Droplet Conductivity Strongly Influences Bump and Crater Formation on Electrodes during Charge Transfer

Abstract

Aqueous droplets acquire charge when they contact electrodes in high voltage electric fields, but the exact mechanism of charge transfer is not understood. Recent work by Elton et al. revealed that electrodes are physically pitted during charge transfer with aqueous droplets. The pits are believed to result when a dielectric breakdown arc occurs as a droplet approaches the electrode and the associated high current density transiently locally melts the electrode, leaving distinct crater-like deformations on the electrode surface. Here we show that the droplet conductivity strongly modulates the pitting morphology but has little effect on the amount of charge transferred. Electron and atomic force microscopy shows that deionized water droplets yield no observable deformations, but as the salt concentration in the droplet increases above 10 M, the deformations become increasingly large. The observed intensity of the flash of light released during the dielectric breakdown arc also increases with droplet conductivity. Surprisingly, despite the large difference in pitting morphology and corresponding arc intensity, droplets of any conductivity acquire similar amounts of charge. These results suggest that the energy transferred during dielectric breakdown is primarily responsible for electrode pitting rather than the total amount of energy released during charge transfer.

Statistical Analysis of Droplet Charge Acquired during Contact with Electrodes in Strong Electric Fields

“…The experimental setup (Figure , top) is similar to those used in previous experiments. ,, Standard photolithography techniques were used to deposit gold electrodes 1 mm wide, 20 mm long, and 50 nm thick onto glass substrates. For each experiment, two new electrodes were placed in a plastic cuvette and separated by nonconducting spacers at the top and bottom.…”

“…A limiting prediction for the amount of charge a droplet should acquire upon contact with an electrode was derived by James Maxwell, who showed that the amount of charge Q a perfectly conducting sphere would acquire from a perfectly conducting planar electrode is given by Here, a is the sphere radius, εε o is the permittivity of the surrounding liquid, and E is the applied electric field. Although the amount of charge the liquid droplets acquire generally follows this dependence on electric field strength and droplet radius, − for unclear reasons droplets have not been observed to acquire as much charge as predicted by Maxwell. − ,,− Solid particles have also been observed to obtain less than the predicted amount of charge, again for unclear reasons. ,,− …”

Wall Effects on Hydrodynamic Drag and the Corresponding Accuracy of Charge Measurement in Droplet Contact Charge Electrophoresis

“…The studies in the microfluidics field started from 2006 in microchannels and from 2009 in a digital microfluidic system . Since 2010, active research on various topics has been reported in industrial − and biological − applications and in microfluidics technology. − Basic research on the CCEP phenomenon also has been actively studied by various groups on droplet dynamics, − electrohydrodynamics, , interfacial phenomena, , and basic principles and charge measurement. − Recent progress in ECD based digital microfluidic technology is noticeable, − and the application to electroporation showed promising results. − These diverse studies of CCEP have improved the underlying understanding even further, suggesting and verifying the potential features of microfluidic technology.…”

Simulation & modelling of dilute solutions in drop-on-demand inkjet printing: a review

“…where is kinematic viscosity [23,24]. There are five parts in the equation 8 including variation ( ), conviction (( ) ), diffusion ( ), internal source ( ) and external source ( ) respectively.…”