Fast nucleation of water by self-arrangement of hydrophilic crystals on a hierarchically structured surface promoting coalescence-induced droplet jumping

Abstract

The enhancement of the condensation efficiency is required for saving energy and resources in industries exploiting condensation. In this regard, the Laplace pressure-driven spontaneous movement of condensed droplets on hierarchical superhydrophobic surfaces has been exploited to increase the condensation efficiency via the fast removal of the condensates from the surfaces. Although the moving droplets enabled the nucleation sites to be exposed to vapor before the droplets were removed from the surface, nucleation after the exposure was delayed by vapor depletion around the moving droplets. Since the heterogeneous nucleation energy barrier depends on the surface wettability, we investigated the effect of the wettability of the nucleation sites on the nucleation after the nucleation sites were exposed owing to the moving droplets. We arranged hydrophilic crystals inside the inner tips of a zigzag-structured surface and adjusted the crystal size with capillary flow-induced self-arrangement. The crystal size was highly related to the nucleation period at the inner tips because the nucleation energy barrier decreased with increasing crystal size. Consequently, the nucleation period reduced by the adequately sized crystals promoted droplet jumping, thereby increasing the frequency and cumulative volume of the jumping droplets by up to 410% and 130%, respectively. Furthermore, we verified that the increased crystal size did not affect the jumping probability on the biphilic-zigzag surfaces; this is because the wetting transition induced by the V-shapes minimized the work of adhesion. These results provide insight into the relationship between the wettability and nucleation period on hierarchically structured surfaces and highlight the synergetic effect between the hierarchical structures and heterogeneous wettability, which promotes droplet jumping.