Non-equilibrium configurations of swelling polymer brush layers induced by spreading drops of weakly volatile oil

TL;DR

This study investigates the non-equilibrium swelling behavior of polymer brush layers exposed to volatile oil drops, revealing the importance of vapor phase transport in dynamic wetting and developing a model that matches experimental observations.

Contribution

The paper introduces a coupled field gradient dynamics model that captures the complex swelling and vapor transport phenomena in polymer brushes exposed to volatile liquids.

Findings

Formation of a swelling halo ahead of the contact line

Long-lived transient swelling profiles observed experimentally

Quantitative agreement between model and experiments enabling solvent diffusion measurement

Abstract

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.