(83b) Hydrogel Swelling and Deswelling in Complex Environments

Hydrogels are cross-linked networks of hydrophilic polymers that can absorb large amounts of water and swell, or conversely release water and deswell, all while retaining their integrity. Their versatility, ease of manufacture, and biocompatibility make them attractive for use in energy, manufacturing, agriculture, and biomedical applications. These uses typically require hydrogel swelling and deswelling to be predictable and controllable. While these processes are well-studied in controlled lab environments, how swelling and deswelling are altered in complex environments -- e.g., characterized by geometric confinement, adhesion to boundaries, and varying humidities/temperatures -- is poorly understood. Here, I will describe our work studying how polymer chemistry, gel microstructure, internal fluid transport, and external constraints together control how hydrogels swell and deswell. First, I will describe how by directly visualizing the swelling of hydrogels confined in three-dimensional granular media, we have shown how a spatially nonuniform stress fundamentally alters hydrogel swelling, and have developed quantitative principles to predict how hydrogels behave in complex confinement. Second, I will describe how by directly visualizing the deswelling of hydrogels upon drying, we have shown how the kinetics of this process are controlled by a sensitive interplay between gel microstructure, internal fluid transport, and adhesion to surrounding boundaries. Third, I will describe our development of Moisture-Absorbent, Temperature-Controlled Hydrogels (MATCHes) that absorb water from air at ambient conditions, and then release it in liquid form upon slight heating. We have shown that tuning the mesoscale porosity of the hydrogels dramatically impacts both the total amount and rate of water absorption and release. Together, by expanding current understanding of hydrogel swelling and shrinking to more complex environments, our results help inform the use of hydrogels in agriculture for water management in soil, in formulations for the development of functional coatings, and in atmospheric water harvesting, as well as other applications such as oil recovery, construction, mechanobiology, and filtration.