Electric fields have long been used to hasten droplet coalescence. In particular, oppositely charged droplets feel an attractive force that brings them together and induces coalescence more readily. Recently, however, it has been shown that under sufficiently large electric fields two oppositely charged droplets will make contact but fail to coalesce, i.e. they "bounce" off one another. This noncoalescence was explained in terms of a critical cone angle; if the cone angle between the drops is too steep, the capillary pressure inside the short-lived liquid bridge becomes higher than the pressure in the droplets, causing the bridge to pinch off and preventing coalescence. However, many aspects of the drop bouncing behavior are not well understood. It was observed that occasionally charged droplets only partially coalesced, with one droplet delivering only a fraction of its volume into the second drop. Additionally, the droplets were seen to decelerate as they moved away from each other, which is not readily explicable in terms of inertial acceleration or electrostatic effects. Perhaps most fundamentally, the mechanism by which drops acquire charge in the first place is not understood.;In this thesis we elucidate four distinct aspects of the droplet behavior. (1) First, we show experimentally that the partial coalescence of charged drops is mediated by convective, rather than conductive, charge transfer, and we provide a scaling prediction for the size of the ejected daughter droplet based on capillary driven inertia. (2) Next, we explain the droplet deceleration following electrode contact in terms of the dynamic rearrangement of a so-called stagnant cap of surfactant molecules on the droplet surface, and we present a scaling analysis that accords qualitatively with the experimental evidence for both ionic and nonionic surfactants. (3) We then develop a methodology by which the charge transferred to a droplet can be determined via integration of chronoamperometric measurements, and we use this technique to demonstrate that the droplet obtains more charge from the anode than the cathode. (4) Finally, we present direct experimental evidence based on scanning electron microscopy and energy-dispersive X-ray spectroscopy that suggests the charge transfer mechanism is electrochemical in nature and hence sensitive to droplet pH. The thesis concludes with a discussion of potential avenues for future exploration of droplet charging and coalescence under large applied electric fields.
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