Phase change in disordered media: intermolecular forces control simultaneous evaporation and condensation

We investigate phase change and fluid-fluid redistribution in porous media using micromodel experiments, in which air is injected to displace water. At late times, long after injection stops, we expect the system to arrive at a steady-state equilibrium configuration. Instead, we observe the condensation and growth of liquid droplets at the expense of the evaporation of liquid bridges. This is surprising, given that the liquid bridges are at lower pressure than the liquid droplets. We show that this counterintuitive behavior emerges from the influence of intermolecular forces on the instability of condensing thin liquid films. Our observations therefore point to a new physical mechanism for phase change in disordered media, which drives fluid organization at the pore scale, and has important implications for multiphase-flow properties at the macroscale.