Tribology: From Atomic Interactions to Macroscopic Response

该奖项支持理论和计算研究，以及摩擦学微观起源的教育，旨在设计具有理想摩擦学性能的材料。摩擦学是对滑动表面的研究，包括附着、摩擦、润滑和磨损。自达·芬奇以来，摩擦的起源一直是重大的智力挑战之一，摩擦学过程对日常生活和技术产生了巨大的影响。事实上，据估计，摩擦消耗的能源占全球能源消耗的20%以上。传统的宏观摩擦学模型必须适合于模拟应用的实验，而最近的研究揭示了分子尺度接触中大量有趣的行为，这两者之间存在很大的差距。这项研究的目标是将这些不同尺度的行为与实际相关的系统联系起来：像有机玻璃这样的聚合物，与固体表面结合的短分子层（表面活性剂），以及在极端压力和剪切下发现的汽车轴承中的小润滑剂分子。之所以选择这些系统，是因为不同尺度的实验和模型中有不同寻常的数据量，而且它们的行为应该受到比金属或其他晶体系统更少的过程的影响。将要解决的具体问题是：i)粘附力和外力如何决定表面相互作用产生摩擦和粘附的接触面积？Ii)表面的永久塑性变形如何影响接触和摩擦？为什么许多系统中的摩擦既取决于当前速度，也取决于过去的历史？润滑油粘度随压力、温度和速率的变化与分子构型和/或热活化的变化有何关系？这些变化对玻璃化转变有何启示？该项目开发的软件将通过公共存储库和一个在线工具与其他研究人员共享，该工具为用户提供计算出的接触特性，并为未来的研究建立一个粗糙表面的数据库。研究将涉及从高中到研究生阶段的学生，并通过外展努力与公众分享摩擦学行为的演示。技术该奖项支持理论和计算研究，以及摩擦学微观起源的教育，旨在设计具有理想摩擦学性能的材料。在过去的几十年里，我们在纳米尺度上探测摩擦学过程的能力得到了迅速发展，揭示了广泛的新物理和现象。然而，将这种丰富的行为与广泛用于描述宏观实验的现象学模型联系起来被证明是极具挑战性的。提出的研究目标是使用模拟来建立特定系统的这种联系：聚合物，自组装单层和小分子润滑剂，其中新的理论方法，计算算法和实验技术能够弥合纳米和微米之间的差距。弹性表面的多尺度模拟将用于研究van der Waals相互作用和粗糙度在几十微米尺度上如何决定表面之间的接触几何形状和宏观粘附力。然后评估基材塑性发生的条件及其对接触、粘附和摩擦的影响。摩擦的历史依赖性通常由速率-状态模型来描述，该模型用一个现象学的“状态”变量来描述滑动界面的演变。将进行聚合物和自组装单层的模拟，以寻找状态变量变化和分子过程之间的定量联系。在纳米到微米尺度上的粗糙度表面的模拟将根据越来越多的实验数据进行测试，包括聚合物和自组装单层的接触面积、应力和宏观摩擦系数。弹性流体动力润滑为远离平衡状态的流变性提供了一个窗口，在极端压力下，流体可能会通过玻璃化转变，并且速率足够高，可以与分子弛豫时间和模拟重叠。模拟将用于定量测试流变模型的假设，流变模型假设粘度是由复杂能量景观中分子顺序的变化或应力偏跳变决定的。他们还将测试关于玻璃化转变的本质和存在的想法。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICALThis award supports theoretical and computational research, and education on the microscopic origins of tribology with an aim toward enabling the design of materials with desired tribological properties. Tribology is the study of sliding surfaces and encompasses adhesion, friction, lubrication and wear. The origins of friction have remained one of the great intellectual challenges since da Vinci, and tribological processes have a tremendous impact on daily life and technology. Indeed, friction is estimated to account for more than 20% of global energy consumption. There is a large gap between traditional macroscopic models of tribology, which must be fit to experiments that closely mimic the application, and recent studies that reveal a wealth of intriguing behavior in molecular scale contacts. The goal of the proposed research is to link behavior at these different scales for systems of practical relevance: polymers like plexiglass, layers of short molecules (surfactants) bound to solid surfaces, and small lubricant molecules under the extremes of pressure and shear found in automobile bearings. These systems have been chosen because there is an unusual amount of data from experiments and models at different scales, and because their behavior should be affected by fewer processes than metals or other crystalline systems. Specific questions that will be addressed are: i) How do adhesive and external forces determine the contact area where surfaces interact to produce friction and adhesion? Ii) How does permanent plastic deformation of the surfaces influence contact and friction? Iii) Why does friction in many systems depend on past history as well as the current velocity? Iv) How are changes in the viscosity of lubricants with pressure, temperature and rate related to changes in molecular configurations and/or thermal activation and what may they teach us about the glass transition? The software developed in the project will be shared with other researchers through public repositories and an online tool that provides users with calculated contact properties and builds a database of rough surfaces for future research. Research will involve students from high school through graduate school levels and demonstrations of tribological behavior will be shared with the public through outreach effortsTECHNICAL This award supports theoretical and computational research, and education on the microscopic origins of tribology with an aim toward enabling the design of materials with desired tribological properties. Our ability to probe tribological processes at the nanometer scale has developed rapidly in the last decades, revealing a wide range of new physics and phenomena. However, connecting this rich behavior to the phenomenological models widely used to describe macroscopic experiments has proved extremely challenging. The goal of the proposed research is to use simulations to make this link for specific systems: polymers, self-assembled monolayers and small molecule lubricants, where new theoretical methods, computational algorithms, and experimental techniques are able to bridge the gap between nanometers and micrometers. Multiscale simulations of elastic surfaces will be used to study how van der Waals interactions and roughness on scales up to tens of micrometers determine the contact geometry and macroscopic adhesive force between surfaces. Then the conditions for substrate plasticity to occur and its effects on contact, adhesion and friction will be evaluated. The history dependence of friction is often described by rate-state models that describe the evolution of the sliding interface with a phenomenological “state” variable. Simulations of polymers and self-assembled monolayers will be performed to search for a quantitative link between the changes in state variable and molecular processes. Simulations of surfaces with roughness on nanometer to micrometer scales will be tested against the growing body of experimental data for contact areas, stresses and macroscopic friction coefficients of polymers and self-assembled monolayers. Elastohydrodynamic lubrication offers a window on rheology extremely far from equilibrium, with extreme pressures that may drive fluids through the glass transition and rates high enough to overlap with molecular relaxation times and simulations. Simulations will be used for quantitative tests of assumptions underlying rheological models that assume viscosity is determined by changes in molecular order or stress-biased hopping in a complex energy landscape. They will also test ideas about the nature and existence of the glass transition.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.