School of Chemical and Biomolecular Engineering, Cornell University , Ithaca, New York 14853, United States.
Langmuir. 2014 Feb 25;30(7):1788-98. doi: 10.1021/la404057g. Epub 2014 Feb 13.
During dropwise condensation from vapor onto a cooled surface, distributions of drops evolve by nucleation, growth, and coalescence. Drop surface coverage dictates the heat transfer characteristics and depends on both drop size and number of drops present on the surface at any given time. Thus, manipulating drop distributions is crucial to maximizing heat transfer. On earth, manipulation is achieved with gravity. However, in applications with small length scales or in low gravity environments, other methods of removal, such as a surface energy gradient, are required. This study examines how chemical modification of a cooled surface affects drop growth and coalescence, which in turn influences how a population of drops evolves. Steam is condensed onto a horizontally oriented surface that has been treated by silanization to deliver either a spatially uniform contact angle (hydrophilic, hydrophobic) or a continuous radial gradient of contact angles (hydrophobic to hydrophilic). The time evolution of number density and associated drop size distributions are measured. For a uniform surface, the shape of the drop size distribution is unique and can be used to identify the progress of condensation. In contrast, the drop size distribution for a gradient surface, relative to a uniform surface, shifts toward a population of small drops. The frequent sweeping of drops truncates maturation of the first generation of large drops and locks the distribution shape at the initial distribution. The absence of a shape change indicates that dropwise condensation has reached a steady state. Previous reports of heat transfer enhancement on chemical gradient surfaces can be explained by this shift toward smaller drops, from which the high heat transfer coefficients in dropwise condensation are attributed to. Terrestrial applications using gravity as the primary removal mechanism also stand to benefit from inclusion of gradient surfaces because the critical threshold size required for drop movement is reduced.
在蒸汽从蒸气滴落到冷却表面的过程中，液滴的分布通过成核、生长和聚结而演变。液滴表面覆盖率决定了传热特性，并且取决于在任何给定时间表面上的液滴大小和液滴数量。因此，操纵液滴分布对于最大化传热至关重要。在地球上，通过重力来实现操纵。但是，在小尺寸应用或低重力环境中，需要其他去除方法，例如表面能梯度。本研究研究了冷却表面的化学修饰如何影响液滴生长和聚结，这反过来又影响了液滴群体的演变。蒸汽凝结到水平取向的表面上，该表面已通过硅烷化处理以提供空间均匀的接触角（亲水，疏水）或连续的接触角径向梯度（从疏水到亲水）。测量了数密度和相关液滴尺寸分布的时间演化。对于均匀表面，液滴尺寸分布的形状是独特的，可以用来识别冷凝的进展。相比之下，对于梯度表面，相对于均匀表面，液滴尺寸分布向小液滴群体转移。液滴的频繁扫除截断了第一代大液滴的成熟过程，并将分布形状锁定在初始分布。没有形状变化表明点滴冷凝已达到稳定状态。先前关于化学梯度表面上传热增强的报告可以通过这种向较小液滴的转移来解释，这归因于点滴冷凝中的高热传递系数。由于减少了用于液滴移动的临界阈值尺寸，因此使用重力作为主要去除机制的地面应用也将从包括梯度表面中受益。
