Condensation and Droplet Dynamics Under Shear at Superhydrophobic Surfaces

当水滴碰到物体表面时，它会扩散并弄湿物体表面，或者凝结起来。这取决于表面的润湿特性。在给汽车打蜡时，最好准备好表面，使水滴向上聚集并滚下。然而，如果在表面喷涂油漆，液滴扩散是可取的。使水滴聚集的表面称为超疏水表面。这种表面存在于自然界中（例如，荷叶），也可以人工改造。如果冷的表面与热的蒸气接触，水滴就会在表面凝结。这是在许多使用冷凝器的工业过程中使用的原理，例如在发电厂。这些液滴可以合并形成一层薄膜。如果冷凝器表面被设计成超疏水的，冷凝的液滴会滚掉，不会形成薄膜。这将提高冷凝器的效率。本项目研究超疏水表面上的冷凝过程。它探索液滴的形成和生长，液滴脱离，以及超疏水表面的传热。该奖项为研究生和本科生提供参与研究的机会。调查小组还将为教师准备学习模块。本文主要研究了剪切流动中超疏水液滴的对流热传递和流体动力学特性。由于表面边界条件的明显改变，与SH壁面接触的液体的基本流体动力学和对流热输运物理表现出与经典行为的根本偏离。先前在首席研究员实验室的工作表明，静止液滴的行为与经典行为有很大的不同，并且具有超疏水边界的绝热两相流的压降也有所降低。由于两个原因，预计与SH表面接触的两相冷凝流将经历与经典行为的重大偏离。首先，超疏水性对液固表面张力有巨大的影响，尤其是薄膜和液滴，它们在两相流体系中占主导地位。其次，当水滴式冷凝发生时，冷凝传热系数比与膜式冷凝相关的传热系数增加一个数量级。如果在发生冷凝的地方使用SH壁，由于表面附着力的改变，液滴尺寸的减小和液滴流动性的增加，液滴向冷凝的通道长度可能会显著增加。这些影响有可能为下一代冷凝器技术提供变革性突破。目前还没有研究系统地探讨了SH型通道壁面对冷凝流动中剪切作用下液滴行为的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

When a water droplet meets a surface, it can spread and wet the surface or bead up. This depends on the wetting property of the surface. When waxing a car, it is desirable to prepare the surface such that droplets bead up and roll off. Droplet spreading, however, is desirable if paint is sprayed on a surface. A surface that causes the droplets to bead up is called superhydrophobic. Such surfaces exist in nature (for example, the lotus leaf) and can be also engineered artificially. If a cold surface is in contact with hot vapor, water droplets will condense on it. This is the principle used in many industrial processes that use condensers, such as in power plants. These droplets can merge to form a thin film. If the condenser surface is designed to be superhydrophobic, the condensed droplets will roll off and do not form a film. This will enhance the efficiency of the condenser. This project investigates the condensation process on superhydrophobic surfaces. It explores droplet formation and growth, droplet detachment, and heat transfer on superhydrophobic surfaces. This award provides opportunities for graduate and undergraduate students to participate in research. The team of investigators will also prepare learning modules for teachers.This research focuses on an investigation to characterize convective thermal transport and fluid dynamics of condensed droplets at superhydrophobic (SH) surfaces in shear flow. The fundamental hydrodynamic and convective thermal transport physics for liquid in contact with SH walls exhibit radical departures from classical behavior due to distinct alterations to the boundary conditions at the surface. Previous work in the principal investigator's lab has shown dramatic departures from classical behavior for quiescent droplets and reduced pressure drop for adiabatic, two-phase flows with superhydrophobic boundaries. It is expected that two-phase condensing flows in contact with SH surfaces will experience significant departure from classical behavior for two reasons. First, superhydrophobicity exerts enormous influence on the liquid-solid surface tension, especially for thin films and droplets, which dominate large portions of two-phase flow systems. Second, when drop-wise condensation occurs, the condensing heat transfer coefficient experiences an order of magnitude increase compared to that associated with film-wise condensation. If SH walls are employed where condensation is occurring, the length of channel over which drop-wise condensation prevails may be significantly increased due to alteration of surface adhesion forces, reduction in nominal droplet size, and increased droplet mobility. These effects have the potential to provide transformative breakthroughs in next generation condenser technologies. No previous study has systematically explored the influence of SH channel walls on the droplet behavior under shear in condensing flows.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.