Electroneutral Layer Dynamics Drive Ion Migration in Low Frequency AC Electrophoresis Below the Water Electrolysis Threshold.

Abstract

Low-frequency AC electrophoresis lies in a regime between DC microchannel electrophoresis and dielectrophoresis, which typically utilizes frequencies above 1000 Hz. Although few electrophoretic methods have been reported in this ≤100 Hz range, traveling wave electrophoresis (TWE) and transverse AC electrophoresis (TrACE) operate in this frequency range, and use low voltages to avoid bubble formation from water electrolysis. TWE provides molecular separations with enhanced control and TrACE provides multiplexed, multiparameter particle characterization. However, two related fundamental questions remain about the mechanisms of electrophoretic migration in these systems. First, particle electrophoresis in TrACE is largely captured by a simple model that combines the alternating electric field with DC electrokinetics, but a deviation from the model is observed for applied square electric field waves that increases with decreasing frequency. Second, although electrode charging is believed to drive ion migration in TWE, the estimated electrode charging time is about 2-3 orders of magnitude faster than the wave period. In this study, a 1D finite numerical model that excludes Faradaic reactions simulates ion and particle migration across the microchannel width in TrACE. The 1D model results show good agreement with both particle and ion migration in TrACE systems. Furthermore, although ion migration between the pair of electrodes slows during each excursion of a 1 Hz square wave, there is substantial ion migration throughout the 0.5 s half-period. This modeling result agrees with experimental observations in TWE. Therefore, the clarification of the mechanisms of ion migration in these low-frequency and low-voltage AC electrophoresis is expected to expand their applications.