A molecular simulation study on wetting of octane on nanorough reentrant surfaces
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Superoleophobic surfaces have generated a lot of interest due to their tendency to repel low surface tension fluids such as alkanes. When a fluid droplet is placed on a geometrically structured surface, different wetting states (Cassie or Wenzel) may be observed depending on the relative free energies of these states. It is desired to achieve a Cassie like state (fluid remains suspended on top of the geometrical structures) to observe oleophobic behavior. Using molecular simulation, we try to understand the role of geometry of these structural features in promoting the formation of cassie like states, by studying the evolution of free energies with geometry of the structures. We use Grand Canonical Transition Matrix Monte Carlo (GCTMMC) simulation to learn about the wetting characteristics of n-octane. The United atom description of n-alkanes is used to model the fluid. We study two types of surfaces: (1) surface with rectangular stripes and (2) surface with T-shaped re-entrant structures. We investigate the effect of the size of these structural features and fractional coverage. By using various simulation techniques, we capture the different wetting states and calculate the free energy barrier between them. We observe a greater free energy barrier for the Cassie-Wenzel transition for the re-entrant T-shaped structures over rectangular stripes of the same size. This confirms that the T-shaped re-entrant structures facilitate in inducing fluid repellent behavior over rectangular stripes.