Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LISs) having ultralow contact angle hysteresis (<3°) for stable dropwise condensation of ethanol (γ ≈ 23 mN/m) and hexane (γ ≈ 19 mN/m). Using a combination of optical imaging and rigorous heat transfer measurements in a controlled environmental chamber free from noncondensable gases (<4 Pa), we characterize the condensation behavior of ethanol and hexane on ultrascalable nanostructured CuO surfaces impregnated with fluorinated lubricants having varying viscosities (0.496 < μ < 5.216 Pa•s) and chemical structures (branched versus linear, Krytox and Fomblin). We demonstrate stable dropwise condensation of ethanol and hexane on LISs impregnated with Krytox 1525, attaining about 200% enhancement in condensation heat transfer coefficient for both fluids compared to filmwise condensation on hydrophobic surfaces. In contrast to previous studies, we use 7 h of steady dropwise condensation experiments to demonstrate the importance of rational lubricant selection to minimize lubricant drainage and maximize LIS durability. This work not only demonstrates an avenue to achieving stable dropwise condensation of ethanol and hexane, it develops the fundamental design principles for creating durable LISs for enhanced condensation heat transfer of low-surfacetension fluids.
Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( t), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.
Liquid-vapor phase-change cooling has a significant potential to facilitate the development of highly dense electronics by leveraging latent heat during the phase transition to remove heat from hotspots. A promising form of liquid–vapor phase-change cooling is coalescence-induced jumping droplet condensation, where droplet growth results in coalescence and gravity-independent jumping from the cold surface due to capillary-inertial energy conversion. Once the departed droplets reach the hotspot, heat is extracted via evaporation and through vapor return, subsequently spreading to the cold surface via condensation. Realizing the full potential of jumping droplet cooling requires a detailed understanding of the physics governing the process. Here, we examine the fundamental thermal and hydrodynamic limits of jumping droplet condensation. We demonstrate that jumping is mainly governed by the rate of droplet growth and fluid thermophysical properties. Timescale analysis demonstrates that the upper bound of water vapor jumping droplet condensation critical heat flux is ∼ 20 kW/cm2, significantly higher than that experimentally observed thus far due to surface structure limitations. Analysis of a wide range of available working fluids shows that liquid metals such as Li, Na, and Hg can obtain superior performance when compared to water.
Electrifying both stationary and mobile systems requires ultra-compact, lightweight power electronics and electric machines. Increasing the volumetric and gravimetric density of these systems is constrained, however, by the capacity to remove heat from these assemblies. A promising method for extracting heat is jumping droplet condensation, which can address both spatially and temporally changing hotspots. Yet, disagreement exists in the literature about the maximum attainable heat flux for water-based, droplet jumping devices such as vapor chambers, with values ranging from 5 to 500 W/cm2. Here, using thermal measurements and optical imaging in pure vapor conditions, we directly observe the hydrodynamics occurring inside of a jumping droplet vapor chamber. Our experiments show that flooding is the key obstacle limiting jumping droplet mass flux to hot spots, limiting heat transfer to less than 15 W/cm2. These results indicate that past works reporting high heat fluxes benefited from other hot spot cooling pathways such as previously observed liquid bridges formed due to flooding. To test our hypothesis, we characterize progressive flooding on a variety of structured surfaces ranging in length-scale from 100 nm to 10 μm. Progressive flooding was delayed by decreasing the length-scale of the surface structures, which supports recent observations in the literature. Our work not only helps to understand the wide variability of past results quantifying droplet jumping heat transfer, but also provides design guidelines for the development of surfaces that are capable of maintaining enhanced jumping droplet condensation.
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