Condensate droplet size distribution and heat transfer on hierarchical slippery lubricant infused porous surfaces

In recent years, slippery lubricant infused porous surfaces (SLIPSs) have received important attention due to their excellent performance in applications such as condensation, low friction, self-cleaning and anti-icing, which is owed to the presence of an infused lubricant or oil effectively decreasing the liquid-solid interactions and enhancing droplet mobility when compared to hydrophobic and/or to superhydrophobic surfaces. In this work, we fabricate and make use of hierarchical micro-/nano-structured and nano-structured SLIPSs for condensation phase-change. Optical microscopy and macroscopic experimental observations are coupled to extract the droplet size distribution at different condensation times. Heat transfer resistance model through individual condensing droplets is further extended here to account for the presence of both micro- and nano-structures. Then, heat transfer through individual droplets is coupled to the droplet number density to estimate the heat transfer at different condensation times and their overall performance. A 100% greater heat transfer performance is reported on nano-structured SLIPSs when compared to hierarchical micro-/nano-structure SLIPSs due to the greater thermal resistance imposed by the micro-structures and the lubricant present within the structures. We conclude that although the presence of micro-structures shifts the droplet number density towards greater population of smaller sized droplets, this effect is not enough to overcome the greater heat transfer predicted on solely nano-structured SLIPSs. Findings presented here complement current research on SLIPSs and condensation phase-change heat transfer and are of great importance for the effective design of SLIPSs with enhanced condensation heat transfer performance.