Materials Study for Optimization of Redox-Magnetohydrodynamics (R-MHD) for Pumping in Microfluidics Systems
Authors: Khan Foysal Z , Fritsch Ingrid 
Published: February 2020

Magnetohydrodynamics (MHD) is a unique pumping approach that can be used in Lab-On-A-Chip microfluidics and offers several features that either complement or improve upon other microfluidic pumping methods, including a flat flow profile, low voltage requirements, portability and programmability, and rotational flow.
1
MHD generates a body force (FB
) from the interaction of net ionic current in the solution (j) in the presence of magnetic flux density (B), by following the equation FB
= j X B.
2, 3
Ionic current can be generated between electrodes in a solution by passing electronic current through the electrodes in the external circuit to oxidize or reduce electroactive species called redox species. Redox species can be confined onto the electrode surface or mixed with electrolyte solution.  The latter may produce a larger interference with analytes. In addition to improved compatibility with analysis, the advantage of immobilized redox species is that of easily-accessible, high concentration of charge at the electrodes that offers high currents, and therefore high fluid velocities.  Immobilization through electropolymerization allows controlled deposition of redox species onto the electrode surfaces. The conducting polymer polyethylene dioxythiophene (PEDOT) has been used successfully for this purpose, and has generated MHD flow while maintaining good chemical stability, reversible doping states, and low redox potential, and the monomer (EDOT) is commercially available.
4, 5
The pumping efficiency of MHD can be expressed as how fast and how far fluid will flow; and those features depend on the ionic current density and charge density generated from the polymer film, respectively. Both can be enhanced by optimizing the factors that affect the film morphology such as electrodeposition conditions (cycled or stepped potential, scan rate, and deposition cycles), solvents, electrolytes, and monomer concentrations.
6, 7

Solvent and electrolyte combinations for electrodeposition play an important role in PEDOT morphology. Organic solvents are particularly useful over aqueous solutions because EDOT dissolves more easily in them and produces polymer films with higher conductivity. Organic solvents also provide a hydrophobic medium that facilitates uniform and well-adherent thick films during deposition.
8, 9
In the studies that will be reported, propylene carbonate and acetonitrile were used as solvents and LiClO4 and TBAPF6 as electrolytes for electrochemical deposition at different numbers of deposition cycles and scan rates. The studies reveal the optimized current and charge density to maximize MHD pumping efficiency by comparing electrochemical responses of PEDOT films from different solvent/electrolyte combinations and deposition conditions. Chronoamperometry (CA) and chronocoulometry (CC) were used to determine charge and current densities in different electrolyte solutions. In a MHD fluidics application, the charge and current densities are good predictors of how far and how long the fluid will flow, respectively. The chip design for the MHD experiments contained four gold microband electrodes (each is 1.5 cm X 650 μm X 250 nm). A 2.14 mm polydimethylsiloxane gasket over the chip with a 3 cm x 1.8 cm rectangular opening used to define the cell dimensions. 1100 μL electrolyte solution pipetted into the cell with polystyrene beads as MHD solution. A glass coverslip (24 x 50 mm) placed over the gasket as a ceiling of the cell which also limit the vertical direction of fluid flow. MHD fluid flow obtained by applying current through anode and cathode (two adjacent PEDOT modified electrodes were chosen) and placing a 0.37 T DC magnet under the chip. Fluid flows by following  FB = j X B in between the parallel electrodes and visualized and recorded by using a microscope interfaced with Sony Handycam camera.
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Videos were analyzed by particle tracking velocimetry and particle tracking software to get bead velocities. For a fixed electrode geometry and cell height it was found that bead velocity is proportional to the current applied.

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