Vaibhaw Kumar

Monte Carlo simulation strategies for computing the wetting properties of fluids at geometrically rough surfaces

We introduce Monte Carlo simulation methods for determining the wetting properties of model systems at geometrically rough interfaces. The techniques described here enable one to calculate the macroscopic contact angle of a droplet that organizes in one of the three wetting states commonly observed for fluids at geometrically rough surfaces: the Cassie, Wenzel, and impregnation states. We adopt an interface potential approach in which the wetting properties of a system are related to the surface density dependence of the surface excess free energy of a thin liquid film in contact with the substrate. We first describe challenges and inefficiencies encountered when implementing a direct version of this approach to compute the properties of fluids at rough surfaces. Next, we detail a series of convenient thermodynamic paths that enable one to obtain free energy information at relevant surface densities over a wide range of temperatures and substrate strengths in an efficient manner. We then show how this information is assembled to construct complete wetting diagrams at a temperature of interest. The strategy pursued within this work is general and is expected to be applicable to a wide range of molecular systems. To demonstrate the utility of the approach, we present results for a Lennard-Jones fluid in contact with a substrate containing rectangular-shaped grooves characterized by feature sizes of order ten fluid diameters. For this particular fluid-substrate combination, we find that the macroscopic theories of Cassie and Wenzel provide a reasonable description of simulation data.

Impact of small-scale geometric roughness on wetting behavior.

We examine the extent to which small-scale geometric substrate roughness influences the wetting behavior of fluids at solid surfaces. Molecular simulation is used to construct roughness wetting diagrams wherein the progression of the contact angle is traced from the Cassie to Wenzel to impregnation regime with increasing substrate strength for a collection of systems with rectangularly shaped grooves. We focus on the evolution of these diagrams as the length scale of the substrate features approaches the size of a fluid molecule. When considering a series of wetting diagrams for substrates with fixed shape and variable feature periodicity, we find that the diagrams progressively shift away from a common curve as the substrate features become smaller than approximately 10 fluid diameters. It is at this length scale that the macroscopic models of Cassie and Wenzel become unreliable. Deviations from the macroscopic models are attributed to the manner in which the effective substrate-fluid interaction strength evolves with periodicity and the important role that confinement effects play for substrates with small periodicities.

Monte Carlo simulation strategies to compute interfacial and bulk properties of binary fluid mixtures

We introduce Monte Carlo simulation methods for determining interfacial properties of binary fluid mixtures. The interface potential approach, in which the interfacial properties of a system are related to the surface excess free energy of a thin fluid film in contact with a surface, is utilized to deduce the wetting characteristics of a fluid mixture. The strategy described here provides an effective means to obtain the evolution of interfacial properties with the chemical composition of the fluid. This task is accomplished by implementing an activity fraction expanded ensemble technique, which allows one to obtain elements of the interface potential as a function of composition. We also show how this technique can be utilized to calculate bulk coexistence properties of fluid mixtures in an efficient manner. The computational strategies introduced here are applied to three model systems. One includes an argon-methane fluid mixture that is known to display simple behavior in the bulk. The second fluid model contains a size asymmetric mixture that exhibits azeotropy. The third model fluid is the well-studied size symmetric mixture that displays liquid-liquid-vapor phase coexistence. The techniques outlined here are used to compile the composition dependence of spreading and drying coefficients, liquid-vapor surface tension, and contact angle for these systems. We also compare our surface tension results with values estimated from predictive-style models that provide the surface tension of a fluid mixture in terms of pure component properties. Overall, we find that the general approach pursued here provides an efficient and precise means to calculate the bulk and wetting properties of fluid mixtures.

Understanding wetting of immiscible liquids near a solid surface using molecular simulation

We introduce Monte Carlo simulation methods for determining interfacial properties of fluids that exhibit bulk liquid-liquid immiscibility. An interface-potential-based approach, in which the interfacial properties of a system are related to the surface excess free energy of a thin fluid film in contact with a surface, is utilized to deduce the wetting characteristics of these systems. We present a framework for implementing this general method within both the grand canonical and semigrand isobaric-isothermal ensembles. Tracking the evolution of interfacial properties along various thermodynamic paths is also examined. This task is accomplished by implementing variants of the expanded ensemble technique, which enables one to obtain components of the interface potential along a path of interest. We also discuss how these concepts are employed to calculate bulk liquid-liquid coexistence properties in an efficient manner. The computational strategies introduced here are applied to three model Lennard-Jones systems. For each system, we compile the evolution of the liquid-liquid surface tension and contact angle with temperature or pressure. For one of the model systems we compare our results with literature data. We also examine how interfacial properties evolve upon variation of the relative affinity of the fluid components for the substrate. Overall, we find that the approach pursued here is generally applicable and provides an efficient and precise means to calculate the bulk and interfacial properties of fluids that exhibit liquid-liquid immiscibility.

Application of the interface potential approach to calculate the wetting properties of a water model system

The interface potential approach is used to compute the interfacial properties of a model system consisting of SPC/E water at a structureless non-polar surface. Both the spreading and drying versions of the method, in which one focuses on the growth of a thin liquid and vapour film from the surface, respectively, are employed. We examine the substrate strength dependence of interfacial properties, including the spreading and drying coefficients, contact angle and liquid–vapour surface tension at temperatures between 300 and 500 K as well as the temperature dependence of these properties at various substrate strengths. Two schemes are used to handle electrostatic interactions. In the first, the Coulomb potential is simply truncated, and in the second, Ewald summations are used to compute long-range electrostatic interactions. We find that the two schemes provide quantitatively different, but qualitatively consistent results.