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

A 3D finite element model is used to investigate the behavior of a small sample volume of molecules inside a redox-magnetohydrodynamics (R-MHD) microfluidic chamber (3.0 cm × 1.7 cm, 429 µm-high), enclosing chip-based, coplanar parallel-band electrodes ( ~ 900 µm wide, 1.5 cm long, and 28 µm thick). A 539-pL cylindrical sample plug is introduced, 20 µm radius, spanning the chamber height, containing 0.10 M molecular species with diffusion coefficient of 8.75 × 10⁻¹⁰ m²·s⁻¹. Fluid motion is driven by the magnetic portion of the Lorentz force by applying ± 400 µA between two pumping electrode pairs, separated by 2760 µm and 4441 µm, and positioned above a 0.37 T permanent magnet. The model tracks how plug trajectory, spreading, and deformation under the combined influence of molecular diffusion and R-MHD-driven convection depends on electrode configuration, pumping direction, wall placement and initial plug position. Different scenarios include transporting the plug toward a chamber wall and sending it around electrode ends while maintaining a closed circulating zone rather than reaching chamber boundaries. The findings demonstrate that sample plugs can be steered, retained, or redirected through electrode activation and current polarity, without external pumps or moving parts, enabling programmable sample manipulation for R-MHD-based lab-on-a-chip systems.