Hygroscopic hydrogels hold significant promise for high-performance atmospheric water harvesting, passive cooling, and thermal management. However, a mechanistic understanding of the sorption kinetics of hygroscopic hydrogels remains elusive, impeding an optimized design and broad adoption. Here, we develop a generalized two-concentration model (TCM) to describe the sorption kinetics of hygroscopic hydrogels, where vapor transport in hydrogel micropores and liquid transport in polymer nanopores are coupled through the sorption at the interface. We show that the liquid transport due to the chemical potential gradient in the hydrogel plays an important role in the fast kinetics. The high water uptake is attributed to the expansion of hydrogel during liquid transport. Moreover, we identify key design parameters governing the kinetics, including the initial porosity, hydrogel thickness, and shear modulus. This work provides a generic framework of sorption kinetics, which bridges the knowledge gap between the fundamental transport and practical design of hygroscopic hydrogels.

Evaporation plays a critical role in a range of technologies that power and sustain our society. Wicks are widely used as passive, capillary-fed evaporators, attracting much interest since these devices are highly efficient, compact, and thermally stable. While wick-based evaporators can be further improved with advanced materials and fabrication techniques, modeling of heat and mass transport at the device level is vital for guiding these innovations. In this perspective, we present the design and optimization of capillary-fed, thin film evaporation devices through a heat and mass transfer lens. This modeling framework can guide future research into materials innovations, fabrication of novel architectures, and systems design/optimization for next generation, high-performance wick-based evaporators. Furthermore, we describe specific challenges and opportunities for the fundamental understanding of evaporation physics. Finally, we apply our modeling framework to the analysis of two important applications—solar vapor generation and electronics cooling devices.

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Hygroscopic hydrogels are emerging as scalable and low‐cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and thermal energy storage. However, devices using these materials still exhibit insufficient performance, partly due to the limited water vapor uptake of the hydrogels. Here, the swelling dynamics of hydrogels in aqueous lithiumchloride solutions, the implications on hydrogel salt loading, and the resulting vapor uptake of the synthesized hydrogel–salt composites are characterized. By tuning the salt concentration of the swelling solutions and the cross‐linking properties of the gels, hygroscopic hydrogels with extremely high salt loadings are synthesized, which enable unprecedented water uptakes of 1.79 and 3.86 gg−1 at relative humidity (RH) of 30% and 70%, respectively. At 30% RH, this exceeds previously reported water uptakes of metal–organic frameworks by over 100% and of hydrogels by 15%, bringing the uptake within 93% of the fundamental limit of hygroscopic salts while avoiding leakage problems common in salt solutions. By modeling the salt‐vapor equilibria, the maximum leakage‐free RH is elucidated as a function of hydrogel uptake and swelling ratio. These insights guide the design of hydrogels with exceptional hygroscopicity that enable sorption‐based devices to tackle water scarcity and the global energy crisis.

The heat transfer coefficient (HTC, h) and critical heat flux (CHF, q″ CHF ) are two major parameters that quantify boiling performance. The HTC describes the efficiency of boiling heat transfer, defined as the ratio of heat flux (q″) to the wall superheat (ΔT w ), that is, h = q″/ΔT w . Here ΔT w is the temperature difference between the boiling surface and the saturated liquid. In the nucleate boiling regime, the heat flux increases with the wall superheat. However, when the heat flux is sufficiently high, excessive vapor bubbles nucleated on the boiling surface prevent the liquid from rewetting the surface and, in turn, form an insulating vapor film over the surface. This vapor film becomes a thermal barrier that leads to a drastic increase in wall superheat and burnout of a boiling system. This transition from nucleate boiling to film boiling is known as the boiling crisis, where the maximum heat flux is CHF. Enhancing CHF, therefore, can either enable larger safety margins or extend the operational heat flux range for boiling systems. [5] Recent efforts to enhance boiling heat transfer have focused on engineering the working fluid or surface properties. [6] In particular, engineering surface structures have received greater attention owing to the constraints on chemical compatibility or operational conditions which can limit the choice of the working fluid. Representative examples of surface structures that effectively enhance CHF are known to be hemi-wicking surfaces such as micropillars and nanowires. [7] These structures enhance CHF by harnessing thin-film evaporation around pillars and capillary-fed wicking through the structures. [8] Surfaces with microcavities, on the other hand, have shown improved HTC by trapping vapor embryos that promote nucleation. [9] Recently, a combination of microtube and micropillar structures referred to as tube-clusters in pillars (TIP), has shown the ability to tune the HTC and CHF by controlling bubble coalescence while maintaining capillary wicking. [10] Despite the controllability, achieving extreme enhancement of HTC and CHF simultaneously remains challenging due to the intrinsic tradeoff between HTC and CHF associated with nucleation-site density. For example, high nucleation-site density may increase HTC but decrease CHF because extensive bubble coalescence hinders the capillary wicking performance, while the reduced number of nucleation sites will limit the HTC enhancement.Boiling is an effective energy-transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat-transfer coefficient (HTC) and critical heat flux (CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade-off between them-HTC enhancement requires high nucleation-site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade-off is overcome by designing three-tier hierarchical structures. The bubble coalescence is minimized to enhance the ...