Accelerated Water Transportation Phenomenon through a Hydrophilic Metal Roll.

Passive water transport by taking advantage of capillary forces is vital for various applications such as solar-driven interfacial evaporation, evaporative cooling, and atmospheric water harvesting. Surface engineering and structure design with a hydrophilic surface and enhanced capillary force will facilitate passive water transport. Herein, we demonstrate a hydrophilic Cu/CuO foil-based roll for accelerated water transportation. The roll was fabricated by rolling up a typical 2D Cu/CuO film, which transforms the water climbing behavior by significantly enhancing the capillary force between each Cu/CuO film layer. The simple spatial transformation for a 2D film, from planar foil to 3D structure, has extensively facilitated water transportation performance and broadened its practical application potential. The Cu/CuO film with a blade-like nanostructure and excellent hydrophilicity ensures water supply to a limited area, while the capillary effect between different layers of the Cu/CuO foil extends the water transportation height. Consequently, the Cu/CuO foil-based roll demonstrated a high fluidic transport velocity. This design derived from the 2D planar film can be potentially employed for a large range of applications such as evaporating in a confined space and evaporation-driven energy harvest.

Atmospheric-moisture-induced polyacrylate hydrogels for hybrid passive cooling.

Heat stress is being exacerbated by global warming, jeopardizing human and social sustainability. As a result, reliable and energy-efficient cooling methods are highly sought-after. Here, we report a polyacrylate film fabricated by self-moisture-absorbing hygroscopic hydrogel for efficient hybrid passive cooling. Using one of the lowest-cost industrial materials (e.g., sodium polyacrylate), we demonstrate radiative cooling by reducing solar heating with high solar reflectance (0.93) while maximizing thermal emission with high mid-infrared emittance (0.99). Importantly, the manufacturing process utilizes only atmospheric moisture and requires no additional chemicals or energy consumption, making it a completely green process. Under sunlight illumination of 800 W m-2, the surface temperature of the film was reduced by 5 °C under a partly cloudy sky observed at Buffalo, NY. Combined with its hygroscopic feature, this film can simultaneously introduce evaporative cooling that is independent of access to the clear sky. The hybrid passive cooling approach is projected to decrease global carbon emissions by 118.4 billion kg/year compared to current air-conditioning facilities powered by electricity. Given its low-cost raw materials and excellent molding feature, the film can be manufactured through simple and cost-effective roll-to-roll processes, making it suitable for future building construction and personal thermal management needs.

Dome-arrayed chitosan/PVA hydrogel-based solar evaporator for steam generation.

Water evaporation systems with solar energy as the primary driving energy have received extensive attention in recent years. This work studies the preparation method and performance of hydrogel evaporators using chitosan and polyvinyl alcohol (PVA) as a framework and carbon nanoparticles (CNPs) as the photothermal material. The evaporation rate of CPC (chitosan/PVA and CNPs) hydrogel obtained reaches 2.28 kg m-2 h-1. Simultaneously, a three-dimensional structure is designed based on the two-dimensional double-layer evaporation system in this study. An evaporator with a tiny-pool structure and a hydrogel with a dome-arrayed structure is designed. These two structures achieve highly efficient evaporation rates of 2.28 kg m-2 h-1 and 3.80 kg m-2 h-1, respectively. These optimized designs improve the evaporation rate of the overall system by ~ 66.7%. The developed evaporation devices provide a promising pathway for developing the double-layer evaporators, which promote the new development of water purification with a solar-driven evaporation system.

Effective Approximation Method for Nanogratings-induced Near-Field Radiative Heat Transfer.

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO2) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

Environmentally Friendly and Efficient Hornet Nest Envelope-Based Photothermal Absorbers.

Water shortage is a critical global issue that threatens human health, environmental sustainability, and the preservation of Earth's climate. Desalination from seawater and sewage is a promising avenue for alleviating this stress. In this work, we use the hornet nest envelope material to fabricate a biomass-based photothermal absorber as part of a desalination isolation system. This system realizes an evaporation rate of 3.98 kg m-2 h-1 under one-sun illumination, with prolonged evaporation rates all above 4 kg m-2 h-1. This system demonstrates a strong performance of 3.86 kg m-2 h-1 in 3.5 wt % saltwater, illustrating its effectiveness in evaporation seawater. Thus, with its excellent evaporation rate, great salt rejection ability, and easy fabrication approach, the hornet nest envelope constitutes a promising natural material for solar water treatment applications.

Mechanically stretchable metamaterial with tunable mid-infrared optical properties.

Over the past decade, tremendous efforts have been devoted to the design of metamaterials with ultrahigh absorption. These perfect absorbers can realize the annihilation of incident electromagnetic waves by eliminating reflection and transmission of microwaves, infrared, visible, and ultraviolet. However, the optical properties are usually unchanged due to a rigid structure. In this work, we propose a mechanically stretchable metamaterial composed of polydimethylsiloxane and gold with tunable optical properties in the mid-infrared region. A large variation of absorptances with different gold filling ratios is demonstrated as well as the corresponding electric field distributions. Under moderate uniaxial and biaxial tensions, the proposed two-dimensional grating structure has achieved a dynamic tuning of infrared thermal properties, including a sharp reflectance-absorptance switch. This mechanically stretchable metamaterial can serve different optical and sensing functions due to its facile tunability.

Forest waste to clean water: natural leaf-guar-derived solar desalinator.

Water scarcity and waste mismanagement are global crises that threaten the health of populations worldwide and a sustainable future. In order to help mitigate both these issues, a solar desalination device composed entirely of fallen leaves and guar - both natural materials - has been developed and demonstrated herein. This sustainable desalinator realizes an evaporation rate of 2.53 kg m-2 h-1 under 1 sun irradiance, and achieves consistent performance over an extended exposure period. Furthermore, it functions efficiently under a variety of solar intensities and in high salinity environments, and can produce water at salinities well within the acceptable levels for human consumption. Such strong performance in a large variety of environmental conditions is made possible by its excellent solar absorption, superb and rapid water absorption, low thermal conductivity, and considerable salt rejection abilities. Composed primarily of biowaste material and boasting a simple fabrication process, this leaf-guar desalinator provides a low-cost and sustainable avenue for alleviating water scarcity and supporting a green path forward.

Fully Biomass-Based Hybrid Hydrogel for Efficient Solar Desalination with Salt Self-Cleaning Property.

Solar-driven interfacial steam generation provides an opportunity for solar harvesting and freshwater yield as a promising and eco-friendly technology. Here, we demonstrate a sustainable, nontoxic, and highly efficient fully biomass-based GG/CI hydrogel evaporator consisting of gellan gum (GG) hydrogel as the matrix and cuttlefish ink (CI) as the photothermal material. Induced by the ice-template method and freeze-drying method, vertically aligned microchannels are generated along the ice crystal growth direction. Efficient photothermal conversion is enabled by the natural black cuttlefish ink powder and enhanced by the light trapping effect within vertical microchannels. The hydrophilic property of the gellan gum hydrogel and water capillary force in those microchannels boost water pumping to the top interfacial evaporation region. Effective rapid salt self-cleaning behavior is achieved due to the rapid ion diffusion within vertical microchannels. An evaporation rate of 3.1 kg m-2 h-1 under one sun irradiance is demonstrated by this fully biomass-based GG/CI hydrogel evaporator. This work offers a promising alternative for eco-friendly and sustainable freshwater generation with abundant natural biomasses.

Superhydrophobic and Recyclable Cellulose-Fiber-Based Composites for High-Efficiency Passive Radiative Cooling.

Passive daytime radiative cooling (PDRC) involves cooling down an object by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earth's atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. Herein, we develop scalable cellulose-fiber-based composites with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic poly(tetrafluoroethylene) (PTFE) microparticle suspensions embedded partially within the microsized pores of the cellulose fiber to form a dual-layered structure with PTFE particles atop the paper. The formed superhydrophobic PTFE coating not only protects the cellulose-fiber-based paper from water wetting and dust contamination for real-life applications but also reinforces its solar reflectivity by sunlight backscattering. It results in a subambient cooling performance of 5 °C under a solar irradiance of 834 W/m2 and a radiative cooling power of 104 W/m2 under a solar intensity of 671 W/m2. The self-cleaning surface of composites maintains their good cooling performance for outdoor applications, and the recyclability of the composites extends their life span after one life cycle. Additionally, dyed cellulose-fiber-based paper can absorb appropriate visible wavelengths to display specific colors and effectively reflect near-infrared lights to reduce solar heating, which synchronously achieves effective radiative cooling and esthetic varieties.

Harvesting energy from sun, outer space, and soil.

While solar power systems have offered a wide variety of electricity generation approaches including photovoltaics, solar thermal power systems, and solar thermoelectric generators, the ability to generate electricity at both the daytime and nighttime with no necessity of energy storage remains challenging. Here, we propose and verify an environment-friendly, sustainable, and cost-effective strategy of harvesting solar energy by solar heating during the daytime and harnessing the coldness of the outer space through radiative cooling to produce electricity at night using a commercial thermoelectric module. It enables electricity generation for 24 h a day. We experimentally demonstrate a peak power density of 37 mW/m[Formula: see text] at night and a peak value of 723 mW/m[Formula: see text] during the daytime. A theoretical model that accurately predicts the performance of the device is developed and validated. The feature of 24-h electricity generation shows great potential energy applications of off-grid and battery-free lighting and sensing.

A Novel Probe-to-Probe Method for Measuring Thermal Conductivity of Individual Electrospun Nanofibers.

Polymer nanofibers have the ability to replace expensive materials, such as metals, ceramics and composites, in specific areas, such as heat exchangers, energy storage and biomedical applications. These properties have caused polymer nanofibers to be explored as solutions to a growing list of thermal management problems, driving an even greater need to better measure and understand the thermal properties of these nanofibers. This study intends to further the understanding of the thermal properties of polymer nanofibers through the use of a novel Probe-to-Probe measurement method. Polycaprolactone nanofibers fabricated using the electrospinning method can be easily collected and loaded into a traditional atomic force microscope through a mechanical design for thermal measurement. This Probe-to-Probe method demonstrates the ability to accurately measure the thermal boundary conditions about a polymer nanofiber with a heating prong temperature up to 400 ∘C and assists in characterizing its thermal properties.

Blackbody-cavity ideal absorbers for solar energy harvesting.

Spectrally selective solar absorbers (SSAs), which harvest heat from sunlight, are the key to concentrated solar thermal systems. An ideal SSA must have an absorptivity of unity in the solar irradiance wavelength region (0.3-2.5 [Formula: see text]m), and its infrared thermal emissivity must be zero to depress spontaneous blackbody irradiation (2.5-25 [Formula: see text]m). Current SSA designs which utilize photonic crystals, metamaterials, or cermets are either cost-inefficient due to the complexity of the required nanofabrication methods, or have limited applicability due to poor thermal stability at high temperatures. We conceptually present blackbody-cavity solar absorber designs with nearly ideal spectrally selective properties, capable of being manufactured at scale. The theoretical analyses show that the unity solar absorptivity of the blackbody cavity and nearly zero infrared emissivity of the SSA's outer surface allow for a stagnation temperature of 880 [Formula: see text]C under 10 suns. The performance surpasses state-of-the-art SSAs manufactured using nanofabrication methods. This design relies only on traditional fabrication methods, such as machining, casting, and polishing. This makes it suitable for large-scale industrial applications, and the "blackbody cavity" feature enables easy integration with existing concentrated solar thermal systems using the parabolic reflector and Fresnel lens as optical concentrators.

Highly effective photon-to-cooling thermal device.

Photon-to-cooling phenomenon relies on the atmospheric transparency window to dissipate heat from the earth into outer space, which is an energy-saving cooling technique. This work demonstrates a highly effective aluminized Polymethylpentene (PMP) thin-film thermal structure. The emissivity of aluminized PMP thin films matches well to the atmospheric transparency window so as to minimize parasitic heat losses. This photon-to-cooling structure yields a temperature drop of 8.5 K in comparison to the ambient temperature and a corresponding radiative cooling power of 193 W/m2 during a one-day cycle. The easy-to-manufacture feature of an aluminized PMP thin film makes it a practically scalable radiative cooling method.

Spectral selectivity of multiple nanoparticles doped thin films.

Microscopic thin film doped with different species of nanoparticles displays a unique wavelength selectivity in the context of micro/nanoscale radiative heat transfer. We propose a methodology to shift, broaden, and suppress the thermal radiative selectivity in the desired wavelength ranges. Measured transmittance spectra of potassium bromide pellet doped with a single species of nanoparticles are compared with the theoretical prediction using refractive indices that are extracted by refitting transmittance spectra curve according to the Lorentz-Drude model. For a media doped with more than two species of nanoparticles, a successive effective dielectric function using the refitted complex refractive indices and Maxwell Garnett theory is used to evaluate the thermal radiative selectivity of the composites. It has been confirmed theoretically and experimentally that the wavelength selectivity in the transmittance spectra can be influenced by choosing proper species of materials and varying volume fractions of multiple nanoparticles. This work has shed light on the design and fabrication of novel composites doped with multiple particles for applications such as thermophotovoltaics, radiative cooling, and biosensing.

Near-infrared optics of nanoparticles embedded silica thin films.

This work investigates experimentally the near-infrared optical properties of SiO2 thin film embedded with tungsten (W) nanoparticles at varying volume fractions. The samples are prepared by using the technique of magnetron sputtering. The formation and distribution of W nanoparticles are characterized using transmission electron microscopy, and the volume fraction of W nanoparticles is validated by Auger electron spectroscopy. Near- and mid-infrared diffuse reflectance measurements are conducted using Fourier transform infrared spectroscopy. The samples exhibit wavelength selective optical response in the near-infrared region and are suitable for applications involving selective thermal emitters/absorbers. Measured reflectance data is utilized to estimate the effective dielectric function of the nano-composites. Calculated reflectance spectra in different samples are compared to the measured spectra using the experimentally measured dielectric function of these samples in the near-infrared region. Reflectance spectra after thermal annealing at different temperature are compared to show how the thermal treatment affects the optical properties of samples. Optimized structures are proposed for thermal emitters and absorbers with different volume fractions of W nanoparticles.

Performance enhancement of near-field thermoradiative devices using hyperbolic metamaterials.

We analyze a near-field thermoradiative device that consists of an indium arsenide-based photodiode under negative illumination. We analyze a possible enhancement of conversion efficiency by use of hyperbolic metamaterial (HMM) in place of bulk metallic heat sink. A stack of alternating thin-films of metal [zirconium carbide (ZrC)] and dielectric [silicon dioxide (SiO2)] is chosen to be the HMM under investigation. The presence of hyperbolic modes creates additional channels of near-field radiative transfer. An increased power density is predicted without a compromise in system efficiency.

Tunable wavelength selectivity of photonic metamaterials-based thermal devices.

Wavelength-selective thermal devices have great applications in concentrating solar power systems, high-temperature thermoelectric systems, and solar thermophotovoltaics (STPVs). Lack of high-temperature stability and spectrally selective emissivity in different wavelength regions limits their efficiency. We propose a one-dimensional HfO2/Al2O3-W nanocomposites/W/Al2O3/W multilayered photonic structure as potential wavelength selective thermal devices, and theoretically investigate the emission properties of the proposed Mie-resonance metamaterials from visible (VIS) to midinfrared (MIR) region. HfO2 thin layer is introduced to serve as an antireflection coating film and W layer acts as an IR reflection layer that enhances the absorptivity/emissivity in VIS and near-infrared (NIR) region while reducing the MIR emission simultaneously. Effects of geometric parameters are discussed, such as different radii and volume fractions of W nanoparticles, the thickness of Al2O3-W nanocomposites, and HfO2 thin film. The proposed thermal absorber and emitter exhibit nearly unity absorptance in both VIS and NIR regions, while the emittance approaches zero in the MIR region. The selective absorption/emission window is tunable by varying geometric parameters. The proposed solar thermal devices have great potentials in engineering applications such as STPVs and solar thermoelectric generator due to flexibility of geometric parameters and ease of fabrication.

Strain-induced modulation of near-field radiative transfer.

In this theoretical study, we present a near-field thermal modulator that exhibits change in radiative heat transfer when subjected to mechanical stress/strain. The device has two terminals at different temperatures separated by vacuum: one fixed and one stretchable. The stretchable side contains one-dimensional grating. When subjected to mechanical strain, the effective optical properties of the stretchable side are affected upon deformation of the grating. This results in modulation of surface waves across the interfaces influencing near-field radiative heat transfer. We show that for a separation of 100 nm, it is possible to achieve 25% change in radiative heat transfer for a strain of 10%.

A Review of Tunable Wavelength Selectivity of Metamaterials in Near-Field and Far-Field Radiative Thermal Transport.

Radiative thermal transport of metamaterials has begun to play a significant role in thermal science and has great engineering applications. When the key features of structures become comparable to the thermal wavelength at a particular temperature, a narrowband or wideband of wavelengths can be created or shifted in both the emission and reflection spectrum of nanoscale metamaterials. Due to the near-field effect, the phenomena of radiative wavelength selectivity become significant. These effects show strong promise for applications in thermophotovoltaic energy harvesting, nanoscale biosensing, and increased energy efficiency through radiative cooling in the near future. This review paper summarizes the recent progress and outlook of both near-field and far-field radiative heat transfer, different design structures of metamaterials, applications of unique thermal and optical properties, and focuses especially on exploration of the tunable radiative wavelength selectivity of nano-metamaterials.

Near-field thermal rectification devices using phase change periodic nanostructure.

We theoretically analyze two near-field thermal rectification devices: a radiative thermal diode and a thermal transistor that utilize a phase change material to achieve dynamic control over heat flow by exploiting metal-insulator transition of VO2 near 341 K. The thermal analogue of electronic diode allows high heat flow in one direction while it restricts the heat flow when the polarity of temperature gradient is reversed. We show that with the introduction of 1-D rectangular grating, thermal rectification is dramatically enhanced in the near-field due to reduced tunneling of surface waves across the interfaces for negative polarity. The radiative thermal transistor also works around phase transition temperature of VO2 and controls heat flow. We demonstrate a transistor-like behavior wherein heat flow across the source and the drain can be greatly varied by making a small change in gate temperature.