Development of free energy methods to study wetting behavior with molecular dynamics
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This dissertation focuses on developing molecular simulation methods to understand the wetting behavior of fluids. Within our group, we have developed an interface potential based approach for determining the interfacial properties. This approach has been implemented within the grand canonical (GC) ensemble. However, there are limitations to working within this ensemble, particularly when applying the method to complex molecules and while working at relatively low temperatures. In this dissertation, we introduce a means to overcome these limitations. We first discuss the development of a free energy based approach using isothermal-isobaric (NPT) Monte Carlo (MC) method to obtain interfacial potential for growing a thin fluid film on the solid substrate. This approach is employed within a “spreading” and “drying” framework to calculate wetting properties such as contact angle and interfacial tension. Next, we discuss how to implement the interface potential approach within a molecular dynamics (MD) framework. In this method, umbrella sampling is used to sample states along the order parameter path (defined by volume). We show ways of processing the acquired data using pymbar and force integration technique to construct interface potential. For both the aforementioned methods, we present results for the Lennard-Jones system. Expanded ensemble (EE) technique are also used to evaluate the interfacial properties over a range of conditions. We then apply the interface potential approach using a canonical ensemble to determine the wetting properties. This method uses various simulations at constant volume along the order parameter path to construct an average force profile. We also explain a way of obtaining interface potential from the force profile. This method is applied for the Lennard-Jones system for spreading and drying method. Later, we apply this method to realistic models such as water in contact with the silica surface and room temperature ionic liquid in contact with the graphite surface. The results are shown for the drying method. We calculate drying coefficient as a function of temperature using the expanded ensemble techniques. Also, wherever possible the results obtained are compared with those obtained from different methods.