The Fate of Chemical Species from a Sample Introduced into a Redox-Magnetohydrodynamics (R-MHD) Microfluidics Chamber: Influence of Diffusion within Flow Fields Near Pumping Electrodes and Walls and Under Different Experimental Conditions

Redox-magnetohydrodynamics (MHD) offers unique capabilities for pumping liquids on a small scale where other microfluidic pumps are ineffective or unsuitable. It can vary speed, change direction, stir and pump in a loop by controlling the magnitude and direction of ionic current, j , between two or more electrodes having desired geometries in the presence of a perpendicular magnetic field, B . This interaction generates a force, F B , orthogonal to j and B , following the right hand rule. The localized volume of liquid then moves in the same direction as F B due to momentum transfer. MHD is compatible with a variety of solvents and solution compositions. In principle, MHD does not require valves or channel sidewalls to move fluid in a path. For applications involving analysis or imaging of single entities, like nanoparticles, microbeads, and biological cells, the focus is on inspection of the entities in a specific region of moving fluid. However, in applications where there is a collection of species, such as a concentration of an analyte in a sample introduced into the fluid having a different composition, it is important to understand how the analyte disperses from both the fluid dynamics and diffusion. Numerical approximations were performed using COMSOL Multiphysics® to analyze the 3D velocity profiles generated experimentally by redox magnetohydrodynamics (R-MHD) microfluidics and to predict the fate of analyte molecules in a sample that is introduced into that fluid. Simulations were first compared to experimental data from parallel band, pumping electrodes, where uniform horizontal flow profiles are possible when sidewalls are far away. Experimentally obtained velocities involved an R-MHD chamber with a rectangular geometry of 30 mm x 17 mm formed from a cutout in a poly(dimethyl siloxane) (PDMS) gasket placed onto a microfabricated chip containing individually-addressable electrodes, filled with an electrolyte solution, and capped with a glass coverslip. The gasket thickness defined the chamber height. To generate an ionic current, an electronic current was applied between two poly(3,4-ethylenedioxythiophene) (PEDOT) modified, parallel band electrodes with dimensions of ~890 µm width and 15 mm length (along the x-direction), separated by a 2.76 mm gap. A magnetic flux density of 0.37 T was produced by a permanent magnet placed beneath the chamber. Astigmatism particle tracking velocimetry (APTV) was used to interrogate the fluid velocity between t