Evaporation from spherical chitosan polymer gels

Water retention is an important feature in the design and use of biocompatible polymers, such as chitosan, which is often used as a hydrogel for biomedical and pharmaceutical applications. We study water evaporation from chitosan-based hydrogels both for gels in close proximity and for essentially isolated gels. We find that water evaporation from single gels is diffusive, as for pure water drops, but that it happens more slowly. We interpret this difference in terms of a steady-state where the equilibrium properties entering the problem are effectively lower. In the case of many gels, the evaporation process becomes one-dimensional due to water-vapor saturation of the local environment of the gels. Using an electric analogy, we model the transition between the two cases, ultimately illustrating pathways for tuning the water retention properties of polymer hydrogels.