Influence of mass transport near parallel band pumping electrodes and walls on the fate of chemical species in a sample plug introduced onto a redox-magnetohydrodynamics microfluidics chip

Abstract This work presents a three-dimensional, time-resolved finite element model of sample plug evolution under redox-magnetohydrodynamics (R-MHD) microfluidic flow, a problem not addressed by previous computational studies. Geometries and electrochemical conditions reflect existing chip-based R-MHD devices employing poly(3,4-ethylenedioxythiophene) (PEDOT)-modified, coplanar parallel band pumping electrodes (~ 900 μm wide, 1.5 cm long, 28 μm thick) in an enclosed aqueous-electrolyte chamber (3.0 cm × 1.7 cm × 429 μm) on a permanent magnet (surface | B | = 0.37 T). Faradaic processes occur within the PEDOT film, allowing ionic current density ( j ) between electrodes to be treated through bulk electrical properties of supporting electrolytes, rather than tracking redox-species transport, freeing computational resources for time-resolved plug dynamics. The magnetic component of the Lorentz force, F B = j × B , drives fluid motion with ± 400 µA applied across pumping electrode pairs at two separations (2760 and 4441 μm). Full three-dimensional, three-component astigmatism particle tracking velocimetry measurements at mid-path and at electrode ends validate the simulated flow field. After introducing a 539-pL cylindrical plug (20 μm radius, 0.10 M solute, D  = 8.75 × 10⁻¹⁰ m²·s⁻¹), its trajectory, spreading, and deformation are tracked as functions of electrode configuration, pumping direction, wall proximity, and initial plug position. The results show that plugs can be steered, retained, or redirected, including transport toward a wall and recirculation around electrode ends, by electrode activation and current polarity alone, without external pumps or moving parts. This programmable, channel-free manipulation is directly relevant to chemical separations, microfluidic mixing and reaction processes, and multiplexed analytical platforms.