The effect of nanoscale chemical heterogeneities on wetting behavior
Kamakshi Nadha, Aravindh
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The effect of Nanoscale heterogeneities on wetting has gained a lot of importance with advancement in colloidal materials and micro contact printing. Especially, chemically heterogeneous surfaces promote several interesting wetting phenomena. A variety of morphologies and transitions between them have been observed. Natural systems, such as proteins and amphiphilic molecules constitute surfaces that are chemically heterogeneous. We aim to investigate the wetting characteristics of a surface with chemical heterogeneities at the nanometer scale. We study fluid over an atomistically detailed Lennard-Jones model system with alternating rectangular striped surfaces which differ in the substrate-fluid interaction strength. We employ an interface potential approach to deduce interfacial properties of the model system. Grand canonical transition matrix Monte Carlo simulation is used to deduce the surface density dependence of the surface energy, from which the spreading coefficient is obtained. Expanded ensemble simulations are used to determine how interfacial properties vary with both overall substrate strength and contrast between the substrate stripes. The periodicity of the heterogeneities seems to have a profound effect on the wetting behavior of the fluid. We observe a transition between the homogeneous vapor and "fluid stripe" morphologies, preceding complete wetting. An unbending transition is also observed when this fluid stripe morphology gives way to a uniformly thick wetting layer. Previous studies, both theoretical and computational have been carried out on surfaces with micro and nanoscale chemical heterogeneities. Our investigations are performed on surfaces where the length scales of the chemical heterogeneities span up to 20 fluid diameters. The Cassie equation provides a means to estimate the interfacial properties of a composite surface based on that of the constituent homogeneous surfaces. We seek to analyze the validity of the Cassie equation and the evolution of contact angles with change in surface parameters. Also, we provide a density profile analysis into the contour of the "fluid stripes" that form in certain cases. The shapes of these contours are dominated by the underlying stripe pattern. We visually analyze the structural aspects of these stripes. The next step in the future is to extend the same analyses to substrates with two dimensional heterogeneities. In addition, this study also provides future scope to analyze the nature of the stripes in terms of the layering phenomena that occur.