Indexed on: 20 Sep '17Published on: 20 Sep '17Published in: ACS Applied Materials & Interfaces
Superhydrophobic surfaces are receiving increasing attention due to the enhanced condensation heat transfer, self-cleaning and anti-icing properties by easing droplet self-removal. Despite the extensive research carried out in this topic, the presence or absence of microstructures on droplet adhesion during condensation has not been addressed yet. In this work we, therefore, address the condensation behavior on engineered superhydrophobic copper oxide surfaces with different structural finishes. More specifically, we investigate the coalescence-induced droplet-jumping performance on superhydrophobic surfaces with structures varying from the micro- to the nano-scale. The different structural roughness is possible due to the specific etching parameters adopted during the fabrication process. A custom-built optical microscopy setup inside a temperature and relative humidity controlled environmental chamber was used for the experimental observations. By varying the structural roughness, from the micro- to the nano-scale, important differences on the number of droplets involved in the jumps, on the frequency of the jumps and on the size distribution of the jumping droplets were found. In the absence of microstructures, we report an enhancement of the droplet-jumping performance of small droplets with sizes in the same order of magnitude as the microstructures. Microstructures induce the droplet angular deviation from the main surface normal increasing the droplet adhesion. As a consequence, upon coalescence, there is a decrease in the net momentum in the out-of-plane direction and the jump does not ensue. We demonstrate that the absence of micro-structures has therefore a positive impact on the coalescence-induced droplet-jumping and on the heat transfer performance. Microstructures are then rule out for the optimum design of superhydrophobic surfaces with enhanced droplet mobility of micrometer droplets.