Surface modifications to enhance dropwise condensation

Abstract

Condensation is of paramount importance in numerous technological applications where phase change processes take place. The physical nature of condensation from vapor to liquid can impose limits on the speed of the process, especially at the interface of the vapor, surface, and liquid. However, the nature of the surfaces themselves, especially those with low wettability, have been shown to enhance and promote condensation. This review is focused on the advances in surface wettability related to enhancement of condensation processes and while both filmwise and dropwise modes are addressed, emphasis is given to the enhancement of drop-wise condensation which produces higher heat transfer. Fundamental theoretical models governing surface wetting characteristics and heat transfer in condensation are presented. Methods to reduce wetting including coatings of low surface energy materials such as polymers, noble metals, rare-earth oxides, organic monolayers, switchable wettability surfaces, and ion-implantation are discussed. Certain surface features along with low surface energy coatings can enable droplet removal at much smaller sizes and can even control the droplet movements during condensation. Special emphasis is given to characteristics of these surface features including roughness, porous structures, hybrid surfaces, micro-grooved surfaces, and lubricant infused surfaces. Additionally, challenges and opportunities associated with the use of these surfaces in condensation are discussed. It is noted that dropwise condensation is prone to surface flooding at higher subcoolings. Moreover, the surface design is critical to choice of working fluid and further developments are required to achieve dropwise condensation of working fluids of low surface tension. The current review focusses on recent technological advances that may assist with the development of dropwise condensation for industrial, domestic and consumer related heat transfer devices, along with the concomitant superior heat transfer capabilities over the filmwise mode.