Electric fields have long been used to manipulate water droplets in insulating oils. In the presence of an electric field, charged droplets will move between electrodes and contact each electrode to exchange charge. This induced motion has traditionally been used to drive droplet coalescence in electrostatic dehydrators and, more recently, microfludic devices. In addition, charged droplets are also studied in atmospheric sciences, where their coalescence and motion is key to the development of rain in clouds, and have found application in electrospray mass spectrometers, inkjet printers, and other devices which use electric fields to dispense and control the motion of small droplets. Despite the applications and widely reported observations surrounding charged droplets, there is no comprehensive understanding of the charge transfer mechanism between droplets and electrodes. An understanding of the charge transfer mechanism is also important to understand the root causes of several widely reported phenomena, including why droplets acquire more positive charge than negative charge, and why droplets do not acquire as much charge as theoretically predicted. This dissertation provides insight into the charge transfer mechanism in three ways. First, we demonstrate that charge transfer between droplets and electrodes physically changes the surface of the electrode. As droplets approach electrodes, a dielectric breakdown event occurs as evidenced by flashes of light emitted just prior to droplet contact. The high current density and plasma present in the breakdown locally melts the electrode and physically moves the molten material radially outward to form craters on the surface of the electrode. The observed crater size scales in accordance to the amount of energy released due to Joule heating during the charge transfer event. Second, we show that different conductivity droplets produce dramatically different deformation morphology on the electrode surface during charge transfer. Low conductivity droplets produce small "bumps" while increasing the droplet conductivity produces increasingly large craters. The strength of the observed dielectric breakdown also increases as the droplet conductivity increases. Surprisingly, despite the differences in dielectric breakdown and deformation morphology, droplets of all conductivities acquire similar amounts of charge. Finally, we develop a method to use the observed current to find the droplet charge independent of a knowledge of any forces acting on the droplet. The results of this analysis indicate that the droplet acquires more positive charge than negative charge regardless of the measurement method. This dissertation concludes with a discussion of potential avenues for future exploration of droplet charging as well as some suggested applications for the phenomena observed here.
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- Chemical engineering;Fluid mechanics;Physics