Nanofibrous Biomaterial-Based Passive Cooling Paint Structurally Linked by Alkane-Oleate Interactions.

Passive radiative cooling materials, which provide cooling without consuming electricity, are widely recognized as an important technology for reducing greenhouse gas emissions and delivering thermal comfort to less industrialized communities. Optimizing thermal and optical properties is of primary importance for these materials, but for real-world utilization, ease of application and scalability also require significant emphasis. In this work, we embed the biomaterial hydroxyapatite, in the form of nanoscale fibers, within an oil-based medium to achieve passive cooling from an easy-to-apply paint-like solution. The chemical structure and bonding behaviors of this mixture are studied in detail using FTIR, providing transferable conclusions for pigment-like passive cooling solutions. By reflecting 95% of solar energy and emitting 92% of its radiative output through the atmospheric transparency window, this composite material realizes an average subambient cooling performance of 3.7 °C in outdoor conditions under a mean solar irradiance of 800 W m-2. The inflammability of the material provides enhanced durability as well as unique opportunities for recycling which promote circular economic practices. Finally, the surface structure can be easily altered to tune bonding behaviors and hydrophobicity, making it an ideal passive cooling coating candidate for outdoor applications.

Optical Epidermal Mimicry from Ultraviolet to Infrared Wavelengths.

Optical tissue phantoms present substantial value for medical imaging and therapeutic applications. We have developed an epidermal tissue phantom to mimic the optical properties of human skin from the ultraviolet to the infrared region, exceeding the breadth of existing studies. An epoxy matrix is combined with melanin-mimicking polydopamine via a cost-effective fabrication strategy. Reflectance and transmittance measurements enable calculation of the wavelength-dependent complex refractive index and absorption coefficient. Results are compared with literature data to establish agreement with a real human epidermis. By analyzing emissive power at a typical skin temperature, the epidermal tissue phantom is shown to accurately mimic the radiative properties of human skin. This simple, multifunctional material represents a promising substitute for human tissue for a variety of medical and bioengineering applications.

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.

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.

Piston-Type Optical Modulator for Dynamic Thermal Radiation Tuning Applications.

This study introduces a movable piston-like structure that provides a simple and cost-effective avenue for dynamically tuning thermal radiation. This structure leverages two materials with dissimilar optical responses-graphite and aluminum-to modulate from a state of high reflectance to a state of high absorptance. A cavity is created in the graphite to house an aluminum cylinder, which is displaced to actuate the device. In its raised state, the large aluminum surface area promotes a highly reflective response, while in its lowered state, the expanded graphite surface area and blackbody cavity-like interactions significantly enhance absorptance. By optimizing the area ratio, reflectance tunability of over 30% is achieved for nearly the entire ultraviolet, visible, and near-infrared wavelength regions. Furthermore, a theoretical analysis postulates wavelength-dependent effectivenesses as high as 0.70 for this method, indicating that tunabilities approaching 70% can be achieved by exploiting near-ideal absorbers and reflectors. The analog nature of this control method allows for an infinitely variable optical response between the upper and lower bounds of the device. These valuable characteristics would enable this material structure to serve practical applications, such as reducing cost and energy requirements for environmental temperature management operations.

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.