The effect of droplet morphology on the heat transfer performance of micro-/nanostructured surfaces in dropwise condensation

Abstract

It has been proved that dropwise condensation (DWC) has better heat transfer than filmwise condensation. DWC occurs on hydrophobic or superhydrophobic surfaces (SHS). Low surface energy and surface roughness are two key specifications of SHS. Roughness leads to appearing of Cassie and Wenzel morphologies on SHS. In the Cassie state, droplet suspends over roughness while in the Wenzel state droplets penetrates through roughness. Single Cassie droplets have higher mobility (an advantage) than Wenzel droplets while having a lower heat transfer rate (a disadvantage). The advantage of Cassie droplets leads to that droplets depart the surface at lower radius, sweep other droplets and prepare fresh surface for nucleating of new droplets. Some researchers have shown that surfaces with Wenzel droplets have better heat transfer and some have indicated that surfaces with Cassie droplets have better performance. Hence, a numerical investigation has been done in this study to explore which of these two morphologies have better performance in DWC. Results show that advantage and disadvantage of the morphologies can prevail on the other according to surface conditions. For example, a surface with Cassie state droplets may have higher or lower total heat transfer than a surface with Wenzel droplets depending on roughness height.