Molecular Simulation Study of Wetting at Rough Surfaces
Jeffrey Errington Principal Investigator
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CBET-0828979<br/>Errington<br/><br/>Intellectual Merit<br/><br/>In 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.<br/><br/>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.<br/><br/>Broader Impact<br/>Development 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.