October 14, 2016

Coalescence-Induced Nanodroplet Jumping

  • Cha H.
  • Chun J.
  • Enright R.
  • Miljkovic N.
  • Sotelo J.
  • Xu C.
  • Yokoyama Y.

Water vapor condensation on superhydrophobic surfaces has received much attention in recent years due to the ability of such surfaces to shed microscale water droplets via coalescence-induced droplet jumping, resulting in heat transfer, anti-icing, and self-cleaning performance enhancement. This Letter reports coalescence-induced removal of water nanodroplets (R approx 500 nm) from superhydrophobic carbon nanotube (CNT) surfaces. The two-droplet coalescence time is measured for varying droplet Ohnesorge numbers, confirming that coalescence prior to jumping is governed by capillary-inertial dynamics. By varying the conformal hydrophobic coating thickness on the CNT surface, the minimum jumping droplet radius was shown to increase with increasing solid fraction and decreasing apparent advancing contact angle, indicating that hydrodynamic limitations stemming from viscous dissipation do not limit the minimum droplet jumping size even for the smallest nanostructure length scale (leq 100 nm) and surface adhesion. Rather, a surface interaction mechanism stemming from the evolved droplet morphology plays the defining role in limiting the minimum size for jumping. The outcomes of this work demonstrate the ability to passively shed nanometric water droplets, which has the potential to further increase the efficiency of jumping droplet applications.

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