(221d) Molecular Modeling of Three Phase Contact for Static and Dynamic Contact Angle Phenomena

Interfacial phenomena arise in a number of industrially important situations, such as multiphase flows in pipes, repellency of liquids on surfaces, condensation, particulate dispersants, etc. In designing materials for such applications, the key component is their wetting behavior, which is characterized by three-phase static and dynamic contact angle phenomena. Molecular modeling has the potential to provide basic insight into the chemical and thermodynamic factors that influence these properties, as well as a detailed picture of the three-phase contact line resolved on the sub-nanometer scale which is essential for the success of these materials. Currently popular simulation methodologies to characterize contact angle phenomena include: a) simulation of a finite sized droplet on a semi-infinite planar plate (“drop-on-plane” geometry); and b) simulation of interfacial free energies between solid/liquid, liquid/vapor and solid/vapor phases and invoking Young's continuum equation. Even though these methods are very robust, they do not offer practical routes to characterization of dynamic contact angle phenomena. We have proposed a computational strategy to study three-phase contact using a configuration that in some sense “inverts” the conventional drop-on-plane geometry. In our “inverted model” approach, buoyancy of a solid rod or particle is studied in a planar liquid film. Such solids have real counterparts in the form of carbon nanotubes, fibrous materials such as those formed by electrospinning or electroblowing, and particle-based dispersants. The contact angle is readily evaluated by measuring the position of solid and liquid interfaces. This strategy provides a simple and efficient method for studying contact angle behavior. The system can be studied using molecular dynamics as well as Monte Carlo methods. As proof of concept, the methodology has been validated extensively using a simple Lennard-Jones (LJ) fluid in contact with a face-centered cubic LJ crystal surface. Excellent agreement is observed between the static (equilibrium) contact angles obtained by the new method and those reported in the literature. In addition, the dynamic contact angle analysis is performed for two selected cases. The evolution of contact angle as a function of force applied to the rod or particle is characterized by the pinning and slipping of the three phase contact line and changes in radius of curvature of the liquid-vapor interface. Ultimately, complete wetting or de-wetting is observed, allowing molecular level characterization of the contact angle hysteresis.