水平的摩擦机制，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)实验尽可能匹配摩擦学条件，将允许直接比较两者的结果。目前，将原子力显微镜实验与包括分子动力学(MD)在内的动态原子模拟进行比较的能力有限。人们无法在原子力显微镜实验中看到原子的位置和速度，这激发了MD研究。然而，MD的结果不能与AFM的结果直接比较，因为传统的模拟必须以比AFM实验快几个数量级的速度运行。此外，许多MD和AFM研究在其他重要条件上也不同，例如材料、负载和尖端大小。在这项工作中，速度将通过同时使用由主要研究人员合作开发的新方法来匹配，这些方法专门用于原子规模的摩擦研究。贵金属的使用将确保界面定义良好，建模可靠。其他关键参数，包括刚度、针尖大小和形状、环境、晶体表面和滑动方向，也是匹配的。如果成功，这将使接触的原子结构、力学和动力学直接与相应的摩擦力和能量耗散联系在一起。具体地说，通过缩小模拟和实验之间的差距，只有原子模拟才能解决的详细结果和机理可以通过实验来验证，实验观察到的现象可以参考模拟来解释，这两者都可以形成描述纳米级摩擦滑动的可靠预测模型。这将提供对单个粗糙摩擦的深入和可靠的理解，这是朝着完全理解在更大范围接触中遇到的粗糙集合的行为迈出的重要一步？纳米摩擦学研究的长期目标。从技术角度来看，这项工作可以为合理设计涉及接触、滑动表面的纳米机械设备提供所需的知识库。从教育的角度来看，通过两位首席调查员之间的合作努力，将产生重大影响。这包括开发基于原子力显微镜的多功能演示模块，让本科生和高中生参与，组织和向研究生提供纳米摩擦学的短期课程，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.