BIRS Workshop Lecture Videos
Wetting at the Nanoscale. Equilibrium and Dynamics Yatsyshin, Peter
The most exciting effects associated with wetting and adsorption are caused by the fluid inhomogeneity at the nanoscale and the nonlocality of the intermolecular fluid–fluid and fluid–substrate interactions. Fluids adsorbed at walls, in capillary pores and slits and in sculpted geometries such as grooves and wedges can form different thermodynamic phases (e.g., figure 1) and exhibit many new phase transitions compared to their bulk counterparts. As well as being of fundamental interest to the modern statistical mechanical theory of inhomogeneous fluids, these are also relevant to nanofluidics, chemical- and bioengineering, design of surfaces with tunable wetting properties and lab-on-a-chip devices. In this talk we will discuss novel, first-order and continuous, interfacial transitions, including wetting, pre-wetting, capillary-condensation and filling, the formation of droplets and liquid bridges [1, 2], which can occur in sculpted pores with one or more dimensions on the order of several nanometers. These transitions are sensitive to both the range of the intermolecular forces and the interfacial fluctuation effects. Our methodology is based on the density functional theory (DFT) formulation of statistical mechanics of classical fluids. Within DFT, the grand free energy of a classical soft-matter system is expressed as a functional of the system’s one-body density field. In this way, DFT elegantly captures the small-scale inhomogeneity of the fluid structure in a theoretically consistent and computationally accessible manner, and can be viewed as a means to include the fluid structure into the thermodynamic equation of state. Dynamic DFT (DDFT) in its simplest form is a generalized diffusion equation corresponding to the Smoluchowsky picture of the dynamics of colloidal particles in a solvent. We will demonstrate how DDFT can be used effectively to study diffusion-driven spreading and coalescence of sessile nanodroplets. Our computations may provide insight into the dynamics of the three-phase contact line and static and dynamic contact angles of small nanodroplets, which remain in debate.
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