Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects

Abstract

Evaporation of a sessile droplet is a complex, nonequilibrium phenomenon. Although evaporating droplets upon superhydrophobic surfaces have been known to exhibit distinctive evaporation modes such as a constant 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 surface roughness. Through global interfacial energy analysis we reveal that, when the size of the evaporating 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 Cassie-Wenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.

Mechanism Study on Transition of Cassie Droplets to Wenzel State after Meniscus Touching Substrate of Pillars

“…Micro-nano hierarchical textures have been proved to be able to effectively prevent the C-W transition by experiments 16,22,[28][29][30]39,40 . But detailed theoretical analysis is very limited.…”

Influence of structured sidewalls on the wetting states and superhydrophobic stability of surfaces with dual-scale roughness

“…Compared with surfaces with single-scale roughness, higher free energy and higher ACA for given ˛1 and x 1 are obtained in surfaces with dual-scale roughness, particularly those with structured sidewalls. External pressures, such as the vibrations and evaporation of a droplet [44,45], are required for a system at rest to start moving toward higher free energy states. The normalized minimum free energy (G * min ) CB-CB→CB-W for the transition from the CB-CB state to the CB-W state consists of two parts.…”

Experimental study of evaporation of sessile water droplet on PDMS surfaces

“…When water droplet evaporates in the CCR mode, attributing to contact angle hysteresis (CAH) [11] (Fig. 1a) which is mainly associated with line tension [12,13], there is a pinning force per unit length acting at the three-phase contact line in the lateral direction f pin = γ lv (cos θ r − cos θ a ),…”