Dropwise condensation on hydrophobic bumps and dimples

Yuehan Yao,Joanna Aizenberg,Kyoo-Chul Park

doi:10.1063/1.5021343

Abstract

Surface topography plays an important role in promoting or suppressing localized condensation. In this work, we study the growth of water droplets on hydrophobic convex surface textures such as bumps and concave surface textures such as dimples with a millimeter scale radius of curvature. We analyze the spatio-temporal droplet size distribution under a supersaturation condition created by keeping the uniform surface temperature below the dew point and show its relationship with the sign and magnitude of the surface curvature. In particular, in contrast to the well-known capillary condensation effect, we report an unexpectedly less favorable condensation on smaller, millimeter-scale dimples where the capillary condensation effect is negligible. To explain these experimental results, we numerically calculated the diffusion flux of water vapor around the surface textures, showing that its magnitude is higher on bumps and lower on dimples compared to a flat surface. We envision that our understanding of millimetric surface topography can be applied to improve the energy efficiency of condensation in applications such as water harvesting, heating, ventilation, and air conditioning systems for buildings and transportation, heat exchangers, thermal desalination plants, and fuel processing systems.

Highlights

Controlling condensation and droplet growth is fundamentally important in a wide spectrum of industrial applications including water harvesting from thin air, multi-stage flash desalination, heat exchangers, and thermal power generation.[1–5]
While recent studies have shown that droplet growth on millimetric convex surfaces by condensation is facilitated,[11-13] a more detailed study on droplet growth on textured surfaces is required to systematically understand the impact of the sign and the magnitude of the radius of curvature of surface features
We provide a quantitative analysis of droplet growth dynamics on hydrophobic surfaces with a wide range of millimeter-scale surface topographies ranging from convex textures to concave textures

Summary

Introduction

Controlling condensation and droplet growth is fundamentally important in a wide spectrum of industrial applications including water harvesting from thin air, multi-stage flash desalination, heat exchangers, and thermal power generation.[1–5] Extensive research efforts have been devoted to promote dropwise condensation of water by employing various surface chemistries and micro-/nano-textures.[2–10] While recent studies have shown that droplet growth on millimetric convex surfaces by condensation is facilitated,[11-13] a more detailed study on droplet growth on textured surfaces is required to systematically understand the impact of the sign and the magnitude of the radius of curvature of surface features. The dimple with the smallest magnitude of radius of curvature ( = −4.44 mm, Fig. 1(d)) shows the smallest condensed water droplets (diameter ~ 1 102 μm) at the center.

Results

Conclusion