Modelling of water evaporation in hydrogels considering the state of water in tension

Water evaporation-induced drying is critically important for hydrogels in open-air applications, but theoretically, modelling is difficult due to the complicated intermolecular interactions and structural deformation. Herein, we have constructed a simplified model to describe the evaporation of water inside the hydrogel by considering the state of water in tension. We employed "negative pressure" to bridge the stretching forces in water and the elastic forces generated by the polymer network. Combined with a constitutive equation of elasticity for hydrogels and a diffusive equation, this model provides a feasible approach to calculate the saturated vapour pressure, dynamic evaporation rates, and deformation of different hydrogels. The calculated results agree well with experimental results, both in steady-state and dynamic processes, for commonly used poly 2-hydroxyethyl methacrylate and polyacrylamide hydrogels with diverse components. In addition, the model predicts that hydrogels with high moduli show a stronger ability to retain water in an open environment.