Condensation and Droplet Dynamics Under Shear at Superhydrophobic Surfaces

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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.