Non-coalescence of oppositely charged drops

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Original entry: Sujit S. Datta, APPHY 225, Fall 2009.

Reference

W. D. Risternpart, J. C. Bird, A. Belmonte, F. Dollar and H. A. Stone, Nature 461, 377 (2009).

Keywords

electric, droplet, coalescence, dielectrophoresis

Key Points

The objective of this work was to study the coalescence behavior of water droplets (dispersed in oil) under the influence of an applied electric field. The experimental setup was simple: the bottom half of a container was filled with water and was electrically grounded, while the top half was filled with a poorly conducting oil, with an electrode inserted in the liquid. This allows an inhomogeneous electric field (of strength up to 100,000V/m) to be applied across the sample chamber. After this field is applied, a small water droplet is pipetted into the oil at the top of the chamber. Because the electric field is inhomogeneous, the droplet moves by dielectrophoresis towards the top electrode. Upon contact, the droplet picks up a charge (say positive) and is subsequently repelled from the top electrode and attracted toward the water in contact with the bottom electrode. What happens at this point?

The central result of this work is the following surprising result: for sufficiently small electric force and salt concentration of the water droplet, the droplet contacts and coalesces with the water filling the bottom half of the sample chamber (this itself is unsurprising, because the droplet is attracted to the `grounded' water), while for an electric force larger than a critical value (that is a function of the salt concentration), the droplet bounces off the lower oil-water interface. This is surprising: while the droplet is electrically attracted to the oppositely-charged lower large water droplet, upon contact, it does not coalesce but is, in fact, repelled. This effect can be generalized: for example, the authors of this paper show a cute example of how individual droplets in a "train" of droplets can "shuttle" charge back and forth between each other via this non-coalescence bouncing process.