(443d) Solution pH Changes in Non-Uniform AC Electric Fields Above the Electrode Charging Frequency

AC Faradaic reactions have been attributed as the mechanism inducing non-ideal phenomena such as flow reversal and cell deformation in microfluidics systems. Prior published work examined uniform electric fields below the electrode charging frequency (f_c), the frequency for electric double layer charging at the electrode.  Spatially non-uniform AC electric fields are required for applications such as AC electroosmosis, AC electrothermal pumps, and dielectrophoresis. Many such applications utilize the AC frequency around or above f_c. In this work, fluorescein sodium salt, a pH sensitive dye, was used to detect [H⁺] in order to follow Faradaic reactions in aqueous solutions within non-uniform electric fields. Comparison experiments with a) parallel (uniform fields) electrodes and b) organic media were employed to test the electrode charging mechanism at 1.5f_c. Time dependency analysis was processed and illustrated Faradaic reactions existence above the theoretically predicted electrode charging frequency. Further, spatial analysis revealed that the Faradaic reactions varied spatially due to the non-uniformity of the electric field and influenced local pH >50 µm away from the electrode edge.  The non-uniform AC fields yielded pH gradients while uniform fields did not yield pH gradients, consistent with prior published data. Faradaic reaction frequency dependencies were examined from f_c to 3f_c at 11V peak-to-peak potential and voltage dependency was explored from 7 to 15V_pp potential at 1.5f_c. Results imply that Faradaic reactions can still proceed within electrochemical systems in the absence of a well-established electric double layers. This work also illustrates that in microfluidics systems, medium variations need to be considered as a function of experiment time, initial medium conditions, electric signal potential, frequency, and spatial location in the fluid system.