(290f) Molecular Level Modeling of Three-Phase Contact

Three-phase contact lines, between solid, liquid and vapor phases, occur in a variety of scientifically and industrially important situations. These include, for example, the repellency of liquids on smooth, rough or patterned solid surfaces, responsible for superhydrophobicity or oleophobicity, as well as the collection of liquid aerosols on filter media and dispersion of particles at liquid-vapor interfaces. The nature of liquid-solid interactions are commonly characterized by the contact angle, and contact angle hysteresis, measured at the line of contact between solid, liquid and vapor phases. The details of these properties are sensitive to the specific structure and interactions between molecules in the vicinity of the three-phase contact, and some ?pre-wetting? phenomena are thought to occur at the molecular length scale. Molecular simulations are particularly well-suited to studies of these phenomena and their dependence on molecular structure and chemical composition. A direct approach is to simulate a ?nanodrop? of fluid on a solid surface in the presence of a large volume of vapor. However, such simulations are complicated by finite size effects and computational inefficiencies associated with equilibration between the vapor and a liquid phase with curved interface. We describe an ?inverted? approach that allows to obtain the same information without such complications. In this approach, the solid phase, whose vapor pressure is essentially zero, is employed as the phase with a curved interface. The flotation of such solids with curved surfaces are studied at a planar liquid-vapor interface. Interfacial forces provide the necessary buoyancy, and equilibration is realized by satisfying Young's equation locally at the three-phase contact line. We have applied this method to the study of single walled carbon nanotubes (CNT) at the interface of liquid hexane in equilibrium with its own vapor. This simulation closely mimics the wetting of oil drops on a CNT ?paper?. Measurements of contact angles, contact angle hysteresis, interfacial tensions, and wetting phenomena will be discussed.