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43452-AC5
Investigation of the Effect of Substrate Molecular Roughness on Surface Phase Behavior
The goal of this project is to enhance our understanding of the effect of molecular disorder (roughness) within a substrate's surface on the behavior of a fluid in contact with it. Over the past year we have completed a project focused on the development of an efficient computational technique for calculating the interfacial properties of a fluid-substrate combination. We have also significantly advanced a project aimed at understanding how a fluid's wetting behavior depends upon how atoms are organized within a substrate (e.g. fluid adsorbed to a 100 bcc face versus a 111 fcc face).
At the outset of the project, we realized that better methods were needed to characterize the surface phase behavior of a system than were currently available in order to probe the subtle effects of substrate molecular roughness. As a result, we developed a general free-energy-based method for determining contact angles and interfacial tensions of a fluid in contact with any substrate through Monte Carlo simulation. The approach was first tested with Lennard-Jones systems examined previously by Tang and Harris [1] and Nijmeijer et al. [2]. We have recently submitted an article to The Journal of Chemical Physics to describe our work. Eric Grzelak, a Ph.D. student, led this project. To probe the efficacy of the method with more complex molecular interactions, a M.S. student, Wai Keong Choong, performed a series of calculations with a water-graphite model previously investigated by Werder et al. [3]. Overall, the method proved robust, enabling us to calculate contact angles, interfacial tensions, and prewetting coexistence points over a range of temperatures and wall strengths.
Our most recent efforts have focused on understanding the role of substrate molecular roughness on wetting behavior. The key aspect of this work is to probe how the organization of molecules within a substrate influences surface properties, such as the wetting temperature and contact angle. As an illustrative example, consider the three common faces of an fcc lattice: 100, 110, and 111. The “roughness” of the surface topology presented to a fluid is slightly different in each case. To what extent does this variation influence the wetting properties? To probe this question we are studying the wetting properties of a Lennard-Jones fluid in contact with various faces of perfect fcc, sc, and bcc lattices and several amorphous substrates. Two approaches are used to estimate the substrate roughness. The first of these focuses on the geometry of the substrate-fluid interface. Voronoi tessellations are performed on configurations with a liquid layered on a substrate, and an estimate of the total substrate-fluid interfacial area is obtained from a summation of the areas of all Voronoi polyhedra that provide a dividing surface between substrate and fluid atoms. The second approach for characterizing roughness is based on the substrate-fluid interaction energy. We construct a fine grid of points in the plane parallel to the substrate, and determine for each point the elevation at which the substrate-fluid interaction energy reaches a minimum value. We then evaluate the area of the surface defined by this set of minimum-energy points. An energy roughness factor is taken to be the area of the minimum-energy surface normalized by the projected planar substrate area. We find that the geometry- and energy-based roughness factors provide a consistent mapping between substrate identity and roughness. We are currently collecting data that will enable us to analyze the relationship between wetting behavior and atomistic substrate roughness for the surfaces specified above. Early results suggest a weak correlation between wetting properties and substrate roughness.
Over the past year we have continued our collaboration with Prof. Thomas Truskett's group at the University of Texas at Austin. One of the aims of this project is to develop a quantitative understanding of the relationship between kinetic and thermodynamic properties of confined systems. The long-term goal of the project is to develop a means for scientist and engineers to predict difficult-to-measure kinetic properties (e.g. diffusivity, viscosity) from knowledge of more accessible thermodynamic properties (e.g. density, entropy). Our approach involves using theory and computer simulation to determine the relationship between kinetic and thermodynamic properties for well-defined molecular models exposed to various degrees of confinement. Such information will enable us to establish trends, which can subsequently be used to predict the properties of real systems.
1. J. Z. Tang and J. G. Harris, J. Chem. Phys. 103, 8201 (1995).
2. M. J. P. Nijmeijer, C. Bruin, A. F. Bakker, , and J. M. J. van Leeuwen, J. Phys. Condens. Matter 4, 15 (1992).
3. T. Werder, J. H. Walther, R. L. Jaffe, T. Halicioglu, and P. Koumoutsakos, J. Phys. Chem. B 107, 1345 (2003).
