Characterization of swelling gradients and evaporation dynamics in polymer brushes using high-resolution colorimetry

We present a white-light interferometry method enabling real-time, full-field measurement of local film thickness with nanometer vertical resolution using a simple optical microscopy set-up. We apply this approach to investigate droplet-induced swelling and vapor-infused deswelling of poly(lauryl methacrylate) (PLMA) brushes in contact with n-alkanes (C12, C14, C16), spanning two orders of magnitude in vapor pressure. For uniformly pre-swollen brushes, we demonstrate that evaporation is governed by vapor-phase transport rather than transport within the brush, remaining remarkably constant over swelling ratios from 4.2 to 1.6. Rescaling time by vapor pressure collapses all deswelling data onto a single master curve, consistent with diffusion-limited gas-phase transport. For droplet-induced swelling on initially dry brushes, we quantify the coupled effects of imbibition, evaporation, and condensation on the formation of swelling halos. More volatile solvents produce faster-growing halos that reach a limited lateral extent in open air. The final width follows from a balance between near-constant liquid influx and size-dependent evaporative outflow. Temporal swelling profiles collapse onto a common functional form, indicating universal behavior governed by solvent transport and evaporation. These results establish colorimetric interferometry as an accessible tool for probing dynamic soft interfaces and a means to constrain key model parameters, notably the grafting density $\sigma$ and the interaction parameter $\chi$ , enabling systematic extraction of material properties from dynamic experiments.