  Polymer brush layers are responsive materials that swell in contact with good
solvents and their vapors. We deposit drops of an almost completely wetting
volatile oil onto an oleophilic polymer brush layer and follow the response of
the system upon simultaneous exposure to both liquid and vapor. Interferometric
imaging shows that a halo of partly swollen polymer brush layer forms ahead of
the moving contact line. The swelling dynamics of this halo is controlled by a
subtle balance of direct imbibition from the drop into the brush layer and
vapor phase transport and can lead to very long-lived transient swelling
profiles as well as non-equilibrium configurations involving thickness
gradients in a stationary state. A gradient dynamics model based on a free
energy functional with three coupled fields is developed and numerically
solved. It describes experimental observations and reveals how local
evaporation and condensation conspire to stabilize the inhomogeneous
non-equilibrium stationary swelling profiles. A quantitative comparison of
experiments and calculations provides access to the solvent diffusion
coefficient within the brush layer. Overall, the results highlight the -
presumably generally applicable - crucial role of vapor phase transport in
dynamic wetting phenomena involving volatile liquids on swelling functional
surfaces.
