ELECTRIC-FIELD-ENHANCED JUMPING-DROPLET CONDENSATION

  • Enright R.
  • Miljkovic N.
  • Preston D.
  • Wang E.

When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of vapor flow towards the condensing surface increases the drag on the jumping droplets, which can lead to droplet reversal of direction and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface that impede heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide surfaces, we show that this vapor flow entrainment dominates droplet motion for droplets smaller than R &#8776; 30 &#956;m at moderate heat fluxes (q” > 2 W/cm2). Subsequently, we demonstrate electric-field-enhanced (EFE) condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these droplets gain a net positive charge due to charge separation of the electric double layer at the hydrophobic coating. As a result, with scalable superhydrophobic CuO surfaces, we experimentally demonstrate a 50% higher overall condensation heat transfer coefficient compared to that on a jumping- droplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement, but also offers avenues for improving the performance of self- cleaning and anti-icing surfaces as well as thermal diodes.

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