Spreading of Nonvolatile Liquids in a Continuum Picture

We discuss wetting criteria for solid/liquid pairs where the long-range interaction is not oscillating and is described by a Hamaker constant A. The key parameters are then A and the spreading coefficient S. Since S contains contributions from short range interactions, S and A are independent variables and can be of either sign. Discussing the resulting four possibilities, we expect three fundamental regimes: (1) complete wetting, a small droplet spreads to become a flat “pancake” surrounded by a dry solid; (2) pseudo partial wetting, a droplet forms a spherical cap with a finite contact angle θ but the surrounding solid is wet, the drop is in equilibrium with a molecular film; (3) partial wetting, the contact angle θ is nonzero and now the solid around the drop is dry. The pseudo partial wetting regime may explain some surprising observations concerning the spreading of silicone droplets.