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Transforming atmospheric water vapor into liquid form can be a way to supply water to arid regions for uses such as drinking water, thermal management, and hydrogen generation. Many current methods rely on solid sorbents that cycle between capture and release at slow rates. We envision a radically different approach where water is transformed and directly captured into a liquid salt solution that is suitable for subsequent distillation or other processing using existing methods. In contrast to other methods utilizing hydrogels as sorbents, we do not store water within hydrogels—we use them as a transport medium. Inspired by nature, we capture atmospheric water through a hydrogel membrane “skin” at an extraordinarily high rate of 5.50 kg m^-2 d^-1 at a low humidity of 35%. and up to 16.9 kg m^-2 d^-1at higher humidities. For a drinking-water application, calculated performance of a hypothetical one-square-meter device shows that water could be supplied to two to three people in arid environments. This work is a significant step toward providing new resources and possibilities to water-scarce regions. 

We find that the collapse of a droplet on a hydrogel is dictated by competing timescales of contact line advancement and water diffusion into the gel. 

From pasta to biological tissues to contact lenses, gel and gel-like materials inherently soften as they swell with water. In dry, low-relative-humidity environments, these materials stiffen as they de-swell with water. Here, we use semi-dilute polymer theory to develop a simple power-law relationship between hydrogel elastic modulus and swelling. From this relationship, we predict hydrogel stiffness or swelling at arbitrary relative humidities. Our close predictions of properties of hydrogels across three different polymer mesh families at varying crosslinking densities and relative humidities demonstrate the validity and generality of our understanding. This predictive capability enables more rapid material discovery and selection for hydrogel applications in varying humidity environments. 

Hydrogels hold promise in agriculture as reservoirs of water in dry soil, potentially alleviating the burden of irrigation. However, confinement in soil can markedly reduce the ability of hydrogels to absorb water and swell, limiting their widespread adoption. Unfortunately, the underlying reason remains unknown. By directly visualizing the swelling of hydrogels confined in three-dimensional granular media, we demonstrate that the extent of hydrogel swelling is determined by the competition between the force exerted by the hydrogel due to osmotic swelling and the confining force transmitted by the surrounding grains. Furthermore, the medium can itself be restructured by hydrogel swelling, as set by the balance between the osmotic swelling force, the confining force, and intergrain friction. Together, our results provide quantitative principles to predict how hydrogels behave in confinement, potentially improving their use in agriculture as well as informing other applications such as oil recovery, construction, mechanobiology, and filtration.