The effect of surface roughness on evaporation of condensate droplets at structured surfaces

Phase change by evaporation is a highly efficient heat transport mechanism, which is of scientific interest and has a wide spectrum of practical applications. Up to date, almost all the studies of droplet evaporation have been restricted to the use of sessile droplets. Condensate droplets formed on the structured surfaces exhibit wetting behaviors that are quite different from the deposited sessile droplets. In this work, we studied the evaporation of condensate droplets on structured surfaces with different roughness. The condensate droplets on structured surfaces were formed by conducting condensation inside environmental scanning electron microscope. The evaporation of condensate droplets were induced by decreasing vapor pressure in the chamber. We found that the evaporation mode of condensate droplets on the structured surface is highly sensitive to their wetting states. On the one-tier microstructured surface, the Cassie-state droplets formed on the top of pillars exhibit constant contact line mode and mixed mode, while sticky Wenzel-state droplets formed by growing from the bottom of the micropillars exhibit only constant contact line mode. We demonstrated that the contact line pining effect on the microstructured surface can be effectively inhibited by using two ways. One is by engineering surface with gradient microscale roughness; the other one is by engineering surface with nanoscale roughness. We envision that the strategy to control the pinning and depinning of contact line during evaporation of condensate droplets at the pre-determined locations might provide new avenue to construct complicated structures through self-assembly process.