Dropwise Condensation on Hydrophobic Bumps and Dimples

The role of macroscopic surface topography in droplet growth by condensation has been overlooked compared to micro- or nano-texture. In this work, we studied the droplet growth on hydrophobic surfaces with millimeter scale radii of curvature with a positive or negative sign under a supersaturation condition created by controlling the surface temperature lower than the dew point. By analyzing the spatiotemporal droplet size distribution on convex surface structures such as bumps and concave surface structures such as dimples, we show detailed local droplet growth and its relation with the sign and magnitude of surface curvature. In particular, we report unexpected, more unfavorable condensation on dimples with a smaller radius of curvature, in contrast to preferential condensation in micro-cavities (i.e., capillary condensation). To explain these experimental results, we numerically calculated water vapor concentration gradient, showing that the diffusion flux is higher on bumps and lower on dimples at the center of them than a flat surface. We envision that our understanding of millimetric surface topography can be applied to improve the energy efficiency of condensation applications such as water harvesting devices, heat exchangers, and desalination plants.