Plausible photomolecular effect leading to water evaporation exceeding the thermal limit.

We report in this work several unexpected experimental observations on evaporation from hydrogels under visible light illumination. 1) Partially wetted hydrogels become absorbing in the visible spectral range, where the absorption by both the water and the hydrogel materials is negligible. 2) Illumination of hydrogel under solar or visible-spectrum light-emitting diode leads to evaporation rates exceeding the thermal evaporation limit, even in hydrogels without additional absorbers. 3) The evaporation rates are wavelength dependent, peaking at 520 nm. 4) Temperature of the vapor phase becomes cooler under light illumination and shows a flat region due to breaking-up of the clusters that saturates air. And 5) vapor phase transmission spectra under light show new features and peak shifts. We interpret these observations by introducing the hypothesis that photons in the visible spectrum can cleave water clusters off surfaces due to large electrical field gradients and quadrupole force on molecular clusters. We call the light-induced evaporation process the photomolecular effect. The photomolecular evaporation might be happening widely in nature, potentially impacting climate and plants' growth, and can be exploited for clean water and energy technologies.

Kinetics of Sorption in Hygroscopic Hydrogels.

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.

Design of a Compact Multicyclic High-Performance Atmospheric Water Harvester for Arid Environments.

Water scarcity remains a grand challenge across the globe. Sorption-based atmospheric water harvesting (SAWH) is an emerging and promising solution for water scarcity, especially in arid and noncoastal regions. Traditional approaches to AWH such as fog harvesting and dewing are often not applicable in an arid environment (<30% relative humidity (RH)), whereas SAWH has demonstrated great potential to provide fresh water under a wide range of climate conditions. Despite advances in materials development, most demonstrated SAWH devices still lack sufficient water production. In this work, we focus on the adsorption bed design to achieve high water production, multicyclic operation, and a compact form factor (high material loading per heat source contact area). The modeling efforts and experimental validation illustrate an optimized design space with a fin-array adsorption bed enabled by high-density waste heat, which promises 5.826 Lwater kgsorbent -1 day-1 at 30% RH within a compact 1 L adsorbent bed and commercial adsorbent materials.

Wrong expectation of superinsulation behavior from largely-expanded nanocellular foams.

This work aims to predict the thermal conductivity of microcellular and nanocellular thermal insulation foams to explore the correlation between the cellular structure and the thermal insulating properties. Closed-cell foam consisting of cell walls and struts was used as the base geometry for modeling. The mathematical correlations to calculate the thickness of cell walls and the diameter of struts for a given cell size, the void fraction and the volume fraction of polymer located in struts were investigated. Then, a mathematical model for the conductive thermal conductivity including the dependency on the void fraction, the strut fraction and the Knudsen effect for gas was introduced. The radiative thermal conductivity was determined by analyzing the attenuation of radiative energy by absorption and scattering based on Mie's theory together with electromagnetic wave interference, as well as interference of propagating waves and tunneling of the radiative energy by evanescent waves in the cells. The thermal conductivity model was validated by experimental data and used to predict the thermal conductivity of polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at various cell sizes and volume expansion ratios. It was found that the radiative thermal conductivity plays a crucial role in nanocellular foam. The trade-off between the cell size and cell wall thickness when cell walls become thinner and highly transparent to thermal radiation was demonstrated, leading to the optimal volume expansion ratio at which the thermal conductivities were minimized. Perspectives for the manufacture of high-performance thermal insulation foams are also discussed.

Robust, ultra-insulative and transparent polyethylene-based hybrid silica aerogel with a novel non-particulate structure.

Aerogels derived from pre-polymerized vinyl trimethoxy silane (VTMS) precursor with nano-size particles are known to exhibit outstanding mechanical and insulation properties. However, the density reduction has been limited by the poor connectivity. This paper presents an innovative technology to generate a new class of VTMS-based hybrid silica aerogels that possess outstanding non-particulate, reticulated structure and superior properties. This technology relies on spinodal decomposition instead of conventionally exploited binodal decomposition, which leads to a particulate structure. This new aerogel technology has significantly increased the void fraction of the pre-polymerized VTMS-based aerogel, which could not be achieved previously using binodal decomposition. The increased void fraction in the form of nano-pores with an average pore size of 21.75 nm nullifies the gas thermal conductivity effectively. Another consequence of the non-particulate structure is decreased processing time by removing the aging step. These improvements are due to the non-particulate structure's increased connectivity produced by spinodal decomposition. This novel structure was then compared to a particulate counterpart aerogel of the same material derived from the conventional binodal decomposition of the pre-polymerized VTMS precursor. To further decrease the processing cost, a lower molecular-weight polymeric precursor was synthesized under milder polymerization conditions. The effects of the polymeric precursor's molecular weight on the mechanical and thermal properties of the aerogel created via spinodal decomposition were also investigated.

Nanostructure to thermal property relationship of resorcinol formaldehyde aerogels using the fractal technique.

We characterized the morphological features of an organic resorcinol-formaldehyde (RF) aerogel and correlated each feature to the thermal insulation properties. Several RF aerogels were synthesized with different morphological features and structural assemblies. This was done by changing the catalyst percentages and the dilution ratios at the polymerization stage. Then, each morphological feature was assessed and categorized using two scales: the macro scale and the micro scale. We found that the macro-features were independent of the catalyst percentages and depended only on the dilution ratios. By contrast, the micro-features were highly sensitive to any changes during the polymerization process. These changes altered the samples' three main micro-structural factors: (i) the structural assembly, (ii) the porous structure, and (iii) the fractal parameters. Thus, we characterized and quantified each component within these areas. Then, we assessed the structure's heat transfer modes and classified them as follows: (i) solid conductivity through the solid particles, (ii) gas conductivity through the gas molecules, and (iii) thermal radiation. We identified the morphological features in our RF samples and correlated them to each mode of the heat transfer. For example, the samples' solid conductivity was highly dependent on the fractal parameters of our structure; that is, the particles' roughness, the structural complexity, and the structural homogeneity. For those samples with extremely rough particles and a complex structure, the solid conductivity reached the lowest possible point. We also found that the total thermal conductivity was mainly controlled by the micro-morphological features, and that the solid conductivity was the most dominant heat transfer mode.

Development of high-porosity resorcinol formaldehyde aerogels with enhanced mechanical properties through improved particle necking under CO2 supercritical conditions.

A new high porosity resorcinol-formaldehyde (RF) aerogel with improved particle necking is presented in this work. This RF aerogel was developed under CO2 supercritical drying conditions without any structural shrinkage. The water content and the catalyst percentage were varied to modify the particles' nucleation and growth mechanisms and to control particle-particle connections. The nucleation mechanism solely dependent on the initial catalyst percentage; the number of nuclei increased with the catalyst percentage. However, the growth and connection of the particles dependent on both the water content and the catalyst percentage through their effect on the pH value. As the water content increased to have a larger void fraction, the pH value decreased. Consequently, the spherical growth of the particles became dominant and, thereby, the connection of the particles became more difficult. But as the catalyst percentage increased, the pH value increased, and the connection of the particles became facilitated with the formation of necks around the particles. As a result, the semi-fibril-like structure was developed with a high void fraction. A 30% increase in the structural elasticity and a very low thermal conductivity of 0.0249W/mK were obtained.