Yan Xiao, Chen Feng, Sett Soumyadip, Chavan Shreyas, Li Hang, Feng Lezhou, Li Longnan, Zhao Fulong, Zhao Chongyan, Huang Zhiyong, Miljkovic Nenad
Department of Mechanical Science and Engineering , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.
Institute of Nuclear and New Energy Technology, Tsinghua University , Beijing 100084 , China.
ACS Nano. 2019 Jul 23;13(7):8169-8184. doi: 10.1021/acsnano.9b03275. Epub 2019 Jul 11.
With the recent advances in surface fabrication technologies, condensation heat transfer has seen a renaissance. Hydrophobic and superhydrophobic surfaces have all been employed as a means to enhance condensate shedding, enabling micrometric droplet departure length scales. One of the main bottlenecks for achieving higher condensation efficiencies is the difficulty of shedding sub-10 μm droplets due to the increasing role played by surface adhesion and viscous limitations at nanometric length scales. To enable ultraefficient droplet shedding, we demonstrate hierarchical condensation on rationally designed copper oxide microhill structures covered with nanoscale features that enable large (∼100 μm) condensate droplets on top of the microstructures to coexist with smaller (<1 μm) droplets beneath. We use high-speed optical microscopy and focal plane shift imaging to show that hierarchical condensation is capable of efficiently removing sub-10-μm condensate droplets both coalescence and divergent-track-assisted droplet self-transport toward the large suspended Cassie-Baxter (CB) state droplets, which eventually shed classical gravitational shedding and thereby avoid vapor side limitations encountered with droplet jumping. Interestingly, experimental growth rate analysis showed that the presence of large CB droplets accelerates individual underlying droplet growth by ∼21% when compared to identically sized droplets not residing beneath CB droplets. Furthermore, the steady droplet shedding mechanism shifted the droplet size distribution toward smaller sizes, with ∼70% of observable underlying droplets having radii of ≤5 μm compared to ∼30% for droplets growing without shading. To elucidate the overall heat transfer performance, an analytical model was developed to show hierarchical condensation has the potential to break the limits of minimum droplet departure size governed by finite surface adhesion and viscous effects through the tailoring of structure length scale, coalescence, and self-transport. More importantly, abrasive wear tests showed that hierarchical condensation has good durability against mechanical damage to the surface. Our study not only demonstrates the potential of hierarchical condensation as a means to break the limitations of droplet jumping, it develops rational design guidelines for micro/nanostructured surfaces to enable excellent heat transfer performance as well as extended durability.
随着表面制造技术的最新进展，冷凝传热迎来了复兴。疏水和超疏水表面都已被用作增强冷凝液脱落的手段，实现了微米级液滴的脱离长度尺度。实现更高冷凝效率的主要瓶颈之一是由于在纳米长度尺度上表面附着力和粘性限制的作用增加，难以脱落小于10μm的液滴。为了实现超高效的液滴脱落，我们展示了在合理设计的覆盖有纳米级特征的氧化铜微丘结构上的分级冷凝，这种结构使得微结构顶部的大（约100μm）冷凝液滴能够与下方较小（<1μm）的液滴共存。我们使用高速光学显微镜和焦平面位移成像来表明分级冷凝能够通过聚结和发散轨迹辅助的液滴自运输有效地去除小于10μm的冷凝液滴，使其朝向大的悬浮卡西 - 巴克斯特（CB）态液滴，这些大液滴最终通过经典的重力脱落，从而避免了液滴跳跃时遇到的蒸汽侧限制。有趣的是，实验生长速率分析表明，与不存在于CB液滴下方的相同尺寸液滴相比，大的CB液滴的存在使单个下层液滴的生长加速了约21%。此外，稳定的液滴脱落机制使液滴尺寸分布向更小尺寸转变，与无遮蔽生长的液滴相比，约70%可观察到的下层液滴半径≤5μm，而无遮蔽生长的液滴这一比例约为30%。为了阐明整体传热性能，开发了一个分析模型，以表明分级冷凝有潜力通过调整结构长度尺度、聚结和自运输来突破由有限表面附着力和粘性效应所决定的最小液滴脱离尺寸的限制。更重要的是，磨损试验表明分级冷凝对表面机械损伤具有良好的耐久性。我们的研究不仅证明了分级冷凝作为打破液滴跳跃限制手段的潜力，还为微/纳米结构表面制定了合理的设计指南，以实现优异的传热性能以及延长耐久性。
