Molecular Simulation Study of Wetting at Rough Surfaces

在广泛的自然现象和工业过程中，流体与固体基材相互作用。这种流体表现出丰富多样的行为，不同于在散装流体中观察到的行为。流体在表面上的响应在质量上取决于流体-流体和流体-基材相互作用的相对强度和范围以及基材的结构特征。我们在这里关注的是后一种影响。具体来说，该项目将研究纳米级衬底粗糙度和曲率对润湿行为的影响。我们提出的研究的动机是最近的努力，采用可调的纳米结构衬底特征来修改和控制系统的润湿行为。例子包括使用氧等离子体处理产生在50-300纳米范围内具有周期性的聚合物表面形貌，使用排列的碳纳米管阵列构建表面，以及使用直径为20-50纳米的硅纳米线产生分形粗糙表面。该项目提出了两项分子模拟研究，这将补充最近的实验工作，并推进有关流体在非均质和/或曲面上润湿行为的基础知识。在这项工作中将使用三种模型流体：原子伦纳德-琼斯流体、水和正构烷烃链。在我们的第一个一般性研究中，我们将研究以周期和振幅在1-10纳米范围内的地形为特征的基材上流体的界面特性。这一努力将使人们能够在难以实验探测的长度尺度上描述规则粗糙度的影响。为了完成这项任务，研究人员将采用一种模型，使我们能够系统地改变暴露在流体中的基底特征的相对振幅、长度尺度和形状。最近开发的模拟方法将用于定量探索这些纳米尺度特征如何影响重要的界面特性，包括接触角、固体-蒸汽和固体-液体界面张力、线张力以及润湿转变的顺序。在第二个一般研究中，他们将探讨基材曲率对润湿行为的作用。最近一些控制液滴接触角的策略包括将流体暴露在直径为30-100个流体分子直径的圆柱形物镜（例如碳纳米管、硅纳米线）的多孔组件中。在分子水平上，流体粒子与高度弯曲的底物相互作用。简单系统的密度泛函计算表明，这种曲率程度对润湿性能有重大影响。考虑到这些圆柱形物体的直径通常可以控制，曲率影响润湿程度的定量知识将在纳米结构基板的设计中证明是有用的。在这项提议中，研究人员描述了一种定量探测直径从几到数千个流体分子直径的圆柱形物体上流体界面特性的方法。更广泛的影响对存在表面的流体的行为有更深入的基本理解，将对科学家和工程师设计新技术和利用自然系统独特特征的能力产生深远的影响。这项研究的结果将对许多科学学科和工业应用产生广泛的影响。说明需要了解流体如何与表面相互作用的例子很多，包括防水和耐污织物的设计，减少摩擦的表面，传感器，医疗植入物和纳米级光流体器件。在教育方面，该项目将影响小学、本科和研究生阶段的学生。在大学阶段，学生将接受表面热力学、统计力学和计算机模拟等方面的培训。PI将积极招收本科生参与这里提出的项目。此外，PI将继续开展一项外展计划，每年访问大树小学五年级的教室。

CBET-0828979ErringtonIntellectual MeritIn a wide range of both naturally occurring phenomena and industrial processes fluids interact with solid substrates. Such fluids exhibit a rich variety of behavior that is distinct from that observed for bulk fluids. The response of a fluid at a surface depends qualitatively upon the relative strengths and ranges of the fluid-fluid and fluid-substrate interactions and the structural characteristics of the substrate. It is the latter of these influences that we focus on here. Specifically, this project will examine the effect of nanoscale substrate roughness and curvature on wetting behavior. Our proposed studies are motivated by recent efforts that employ tunable nanostructured substrate features to modify and control the wetting behavior of a system. Examples include the use of oxygen-plasma treatment to produce polymer surface topographies with periodicity in the 50-300 nm range, the construction of surfaces with aligned carbon nanotube arrays, and the use of silicon nanowires with 20-50 nm diameters to produce fractally rough surfaces.The project proposes two molecular simulation studies that will both complement recent experimental work and advance fundamental knowledge with respect to the wetting behavior of fluids on heterogeneous and/or curved surfaces. Three model fluids will be utilized in this work: an atomistic Lennard-Jones fluid, water, and n-alkane chains. In our first general study we will examine the interfacial properties of fluids at substrates characterized by topographies with a period and amplitude in the 1-10 nm range. This effort will enable one to describe the influence of regular roughness at a length scale that is difficult to probe experimentally. To accomplish this task, the investigators will employ a model that enables us to systematically vary the relative amplitude, length scale, and shape of substrate features exposed to a fluid. Recently-developed simulation methodologies will be used to quantitatively probe how these nanoscale features influence important interfacial properties, including the contact angle, solid-vapor and solid-liquid interfacial tensions, and the line tension, as well as the order of the wetting transition. In the second general stud, they will probe the role of substrate curvature on wetting behavior. A number of recent strategies for controlling the contact angle of a droplet involve the exposure of a fluid to a porous assembly of cylindrical objectives (e.g. carbon nanotubes, silicon nanowires) with diameters on the order of 30-100 fluid molecular diameters. At the molecular level fluid particles interact with highly curved substrates. Density functional calculations with simple systems suggest that this degree of curvature has a significant impact on wetting properties. Given that the diameters of these cylindrical objects can often be controlled, quantitative knowledge of the extent to which curvature influences wetting will prove useful in the design of nanostructured substrates. Within this proposal, the investigators describe a means to quantitatively probe the interfacial properties of fluids at cylindrical objects with diameters spanning from a few to thousands of fluid molecular diameters.Broader ImpactDevelopment of a deeper fundamental understanding of the behavior of fluids in the presence of a surface would have a profound influence on the ability of scientists and engineers to design novel technologies and harness the unique features of natural systems. The results that emerge from this study will have broad impact on numerous scientific disciplines and industrial applications. Examples that illustrate the need for knowledge of how a fluid interacts with a surface are numerous, and include the design of water-repellent and stain-resistant fabrics, friction-reducing surfaces, sensors, medical implants, and nanoscale optofluidic devices. On the education front, this project will impact students at the elementary, undergraduate, and graduate levels. At the university level, students will be trained in the areas of surface thermodynamics, statistical mechanics, and computer simulation. The PI will actively recruit undergraduate students to participate in the project proposed here. Also, the PI will continue an outreach program in which he annually visits 5th grade classrooms at Big Tree Elementary School.