The Mechanics of Drying Polymer Solutions Explain Observations of Humidity Insensitivity and Transient Buckling

Contrary to traditional theories, recent experiments reveal that immediate skin formation acts as a transport barrier, rendering drying kinetics during the entire drying process insensitive to relative humidity. Herein, we present a framework integrating elasto-visco-plastic constitutive modeling and Flory–Rehner theory within nonequilibrium thermodynamics to elucidate the unresolved evaporation phenomena. Our model reveals that the viscoelastic skin transforms the mass transport from humidity-controlled interfacial evaporation to internal viscoelastic-diffusive transport. The relaxation time of the skin orchestrates the coupling among stress evolution, volumetric deformation, and water diffusion, establishing a universal scaling relationship, where maximum evaporation occurs at Deborah number De ∼ O(1). While previous theories accounted for thickness-induced resistance, we identify plasticization-enhanced transport as the critical, previously overlooked mechanism. It is the competition between these two mechanisms that drive the observed decoupling from ambient conditions. Notably, the peak compressive stress at De ∼ O(1) can also induce transient skin layer buckling. These findings reconcile experimental observations of humidity-insensitive evaporation with theoretical predictions, providing essential insights into drying processes under variable environmental conditions.