68th Annual Meeting of the APS Division of Fluid Dynamics (November 22, 2015 — November 24, 2015)

V0018: Self-propelled droplet removal from hydrophobic fiber-based coalescers

  • Kungang Zhang, Duke University
  • Fangjie Liu, Duke University
  • Adam Williams, Duke University
  • Xiaopeng Qu, Duke University
  • James Feng, University of British Columbia
  • Chuan-Hua Chen, Duke University
DOI: https://doi.org/10.1103/APS.DFD.2015.GFM.V0018

Droplets accumulate on fibers all the time, like morning dew hanging on spider webs, warning those who might walk through them. But what if tiny droplets danced themselves right off the silk strands instead?

We have found that this fantasy is actually a reality on thin fibers coated with Teflon. As droplets accumulate on the fiber by condensation, the growing droplets magically disappear upon coalescence. The mechanism behind the unexpected disappearance is best shown by two droplets on opposite sides of the fiber. As individual droplets form, energy is stored on their growing surfaces. This energy is released when two droplets combine, propelling the merged drop away from the fiber. The dancing motion is clearly shown by the end view. The merged drop first moves toward one side of the fiber with a large momentum. Since the round fiber provides little solid surface area to hold the droplet, the released energy is enough for the merged drop to launch. The round geometry of the fiber is critical for the self-removal. On a flat surface, the adhesion from the solid is too much for the drop to dance away, even with the same Teflon coating.

The dancing droplets on thin fibers is powered by surface energy intrinsic to the droplets, and are therefore independent of external forces such as gravity. The self-removal mechanism can be exploited in industrial fibrous coalescers that accumulate droplets from aerosols and emulsions, with applications ranging from water purification to oil refining. 

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