Evaporation of droplets on superhydrophobic surfaces: surface roughness and small droplet size effects

Evaporation ofa sessiledropletisa complex,nonequilibriumphenomenon. Althoughevaporatingdroplets uponsuperhydrophobicsurfaceshavebeenknowntoexhibitdistinctiveevaporationmodessuchasaconstant contact line (CCL), a constant contact angle (CCA), or both, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive. We show that the onset time for the CCL-CCA transition and the critical base size at the Cassie-Wenzel transition exhibit remarkable dependence on the surfaceroughness.Throughglobalinterfacialenergyanalysis wereveal that, whenthe sizeof theevaporating droplet becomes comparable to the surface roughness, the line tension at the triple line becomes important in the prediction of the critical base size. Last, we show that both the CCL evaporation mode and the CassieWenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.