Scale effect on dropwise condensation on superhydrophobic surfaces.

Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase in droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructure roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.