Contact, Adhesion and Friction: From Atomic Interactions to Macroscopic Behavior

非技术概述摩擦和粘合几乎影响到技术和日常生活的方方面面。尽管它们很重要，但我们对它们的起源和在实际应用中预测它们的能力的基本了解仍然有限。在很大程度上，这是因为长度刻度的范围很大，这很重要。表面之间的粘附力和摩擦力是原子间距小于一纳米的区域相互作用的结果，但这些接触区的几何形状和其中的力由微米和更长尺度的地下变形决定。在过去的十年里，计算算法和硬件的进步已经开始允许我们第一次捕捉到所有这些长度尺度。结果揭示了传统工程模型的定性缺陷，并得到了预测粗糙表面之间接触面积和刚度的简单公式。拟议的研究将扩展这些研究，以计算摩擦力和粘附力。其目的是提供对基本过程的新理解，从而能够开发出表面粗糙度与宏观摩擦和附着力之间关系的预测模型。将开发新的计算工具，并将其作为开源软件共享。技术概述接触、粘合和摩擦是复杂的问题，涉及范围广泛的长度刻度。原子尺度的界面相互作用决定了粘附力和摩擦力，但这些作用力的作用区域由纳米到微米或更长尺度上的表面粗糙度和衬底变形决定。传统的接触、粘着和摩擦模型是建立在宏观连续介质方程基础上的，界面相互作用和宏观变形具有唯象本构关系。就其本质而言，连续介质方程不包括与离散原子相关的影响，但大部分粘合能来自分离几埃的变化，晶体上的横向摩擦力是周期性的，位移为晶格常数。拟议的研究将解决与原子描述和连续描述之间的差距相关的基本问题。其中包括原子结构和剥离力之间的关系，摩擦的起源及其与粗糙度、塑性和无序性的联系，以及界面动力学中的关联程度。目标是为不同尺度的物理过程提供新的理解，并开发包含原子水平过程的宏观模型。有效的多尺度方法在界面保留原子细节并捕捉衬底的远程响应将被扩展，移植到开源模拟包，并用于模拟接触和摩擦。这些模拟将检查平面和球体与自仿射表面粗糙度或实验测量表面粗糙度的接触。晶体、聚合物玻璃和弹性体的响应将在不同原子尺度粗糙度和粘结剂相互作用的弹性和塑性极限下进行研究。对接触和附着力的研究将确定接触面积和分离力与界面相互作用、材料特性和表面粗糙度之间的关系。对摩擦的研究将解决这些接触中的相互作用如何导致静态和动态摩擦，粗糙度和塑性如何导致摩擦，并研究只有一部分表面向前滑动的稳定滑动的先兆。

Nontechnical SummaryFriction and adhesion affect almost all aspects of technology and everyday life. Despite their importance, our fundamental understanding of their origins and our ability to predict them in practical applications has remained limited. In large part, this is because of the wide range of length scales that are important. Adhesion and friction forces between surfaces result from interactions in regions where atoms are separated by less than a nanometer, but the geometry of these contact regions and the forces within them are determined by subsurface deformations on micrometer and longer scales. Over the last decade, advances in computational algorithms and hardware have begun to allow us to capture all of these length scales for the first time. The results have revealed qualitative shortcomings of traditional engineering models and resulted in simple equations that predict the area and stiffness of the contacts between rough surfaces. The proposed research will extend these studies to calculate friction and adhesive forces. The goal is to provide new understanding of fundamental processes that will allow predictive models for the relation between surface roughness and macroscopic friction and adhesion to be developed. New computational tools will be developed and shared as open source software. Technical SummaryContact, adhesion and friction are complex problems that involve a wide range of length scales. Atomic-scale interfacial interactions determine adhesive and frictional forces, but the area where these forces operate is determined by surface roughness and substrate deformation on nanometer to micrometer or longer scales. Traditional models of contact, adhesion and friction are based on macroscopic continuum equations with phenomenological constitutive relations for the interfacial interactions and macroscopic deformations. By their very nature, continuum equations do not include effects associated with discrete atoms, but most of the adhesive energy comes from changes in separation by a few angstroms and lateral frictional forces on a crystal are periodic with displacements by a lattice constant. The proposed research will address fundamental questions associated with the gaps between atomistic and continuum descriptions. These include the relation between atomic structure and detachment forces, the origins of friction and its connection to roughness, plasticity and disorder, and the extent of correlations in the dynamics at the interface. The goal is to provide new understanding of the physical processes at different scales and develop macroscopic models that incorporate atomic level processes.Efficient multiscale methods that retain atomistic detail at the interface and capture the long-range response of the substrate will be extended, ported to open source simulation packages, and used to simulate contact and friction. The simulations will examine contact of planes and spheres with self-affine or experimentally measured surface roughness. The response of crystals, polymer glasses and elastomers will be studied in elastic and plastic limits with different atomic-scale roughness and adhesive interactions. Studies of contact and adhesion will determine how the contact area and detachment force are related to interfacial interactions, material properties and surface roughness. Studies of friction will address how interactions within these contacts lead to static and kinetic friction, how roughness and plasticity contribute to friction, and examine precursors to steady sliding where only part of the surface slips forward.