Molecular Dynamics Simulations of the Influence of Drop Size and Surface Potential on the Contact Angle of Ionic-Liquid Droplets

In this work we have performed molecular dynamics simulations of the room temperature ionic-liquid 1-ethyl-3-methylimidazolium tetrafluoroborate on a surface with graphene structured sheets. Contact angles were calculated via polynomial-fitting of two-dimensional atomic density contours. Drop size and surface interaction were varied to assess their influences on the contact angle. At low graphene interaction potential, no change in contact angle with drop size was found. When increasing the surface interaction potential toward actual graphene, the drops deviated to fully wetting, but the spreading rate and extent became dependent on drop size. The smallest drop formed a single adsorbed layer due to its initial size and the large ratio of ions in the adsorbed layer. Larger drops wetted and formed nonuniform metastable drops upon several layers of spread liquid. These were not true equilibrium structures as when starting from a single adsorbed layer film at room temperature, the metastable layers were not reformed. An increase in temperature resulted in the drop forming a single layer film, which implied that the behavior here can be considered a form of contact hysteresis, enabled by a kinetic barrier between the metastable and film layers. At low surface potential, a single layer reverted to a drop and had the same contact angle as when starting from a drop, which showed that the hysteresis is potential dependent and that low surface potentials more readily form equilibrium structures.