Rational Design of Supericephobic Heat Exchangers based on Condensation Frosting Heat Transfer Studies on Structured Surfaces

Understanding frost formation is essential to a variety of heat transfer applications. Previous work has shown that interfacial wetting states as well as inter-droplet ice-bridging govern frosting on superhydrophobic surfaces (SHS). Yet, a physics based understanding of these two frost governing mechanisms is limited. Furthermore, the utilization of frost-growth physics for nanostructure design and scale up is sorely needed. Here, we begin by elucidating the fundamental thermal physics governing condensation frosting of water slabs, droplets, and ice-bridges. At the macroscale, we show that the superhydrophobic state contributes negligible thermal resistance during freezing of thick (~mm) water slabs. At the microscale, we show that the velocity of the ice bridge formation is independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation is governed solely by vapor pressure gradients. We use our fundamental insights to rationally choose metal based surface structures, and demonstrate frost formation inhibition on meter scale aluminum heat exchangers. Our experiments show that SHS surfaces can achieve a 3X slower frost formation rate and a 50% defrost energy savings when compared to uncoated or superhydrophilic heat exchangers.