Effect of Mini/Micro/Nanostructures on Filmwise Condensation of Low-Surface-Tension Fluids

Micro/nanostructured surfaces have been widely explored to enhance condensation heat transfer over the past decades. When there is no flooding, micro/nanostructures can enable dropwise condensation by reducing solid-droplet adhesion. However, micro/nanostructures have mixed effects on filmwise condensation because the structures can simultaneously thin the condensate film and increase the fluid-solid friction. Although oil infusion of structured surfaces has recently been shown to render filmwise condensation dropwise in many cases, challenges remain in the case of extremely low-surface-tension fluids. This work aims to provide a unified experimental platform and study the impact of mini/micro/nanostructures on condensation heat transfer of low-surface-tension fluids in a customized environmental chamber. We first investigate the effect of microstructures, hydrophobic coating, as well as oil infusion on the filmwise condensation of a low-surface-tension fluid, e.g., refrigerant, on microporous aluminum surfaces. And we show that for low-surface-tension condensates, microstructures, hydrophobic coating, or oil infusion do not play a considerable role in enhancing or deteriorating heat transfer. Next, we study how the addition of nanostructures affects the condensation performance of the refrigerant on copper mini-fin structures. It is found that nanostructures slightly deteriorate the condensation performance due to the dominance of solid-liquid friction, although the performance of these mini-fins with nanostructured surfaces is still better than that of the mini-pin-fins. These results provide guidelines of designing mini/micro/nanoscale surface structures for enhanced condensation applications.