Rooftop solar thermal collectors have the potential to meet residential heating demands if deployed efficiently at low solar irradiance (i.e., 1 sun). The efficiency of solar thermal collectors depends on their ability to absorb incoming solar energy and minimize thermal losses. Most techniques utilize a vacuum gap between the solar absorber and the surroundings to eliminate conduction and convection losses, in combination with surface coatings to minimize reradiation losses. Here, we present an alternative approach that operates at atmospheric pressure with simple, black, absorbing surfaces. Silica based aerogels coated on black surfaces have the potential to act as simple and inexpensive solar thermal collectors because of their high transmission to solar radiation and low transmission to thermal radiation. To demonstrate their heat-trapping properties, we fabricated tetramethyl orthosilicate-based silica aerogels. A hydrophilic aerogel with a thickness of 1 cm exhibited a solar-averaged transmission of 76% and thermally averaged transmission of ≈1% (at 100 °C). To minimize unwanted solar absorption by O-H groups, we functionalized the aerogel to be hydrophobic, resulting in a solar-averaged transmission of 88%. To provide a deeper understanding of the link between aerogel properties and overall efficiency, we developed a coupled radiative-conductive heat transfer model and used it to predict solar thermal performance. Instantaneous solar thermal efficiencies approaching 55% at 1 sun and 80 °C were predicted. This study sheds light on the applicability of silica aerogels on black coatings for solar thermal collectors and offers design priorities for next-generation solar thermal aerogels.

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Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a given time interval. However, the transient nature coupled with the significant mass-transfer-governed gas dynamics occurring at the droplet three-phase contact line makes the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines makes the decoupling of these processes impossible with classical transient methods. Here, we present a method to measure the rate of evaporation of spatially and temporally steady droplets. By utilizing a piezoelectric dispenser to feed microscale droplets (R ≈ 9 μm) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using our steady technique, we studied water evaporation of droplets having base radii ranging from 20 to 250 μm on surfaces of different functionalities (45° ≤ θ ≤ 162°, where θ is the apparent advancing contact angle). We benchmarked our technique with the classical unsteady method, showing an improvement of 140% in evaporation rate measurement accuracy. Our work not only characterizes the evaporation dynamics on functional surfaces but also provides an experimental platform to finally enable the decoupling of the complex physics governing the ubiquitous droplet evaporation process.

Considering partial solubility of BMIm in water (γ l = 42 mN m −1); [13b] b) Considering no solubility of BMIm in water (γ l = 72.7 mN m −1). [25]

The rapid anthropomorphic emission of greenhouse gases is contributing to global climate change, resulting in the increased frequency of extreme weather events, including unexpected snow, frost, and ice accretion in warmer regions that typically do not encounter these conditions. Adverse weather events create challenges for energy systems such as wind turbines and photovoltaics. To maintain energy efficiently and operational fidelity, snow, frost, and ice need to be removed efficiently and rapidly. State‐of‐the‐art removal methods are energy‐intensive (energy density > 30 J cm−2) and slow (>1 min). Here, pulsed Joule heating is developed on transparent self‐cleaning interfaces, demonstrating interfacial desnowing, defrosting, and deicing with energy efficiency (energy density < 10 J cm−2) and rapidity (≈1 s) beyond what is currently available. The transparency and self‐cleaning are tailored to remove both snow and dust while ensuring minimal interference with optical light absorption. It is experimentally demonstrated a multi‐functional coating material on a commercial photovoltaic cell, demonstrating efficient energy generation recovery and rapid ice/snow removal with minimal energy consumption. Through the elimination of accretion, this technology can potentially widen the applicability of photovoltaics and wind technologies to globally promising locations, potentially further reducing greenhouse gas emissions and global climate change.