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

摩擦和粘附几乎影响到技术和日常生活的各个方面。尽管它们很重要，但我们对它们起源的基本理解以及在实际应用中预测它们的能力仍然有限。在很大程度上，这是因为长度尺度的范围很广，这很重要。表面之间的粘附力和摩擦力是由原子间距小于一纳米的区域的相互作用产生的，但是这些接触区域的几何形状和它们内部的力是由微米或更长的尺度上的地下变形决定的。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获所有这些长度尺度。结果揭示了传统工程模型在定性上的缺陷，并导致预测粗糙表面之间接触面积和刚度的简单方程。提出的研究将这些研究扩展到计算摩擦力和附着力。目标是提供对基本过程的新理解，这将允许开发表面粗糙度与宏观摩擦和粘附之间关系的预测模型。新的计算工具将被开发并作为开源软件共享。接触、粘附和摩擦是涉及广泛长度尺度的复杂问题。原子尺度的界面相互作用决定了粘附力和摩擦力，但这些力作用的区域是由纳米到微米或更长的尺度上的表面粗糙度和衬底变形决定的。传统的接触、粘附和摩擦模型是基于宏观连续介质方程，具有表征界面相互作用和宏观变形的现象学本构关系。就其本质而言，连续统方程不包括与离散原子相关的效应，但大多数粘附能来自于几埃的分离变化，晶体上的侧向摩擦力是周期性的，其位移是晶格常数。提出的研究将解决与原子和连续体描述之间的差距有关的基本问题。这些包括原子结构和分离力之间的关系，摩擦的起源及其与粗糙度，可塑性和无序性的联系，以及界面动力学中的相关程度。目标是提供对不同尺度的物理过程的新理解，并开发包含原子水平过程的宏观模型。有效的多尺度方法保留了界面上的原子细节，并捕获了基板的远程响应，这些方法将被扩展，移植到开源仿真包中，并用于模拟接触和摩擦。模拟将检查具有自仿射或实验测量的表面粗糙度的平面和球体的接触。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接相互作用的弹性和塑性极限下进行研究。接触和粘附的研究将决定接触面积和剥离力如何与界面相互作用、材料性质和表面粗糙度相关。对摩擦的研究将解决这些接触中的相互作用如何导致静摩擦和动摩擦，粗糙度和塑性如何促进摩擦，并检查只有部分表面向前滑动的稳定滑动的前兆。

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.