Formation of Molten Nanocraters on Electrodes during Charge Transfer with Conductive Droplets or Particles

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CBET - 1707137PI: Ristenpart, William D.When a conductive droplet or particle contacts an electrode, charge is transferred from the electrode to the droplet or particle. Although the exact mechanism of charge transfer is not well understood, it is usually assumed that the charge transfer does not affect the electrode or the droplet or particle that contacts the electrode. However, recent observations show that charge transfer to water droplets or other particles creates craters in the electrode's surface up to a micron wide that can be detected by electron microscopy and atomic force microscopy. The goal of this project is to understand the mechanism of crater formation and how it affects charge transfer. A series of experiments will test the hypothesis that Joule heating during charge transfer melts the electrode locally. The molten metal is pushed out under pressure due to a plasma jet of the surrounding fluid. The metal then cools and solidifies leaving behind a crater. Understanding these and other details of charge transfer could lead to improved efficiencies in a variety of industrial electrochemical processes, including electrical de-emulsification, which is used to separate water from oil, and electrostatic precipitation, which is used to control particulate emissions. The results of this project will be used in a general education course at UC Davis that introduces beginning students to science and engineering and is taken each year by more than 1500 students. The behavior of charged objects at an electrode will be characterized by a series of in situ experiments, including high speed video, high resolution chronocoulometry, and high sensitivity photon counting. Results will be combined with post in situ characterizations of the resulting craters obtained by optical, electron, and atomic force microscopy. Specific experiments will be aimed at directly testing the proposed mechanism of crater formation, including systematically varying the melting point temperature of the electrode, the dielectric breakdown strength of the surrounding fluid, and the radius of curvature of solid conductive particles. If corroborated, the mechanism proposed here could explain the long-standing difficulty in obtaining quantitative agreement with predicted charge transfer values, since the crater formation physically alters the electrode.