Evaporation of Condensate Droplets on Structured Surfaces with Gradient Roughness

The study of evaporation dynamics of droplets is of scientific interest and has numerous practical applications. Here, we studied the evaporation of small condensate droplets on structured surfaces with one-tier microscale roughness and two-tier micro/nanoscale roughness (the top and valley of micropillars are covered by nanograss), respectively. On both surfaces, the micropillar arrays are arranged in a radical lattice with the decreasing pillar-to-pillar spacing towards the center of the surface (The first figures in Figs. 1 and 2). The condensate droplets on structured surfaces were formed by conducting condensation inside environmental scanning electron microscope (ESEM, Philips XL-30, ~4.9 Torr, stage temperature ~ 3°C). The condensate droplet on the one-tier surface stays in a Cassie-state (0 s in Fig. 1). However, owing to the preferential droplet nucleation on the smooth sidewall of micropillars, the condensate droplet on the two-tier surface maintains in the composite state (0 s in Fig. 2). To visualize the evaporation dynamics of condensate droplet, we gradually decreased the vapor pressure in the chamber from ~4.9 Torr to ~4.2 Torr. On the one-tier surface (Fig. 1), the droplet first evaporates in a constant contact radius mode (CCR, 0-124 s), followed by a constant contact angle mode (CCA, 136-166 s), and a mixed mode of both CCR and CCA (188-224 s). By contrast, on the two-tier surface (Fig. 2), the condensate droplet first evaporates in the CCR mode, with the solid-liquid contact line remain pinned until the formation of a flat liquid-air interface at the top of micropillars at ~86 s. After that, the liquid-air interface at the top of surface remains flat and the liquid evaporation is dominant in the lateral direction (96-170 s), with the liquid cylinder symmetrically shrinking towards the center of the surface. The presence of a stable and flat liquid/air interface at the top of surface is due to the stabilization effect rendered by the nanograss on the micropillars.