Sun Jianxing, Jiang Xinyu, Weisensee Patricia B
Department of Mechanical Engineering & Materials Science, Washington University in St. Louis, Saint Louis, Missouri 63130, United States.
Institute of Materials Science and Engineering, Washington University in St. Louis, Saint Louis, Missouri 63130, United States.
Langmuir. 2021 Nov 9;37(44):12790-12801. doi: 10.1021/acs.langmuir.1c01559. Epub 2021 Oct 26.
Lubricant-infused surfaces (LISs) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation free-energy barrier and high droplet mobility. Recent studies showed that oil menisci surrounding condensate microdroplets form distinct oil-rich and oil-poor regions. These topographical differences in the oil surface cause water microdroplets to rigorously self-propel long distances, continuously redistributing the oil film and potentially refreshing the surface for re-nucleation. However, the dynamic interplay between oil film redistribution, microdroplet self-propulsion, and droplet nucleation and growth is not yet understood. Using high-speed microscopy, we reveal that during water condensation on LISs, the smallest visible droplets (diameter: ∼1 μm, qualitatively representing nucleation) predominantly emerge in oil-poor regions due to a lower nucleation free-energy barrier. Considering the significant heat transfer performance of microdroplets (<10 μm) and transient characteristic of microdroplet movement, we compare the apparent nucleation rate density and water collection rate for LISs with oils of different viscosities and a solid hydrophobic surface at a wide range of subcooling temperatures. Generally, the lowest lubricant viscosity leads to the highest nucleation rate density. We characterize the length and frequency of microdroplet movement and attribute the nucleation enhancement primarily to higher droplet mobility and surface refreshing frequency. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed nonlinearity between LISs and the solid hydrophobic surface, we introduce two dominant regimes that influence the condensation efficiency: mobility-limited and coalescence-limited. We compare these regimes based on droplet growth rates and water collection rates on the different surfaces. Our findings advance the understanding of dynamic water-lubricant interactions and provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies.
注入润滑剂的表面（LISs）由于具有低成核自由能垒和高液滴迁移率，能够促进稳定的滴状冷凝并提高传热速率。最近的研究表明，冷凝微滴周围的油弯月面形成了明显的富油区和贫油区。油表面的这些地形差异导致水微滴能够强劲地自行远距离移动，不断重新分布油膜，并有可能使表面焕然一新以进行再成核。然而，油膜重新分布、微滴自行推进以及液滴成核与生长之间的动态相互作用尚未得到理解。通过高速显微镜，我们发现，在LISs上的水冷凝过程中，由于成核自由能垒较低，最小的可见液滴（直径：约1μm，定性地代表成核）主要出现在贫油区。考虑到微滴（<10μm）显著的传热性能以及微滴运动的瞬态特性，我们在很宽的过冷温度范围内，比较了具有不同粘度的油的LISs以及固体疏水表面的表观成核速率密度和集水速率。一般来说，润滑剂粘度最低时成核速率密度最高。我们对微滴运动的长度和频率进行了表征，并将成核增强主要归因于更高的液滴迁移率和表面刷新频率。有趣且出人意料的是，在高过冷温度下，疏水表面的性能优于高粘度LISs，但在低温差下通常不如任何一种测试的LISs。为了解释在LISs和固体疏水表面之间观察到的非线性关系，我们引入了两种影响冷凝效率的主导机制：迁移率限制机制和聚并限制机制。我们根据不同表面上的液滴生长速率和集水速率对这些机制进行了比较。我们的研究结果推进了对动态水 - 润滑剂相互作用的理解，并为选择用于提高滴状冷凝和集水效率的表面提供了新的设计原理。
