RESUMEN
Surface modification, such as bioinspired nanostructured doubly reentrant surfaces that have presented superhydrophobic wettability even under low-surface-tension liquid, is a very promising technology for controlling droplet dynamics, heat transfer, and evaporation. In this article, we investigate the interfacial effects of nanostructured doubly reentrant surfaces on the flow behaviors and local concentration evolution during the evaporation of an ethanol/water multicomponent droplet. Using particle image velocimetry (PIV) and novel aggregate-induced emission-based (AIE) techniques, the flow patterns and local concentration distributions on both hydrophobic and nanostructured doubly reentrant surfaces were probed and compared. It is found that in addition to the established Marangoni flow-dominated stage, transition stage, and buoyancy-induced flow-dominated stage, a new transition stage and a rolling stage for the nanostructured doubly reentrant surface are detected in the late evaporation period. Differences in the local concentration distribution evolution occur depending on the hydrophobicity of the surface on which the droplet is placed. For the hydrophobic surface, a nonuniform local concentration distribution exists consistently, with a high water fraction in a shell-shaped region near the liquid-air interface and a secondary concentration gradient within this shell-shaped region. The concentration distribution on the nanostructured doubly reentrant surface evolves in a more complex manner, with a strip-shaped region of high water fraction forming in the intermediate stage and then reorganized by rolling flow in the late stage. Finally, theoretical analysis combining PIV and AIE visualization results reveals that the variations in droplet concentration distributions on surfaces with different hydrophobicities exert a significant impact on evaporative behaviors. These behaviors, in turn, affect the evolution of the local concentration distribution.
RESUMEN
Supercooling of water complicates phase change dynamics, the understanding of which remains limited yet vital to energy-related and aerospace processes. Here, we investigate the freezing and jumping dynamics of supercooled water droplets on superhydrophobic surfaces, induced by a remarkable vaporization momentum, in a low-pressure environment. The vaporization momentum arises from the vaporization at droplet's free surface, progressed and intensified by recalescence, subsequently inducing droplet compression and finally self-jumping. By incorporating liquid-gas-solid phase changes involving vaporization, freezing recalescence, and liquid-solid interactions, we resolve the vaporization momentum and droplet dynamics, revealing a size-scaled jumping velocity and a nucleation-governed jumping direction. A droplet-size-defined regime map is established, distinguishing the vaporization-momentum-dominated self-jumping from evaporative drying and overpressure-initiated levitation, all induced by depressurization and vaporization. Our findings illuminate the role of supercooling and low-pressure mediated phase change in shaping fluid transport dynamics, with implications for passive anti-icing, advanced cooling, and climate physics.
RESUMEN
The rapid development of power-intensive and flexible electronic devices requires thinner heat-dissipation devices with better thermal performance. Ultra-thin flat heat pipe (UTFHP) with striped wick structure is a promising candidate for this application, but its wick structure and thermal performance have not yet been thoroughly studied and optimized for the small concentrated heat source, which is commonly encountered in electronics. In this study, several concentrated striped composite wick (CSCW) structures for 0.6 mm thick UTFHPs are proposed and experimentally investigated. The CSCW consists of copper foam with striped passages converging in the heating zone and double layers of copper screen mesh. The thermal performance of UTFHPs with various composite wick structures is experimentally evaluated. UTFHPs with the proposed structures are also compared with a UTFHP with a more conventional parallel passage composite wick structure. Experimental results show that the CSCW with the hollow structure at the evaporation section is preferred, due to the directed liquid working medium reflux and a large vapor-liquid evaporation interface. Besides, the passage width of the copper foam significantly affects the thermal performance. With the best-performing wick structure, the UTFHP gives the lowest thermal resistance of 0.79 °C/W at a heat load of 23.34 W. Its effective thermal conductivity is approximately 7 times that of copper. The proposed striped wick structure for UTFHPs provides an alternative to handle the hot-spot challenge of electronic devices.
RESUMEN
Surface modification, such as hydrophobic network modification, is very promising technology to control droplet dynamics, heat transfer, and evaporation. However, fundamental mechanisms of how these chemically patterned surfaces affect the droplet evaporation dynamics and predictions of evaporation rates are still lacking. In the present work, we systematically investigated the full process of droplet evaporation dynamics on hydrophobic network surfaces and distinguished four different stages: constant contact line (CCL) stage, constant contact angle (CCA) stage, pattern-pinning (PP) stage, and moving contact line (MCL) stage. We further developed a general model considering the pinning and depinning forces to accurately predict the evaporation transition from PP to MCL stages (i.e., critical receding contact angle, θcr). As for the influence of the chemically patterned surface on the evaporation rate, a corrected contact line length was considered and combined with the well-known Rowan and Erbil's models. Finally, a general model was thus proposed and showed successful predictions for the evaporation durations of each stage.
