Electric fields induce motion in many fluid systems, including polymer melts1, surfactant micelles2 and colloidal suspensions3. Likewise, electric fields can be used to move liquid drops4. Electrically induced droplet motion manifests itself in processes as diverse as storm cloud formation5, commercial ink-jet printing6, petroleum and vegetable oil dehydration7, electrospray ionization for use in mass spectrometry8, electrowetting9 and lab-on-a-chip manipulations10. An important issue in practical applications is the tendency for adjacent drops to coalesce, and oppositely charged drops have long been assumed to experience an attractive force that favours their coalescence11,12,13. Here we report the existence of a critical field strength above which oppositely charged drops do not coalesce. We observe that appropriately positioned and oppositely charged drops migrate towards one another in an applied electric field; but whereas the drops coalesce as expected at low field strengths, they are repelled from one another after contact at higher field strengths. Qualitatively, the drops appear to ‘bounce’ off one another. We directly image the transient formation of a meniscus bridge between the bouncing drops, and propose that this temporary bridge is unstable with respect to capillary pressure when it forms in an electric field exceeding a critical strength. The observation of oppositely charged drops bouncing rather than coalescing in strong electric fields should affect our understanding of any process involving charged liquid drops, including de-emulsification, electrospray ionization and atmospheric conduction.
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We thank M. Poitzsch and M. Sullivan at the Schlumberger-Doll Research Center for supplying samples of crude oil, C. Holtze, T. Schneider and D. A. Weitz for feedback and the Harvard Nanoscale Science and Engineering Center for support. A.B. acknowledges support from Harvard University's MRSEC.
Author Contributions W.D.R., J.C.B., A.B. and H.A.S. designed the research; W.D.R., J.C.B., A.B. and F.D. performed research; W.D.R., J.C.B., A.B. and H.A.S analysed data; W.D.R. wrote, and all authors commented on, the manuscript.
This file contains Supplementary Discussions, Supplementary Data, Supplementary Figures 1-2 with Legends, Supplementary References and Legends for Supplementary Movies 1-6. (PDF 277 kb)
This movie shows immediate coalescence at a low field strength - see file s1 for full Legend. (MPG 2011 kb)
This movie shows non-coalescence (bouncing) at a higher field strength - see file s1 for full Legend. (MPG 2964 kb)
This movie shows vigorous bouncing at an even higher field strength - see file s1 for full Legend. (MPG 6394 kb)
This movie shows two droplets bouncing back and forth in a high field strength - see file s1 for full Legend. (MPG 2240 kb)
This movie shows a zoomed-in view of the immediate vicinity at the point of contact for a bouncing drop - see file s1 for full Legend. (MPG 451 kb)
This movie shows an example of partial coalescence, where an oppositely charged 'daughter' droplet is emitted from the point of contact - see file s1 for full Legend. (MPG 3315 kb)
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Ristenpart, W., Bird, J., Belmonte, A. et al. Non-coalescence of oppositely charged drops. Nature 461, 377–380 (2009). https://doi.org/10.1038/nature08294
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