William D. Ristenpart1, Sigolene Lecuyer2, Olivier Vincent2, and Howard A. Stone3. (1) Division of Engineering and Applied Science, Harvard University, 402 Pierce Hall, 29 Oxford St., Cambridge, MA 02138, (2) School of engineering and applied science, Harvard university, 9 Oxford street, Cambridge, MA 02138, (3) Division of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138
We demonstrate that electrohydrodynamic (EHD) flow induces polydisperse suspensions of unilamellar vesicles to aggregate laterally along electrodes. Application of an oscillatory electric field (ca. 50 Hz, 10 V) generates EHD flow around each vesicle close to the electrode. Nearby vesicles are entrained in the flow and the vesicles move toward one another, forming aggregates. Upon aggregation, however, smaller vesicles are observed to move underneath the larger vesicles, gradually lifting the larger vesicles off of the electrode entirely. This ‘vertical size segregation' differs qualitatively from the behavior of rigid colloidal particles, where such vertical motion has not been reported. We investigate the vertical motion in the context of the superposition of EHD drag forces and dipolar interactions, and we demonstrate that this phenomenon provides a route for gently separating giant unilamellar vesicles from polydisperse suspensions.