The main scientific goal of this project is to answer the question: how do droplets acquire charge? Recent work by the PI has revealed the existence of a critical electric charge density above which oppositely charged drops do not coalesce (Ristenpart et al., Nature 2009). The non-coalescence was interpreted in terms of a capillary pressure model, in which a short-lived meniscus bridge pinches off following rapid charge exchange between the drops. This model helps shed light on a wide range of phenomena where plateaus in coalescence efficiency have been observed, including atmospheric charge conduction, de-emulsification of petroleum and vegetable oils, and electrical manipulation of drops in lab-on-a-chip devices. Several fundamental questions, however, remain unanswered. First and foremost: what mechanism governs the amount of charge a droplet acquires? The traditional research focus has been on the generation of charged droplets, where a large droplet is destabilized and broken into smaller charged droplets by application of a sufficiently strong electric field (e.g., electrospray ionization). In many applications, including those listed above, this destabilization mechanism is not operative. Instead, droplets acquire charge when they contact another electrified surface, such as another drop or a solid electrode. Strikingly, there are no extant models that accurately predict the amount of charge transferred to a liquid drop upon contact with an electrified surface. Intellectual merit: The experimental framework proposed here will allow groundbreaking, fundamental studies of charge transfer dynamics in emulsions by combining simultaneous high- speed visualization and chronocoulometric techniques. Previous studies have probed the droplet charge only indirectly, by assuming that the hydrodynamic drag force on a drop is balanced by an electrophoretic driving force. The proposed experimental system instead allows direct measurements of the charge transferred from a drop using a high-resolution electrometer, and will provide the first independent tests of the electrophoretic force on drops with a known charge density. Furthermore, the chronocoulometric measurements will provide crucial information for interpreting the effects of droplet deformation, Faradaic reactions, and transient meniscus bridge dynamics on the amount of charge transferred. In all cases, the PI will emphasize the simplest, most intuitive systems to elucidate key phenomena. Broader impacts: The PI's high-speed camera, and the undergraduate students who are trained to operate it, represent core capabilities of the proposed research program. A key goal of this CAREER proposal is to leverage these capabilities at the elementary school level. Specifically, the PI will collaborate with the Explorit Science Center in Davis, CA to develop an interactive, hands-on exhibition named "Time Warp" that features high-speed video. Elementary school students will personally operate the camera software under the supervision of an undergraduate trained by the PI. Preliminary "trial runs" have confirmed that the students are intrigued as they film themselves and their classmates performing hands-on investigations of physical phenomena (such as popping a water balloon or breaking a board) that illustrate key scientific principles. Education: A consistent theme in the PI's research program is incorporation of undergraduate students directly in cutting-edge research, and accordingly the PI's undergraduate curriculum development is focused on encouraging undergraduate participation in research. The PI is developing two lower-division seminars (one each in chemical engineering and food science) that will highlight opportunities and facilitate placement of undergraduates into research programs. At the graduate level, the PI is developing an elective course titled "Electrofluidics" that will survey recent advances in the field and incorporate state of the art research activities from his own group.
