Se-Yeon Heo

This paper describes laser exposure to tune the infrared (IR) emissivity of a film of eutectic gallium indium (EGaIn) particles. EGaIn – a liquid metal at room temperature – forms a native oxide that keeps particles of the metal from spontaneously percolating. Photothermal energy from a CO₂ laser percolates the particles into a conductive network. Here, it also causes a decrease in the IR emissivity of the film of particles from 0.4 to 0.24 over the range of 7.5–13 µm wavelength (measured by an IR camera) with the increase of laser fluence from 1.4 to 1.9 J cm⁻². The particles percolate most prominently at the bottom of the film, and thus, the apparent surface roughness does not change with laser exposure. This finding suggests the decrease in emissivity is not due to changes in the film's topography. Instead, the change in IR emissivity is attributed to a loss of the surface plasmonic resonance effect of EGaIn particles in the IR range after the sintering, which is confirmed by optical simulations. As a demonstration, it is shown that the ability to change the emissivity makes it possible to encrypt messages and camouflage laser‐processed patterns.

Passive radiative cooling has been spotlighted as a promising energy-saving cooling technology owing to its energy-free and zero-carbon emission for addressing global energy and climate crises. Although radiative cooling can significantly save cooling energy in hot weather, it inevitably accompanies undesirable cooling in cold weather resulting from a single-state of strong thermal emission. Dual-state emitters have recently been developed for self-adaptive thermoregulation, but it still has energy loss in moderate weather. Herein, we report a 'continuous' temperature-regulation system by introducing an infrared (IR) polarization valve as the energy-balancing channel. The proposed scheme controls the emitter temperature simply by in-plane rotation of the IR polarizer as if closing and opening the value, which presents the heating/cooling capabilities (-17 to 51 W/m²) and the energy-saving of >20 GJ/year compared with the conventional emitters in all climate zones. Outdoor experiments demonstrate the precise temperature regulation with the range of ΔT_cool >2ºC. This proof-of-concept demonstration in the outdoors verifies our approach's reliability, suggesting its applicability in residential buildings, farms, and electronic devices.

Recent advances in passive radiative cooling systems describe a variety of strategies to enhance cooling efficiency, while the integration of such technology with a bioinspired design using biodegradable materials can offer a research opportunity to generate energy in a sustainable manner, favorable for the temperature/climate system of the planet. Here, we introduce stretchable and ecoresorbable radiative cooling/heating systems engineered with zebra stripe–like patterns that enable the generation of a large in-plane temperature gradient for thermoelectric generation. A comprehensive study of materials with theoretical evaluations validates the ability to accomplish the target performances even under external mechanical strains, while all systems eventually disappear under physiological conditions. Use of the zebra print for selective radiative heating demonstrates an unexpected level of temperature difference compared to use of radiative cooling emitters alone, which enables producing energy through resorbable silicon-based thermoelectric devices. The overall result suggests the potential of scalable, ecofriendly renewable energy systems.

Radiative cooling, which is based on radiative heat exchange between the universe and Earth, can provide a passive and renewable route to reducing energy consumption. This review highlights recent progress and continued efforts for radiative cooling.

Passive radiative cooling functions by reflecting the solar spectrum and emitting infrared waves in broadband or selectively. However, cooling enclosed spaces that trap heat by greenhouse effect remains a challenge. We present a Janus emitter (JET) consisting of an Ag–polydimethylsiloxane layer on micropatterned quartz substrate. The induced spoof surface plasmon polariton helps overcome inherent emissivity loss of the polymer and creates near-ideal selective and broadband emission on the separate sides. This design results in not only remarkable surface cooling when the JET is attached with either side facing outwards but also space cooling when used as an enclosure wall. Thus, the JET can passively mitigate the greenhouse effect in enclosures while offering surface cooling performance comparable to conventional radiative coolers.