(560g) Nano-Crater Formation on Electrodes during the Electrical Charging of Aqueous Drops

A water drop in an insulating fluid acquires charge when it contacts an electrode, but experimental measurements of the charge acquired by the drop have been plagued by irreproducibility and the exact mechanism of charge transfer has remained obscure. Previous work, dating back to Maxwell, has implicitly assumed that the electrode remains unaltered by the charging process. Here we demonstrate that, contrary to this assumption, water drops and other conducting objects create "nano-craters" on the electrode surface during the charging process. We used optical microscopy, SEM, and atomic force microscopy to characterize the electrode surfaces before and after water drops were electrically bounced on them. We show that each drop contact creates an approximately one micron wide and 50-nm deep crater on the electrode surface. Given enough time, the drop will form enough overlapping nano-craters to effectively "eat through" a sufficiently thin electrode. We provide visual evidence that dielectric breakdown occurs during each charge transfer event, and we interpret the nano-crater formation as a consequence of Joule heating and local melting of the metallic electrode. The observations reported here help explain the long standing discrepancies between theoretical predictions and experimental measurements of the charge transfer, and are of particular interest in the development of microfluidic devices that use electrodes to control the motion of aqueous drops.