Non-Equilibrium Systems: Spreading, Deformation, Adhesion and Friction

技术说明这项研究解决了影响许多技术的基本非平衡过程：铺展、塑性变形、粘合失效和摩擦。研究这些现象的传统工程方法是从宏观连续介质方程出发，这些方程具有唯象本构关系和边界条件，其有效性是值得怀疑的，特别是当维度接近分子尺度时。分子模拟将被用来测试现有的连续统描述，提供分子相互作用和宏观行为之间的联系，开发新的中尺度模型，并为无法从实验中获得的宏观模型提供输入。扩展的模拟将研究在流体界面与固体相交的接触线附近预测存在的奇异应力。已经提出了几种相互竞争的机制来消除奇异性，但很难通过实验来区分它们。分子动力学和原子/连续混合方法将被用来检验基于扩散和界面张力变化的新理论。结果将使建立更大长度和时间尺度的中尺度模式成为可能。这些将应用于更复杂几何形状的动力学，以研究接触角滞后和多孔介质中的多相流动。变形、粘合和摩擦的研究将集中在非晶态聚合物玻璃上，这是许多粘合剂和结构材料的主要成分。分子的连接性导致缠结，从而极大地改变了机械性能，特别是在大应变下。一种新的方法将被用来可视化纠缠，并将其演变与银纹形成和剪切变形过程中的应力/应变曲线相关联。原子化和原子化/连续混合模拟将被用来研究块体和薄膜中的断裂能和断裂机制。将对不同的聚合物进行研究，以确定分子相互作用的哪些方面选择了失效模式。摩擦方面的工作将探索摩擦本构关系和整体变形之间的关系，一些聚合物的纳米级和宏观摩擦系数之间的对比，以及分子在表面附近的排列对摩擦的影响。智力优势：拟议的研究解决了基本问题，即分子水平的过程如何控制宏观性质，以及行为如何随着维度接近原子尺度而变化。这些问题与许多工艺和材料系统有关。这项研究还将作为新的杂交技术的试验场，这些技术结合了非常不同的物质描述。由此产生的原子/连续体算法有望广泛适用于许多其他问题和具有广泛长度和时间范围的系统。从分子模拟发展中尺度和连续介质模型的进展也可能使其他领域的相关工作成为可能。更广泛的影响：更好地理解扩散、变形、粘合和摩擦将对材料涂层、聚合物加工以及改进的粘合剂和结构材料的设计和配方产生广泛的影响。该项目将通过在广泛的跨学科建模方法方面培训学生和博士后，为劳动力做出贡献。将开发一门关于多尺度建模的新课程，让工程系和科学系的学生接触这些技术和相关技术。最后，推广工作将通过约翰·霍普金斯大学一年一度的物理博览会以及与当地高中教师和马里兰科学中心的合作，将研究成果带给更广泛的受众。

TECHNICAL EXPLANATION This research addresses fundamental non-equilibrium processes that impact many technologies: spreading, plastic deformation, adhesive failure and friction. Traditional engineering approaches to these phenomena start from macroscopic continuum equations with phenomenological constitutive relations and boundary conditions whose validity is questionable, particularly as dimensions approach the molecular scale. Molecular simulations will be used to test existing continuum descriptions, provide connections between molecular interactions and macroscopic behavior, develop new mesoscale models, and provide input for macroscopic models that cannot be obtained from experiment. Simulations of spreading will study the singular stresses that are predicted to exist near the contact line where a fluid interface intersects a solid. Several competing mechanisms have been proposed for removing the singularity and it is difficult to distinguish between them using experiments. Molecular dynamics and hybrid atomistic/continuum methods will be used to test new theories based on diffusion and variations in interfacial tension. The results will enable construction of mesoscale models for larger length and time scales. These will be applied to dynamics in more complex geometries to study contact angle hysteresis and multi-phase flow in porous media.NON-TECHICAL EXPLANATION Studies of deformation, adhesion and friction will focus on amorphous polymer glasses, which are a major component of many adhesives and structural materials. The connectivity of the molecules leads to entanglements that dramatically alter mechanical properties, particularly at large strains. A new method will be used to visualize entanglements and correlate their evolution to stress/strain curves during craze formation and shear deformation. Atomistic and hybrid atomistic/continuum simulations will be used to study the fracture energy and mechanisms in bulk and thin films. Different polymers will be studied to determine what aspects of molecular interactions select the failure mode. Work on friction will explore the relation between constitutive relations for friction and bulk deformation, the contrast between nanoscale and macroscopic friction coefficients for some polymers, and the effect of molecular alignment near the surface on friction.Intellectual Merit: The proposed research addresses fundamental issues about how molecular level processes control macroscopic properties and how behavior changes as dimensions approach the atomic scale. These issues are relevant to many processes and materials systems. The research will also serve as a testing ground for new hybrid techniques that couple very different descriptions of matter. The resulting atomistic/continuum algorithms are expected to be widely applicable to many other problems and systems with a wide range of length and time scales. Progress in developing mesoscale and continuum models from molecular simulations is also likely to enable related work in other fields.Broader Impact: An improved understanding of spreading, deformation, adhesion and friction would have broad impact in coating of materials, processing of polymers, and the design and formulation of improved adhesives and structural materials. The project will contribute to the workforce by training students and a postdoc in a wide range of interdisciplinary modeling methods. A new course on multiscale modeling will be developed to expose students from engineering andscience departments to these and related techniques. Finally, outreach efforts will bring research results to a wider audience through the annual Physics Fair at Johns Hopkins and collaborations with local high school teachers and the Maryland Science Center.