Wang Ruisong, Jakhar Karan, Ahmed Shoaib, Antao Dion S
J. Mike Walker'66 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States.
ACS Appl Mater Interfaces. 2021 Jul 28;13(29):34923-34934. doi: 10.1021/acsami.1c08496. Epub 2021 Jul 15.
Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.
由于高效的冷凝水去除能增强传热性能，滴状冷凝有利于众多工业及传热传质应用。有机官能硅烷自组装单分子层（SAM）涂层是用于促进水蒸气滴状冷凝的最常见超薄低表面能材料之一，因为它们具有最小的热阻和可扩展的合成工艺。这些SAM涂层在水蒸气冷凝过程中通常会迅速降解（即冷凝从高效的滴状模式转变为低效的膜状模式）。更重要的是，冷凝介导的涂层降解/失效机制仍然未知和/或未经证实。在这项工作中，我们对水蒸气冷凝介导的有机官能硅烷SAM涂层降解形成了机理认识，并通过在硅/二氧化硅衬底上进行可控的涂层合成程序来验证我们的假设。我们进一步证明，在无水印的涂层环境中形成的原始有机官能硅烷SAM涂层在水蒸气冷凝过程中表现出卓越的长期稳定性。我们的分子/纳米级表面表征以及冷凝前后的传热测试表明，涂层环境中水分的存在会导致衬底出现未涂层区域，这些区域成为涂层降解的成核位点。通过阐明这些降解核形成的原因并展示一种抑制此类缺陷的方法，本研究为低表面能硅烷SAM涂层在水蒸气冷凝过程中降解的原因提供了新的见解。所提出的方法解决了一个关键瓶颈（即涂层失效），该瓶颈阻碍了工业中采用高效的滴状冷凝方法，并且将促进工业应用中强化相变传热技术的发展。
