William D. Ristenpart1, Andrew Belmonte2, J. C. Bird3, and Howard Stone3. (1) Dept. of Chemical Engineering & Material Science, University of California, Davis, 3002 Bainer Hall, 1 Shields Ave., Davis, CA 95616, (2) Pritchard Laboratories, Dept. of Mathematics, Pennsylvania State University, University Park, PA 16802, University Park, PA 16802, (3) School of Engineering and Applied Sciences, Harvard University, 9 Oxford St., 501 McKay Hall, Cambridge, MA 02138
We demonstrate the existence of a critical electric field strength above which oppositely charged drops do not coalesce. Application of an external electric field causes appropriately positioned and oppositely charged drops to migrate toward one another. Upon contact, surface tension acts to pull the drops together. For low applied field strengths, the oppositely charged drops coalesce, but at higher field strengths the drops are repelled from one another after contact. Qualitatively, the drops appear to ‘bounce' off one another. A critical electric force for bouncing is derived based on a competition between the time scales for charge transfer and the action of surface tension. The results have broad implications for applications where charged drops are manipulated by electric fields, including microfluidics, atmospheric conduction and emulsion coalescence.