Enhanced Radiative/Evaporative Cooling from Air Eddies above Zebra Stripes: A Deductive Approach

Passive radiative cooling offers a sustainable way to effective temperature reduction, utilizing thermal imbalance of earth and universe and minimizing heat gain from surroundings. Numerous studies have demonstrated its environmentally friendly nature and high cooling efficiency, establishing it as a highly promising solution for sustainable cooling applications. In recent years, scientific research has unveiled that the cooling potential of evaporation (~320 Wm^-2) surpasses that of radiation (~150 Wm^-2), and implementation of evaporative and radiative cooling approaches has successfully achieved superior cooling performance compared to single reliance on radiative cooling methods. But evaporative cooling in nature exist in way for obtaining homeostasis; During the summer season, both plants exhibit active transpiration, and mammals experience increased sweat evaporation, as a means to regulate their body temperature and counteract the rise caused by the elevated ambient temperature. Conversely, in cold winter conditions, these phenomena occur less frequently as a mechanism to maintain a stable body temperature. Indeed, equation for evaporative rate(m’) express that the rate of evaporation intensifies as temperature increases.<br/><br/>m’ = θ(Χ_s(T_body)-X(T_amb)) (1)<br/><br/>where θ is evaporative coefficient, which equals to 25+19v where v is the velocity of air. X_s denotes the maximum humidity ratio of saturated air at the same temperature as the body. X is humidity ratio of ambient air at the temperature T_amb. Since the X_s increases dramatically with increased body temperature(T_body), smaller Tbody results in decreased m'. In other words, the cooler the body, the less evaporation occurs.<br/><br/>As for maximizing efficiency of evaporation, utilizing the heating power of nature, specifically the sun, has gained significant research interest, in modern society. This method, known as solar-driven interfacial evaporation, focuses on heating only the interface (i.e., the surface) rather than heating the entire medium (i.e., bulk heating). By reducing the thermal losses, an interfacial evaporation approach by selectively heating the air/liquid interface has demonstrated to have 45% higher evaporation efficiency than bulk heating based evaporation. Here we suppose that instead of merely heating the air-sample interface, generating convective heat flow of air around the sample could boost surface evaporation, inspired by the scientific discoveries presented by Cobb et al. (2022); The striking black and white striped pelage of zebras maintain a temperature difference, generating convective airflow as warm air rises from the black stripes and cool air descends onto the white stripes. This airflow, above and between the stripes, can induce chaotic or turbulent air patterns due to convection currents and facilitate the exchange of air and water vapor, thus aiding evaporative cooling.<br/><br/>In order to successfully demonstrate this, we employ zebra- patterned cellulose acetate(CA) fiberous network on the surface of the LiBr hydrogel sample. The hydrogel possesses the ability to absorb moisture from the atmosphere and retain the water within the structure, while the CA networks desired to wick the water for evaporation, facilitating continuous evaporative cooling. Given the numerical study presented by Austin et al. (2022) that zebra-striped roof buildings demonstrate lower internal temperatures in comparison to all-white roof buildings, the proposed statement appears to be plausible. The incorporation of a zebra-pattern atop a hydrogel, accompanied by localized heating of the sample surface corresponding to the regions occupied by the black stripes, paradoxically, has the potential to enhance the cooling performance beyond that achievable through sole radiative cooling or radiative/evaporative cooling methods.