Theory of Absorption Kinetics in Hygroscopic Hydrogels*

Hygroscopic hydrogels have emerged as a scalable material capable of high-performance evaporative cooling and atmospheric water harvesting due to their fast kinetics and high water uptake. However, despite extensive research interest, there is a lack of understanding of the governing fluid and mass transport mechanisms within the hydrogels, which hinders the optimization of their performance. In this work, we present the first-ever model that explains the kinetics of absorption in hygroscopic hydrogels. Our model couples vapor transport within the hydrogel micropores with swelling-induced liquid transport in the polymer nanopores to accurately capture experimentally observed water uptake curves based solely on the knowledge of the hydrogel material properties. With the insights of the model, we determine the dominant properties that govern the absorption kinetics and show that mechanically stiff, thin hydrogels with optimized porosity exhibit the best kinetics. The knowledge obtained from this model can therefore guide the design of hygroscopic hydrogels and enable improved performance in atmospheric water capture and evaporative cooling.