Characterization and pumping - Redox magnetohydrodynamics in a microfluidic channel
by Arumugam, Prabhu U.; Fakunle, Eyitayo S. O.; Anderson, Emily Clary; Evans, Stephanie R.; King, Kevin G.; Aguilar, Zoraida P.; Carter, Christopher S.; Fritsch, Ingrid
By adding redox species, both aqueous and nonaqueous solutions can be pumped along the length of a microfluidic channel when low voltages (to produce current) are applied across the channel's breadth in the presence of a magnetic field applied across its height. Control of flow rate is possible by varying the magnitudes of the voltage and magnetic field and by changing the redox concentration. Direction is determined by the voltage polarity and magnetic field orientation. We have demonstrated a dc redox magnetohydrodynamic (MHD) pump in microchannels (270 mu m wide x 640 mu m deep x 2 cm long) that were constructed from low-temperature cofired ceramics with screen-printed gold electrodes on opposing sidewalls. Redox MHD involves the generation of current between the electrodes in a solution through the oxidation/reduction of redox species in the presence of a magnetic field. When the current and magnetic fields are perpendicular, a Lorentz force results, causing fluid flow along the length of the channel. The pumping performance was investigated as a function of type and concentration of redox species, magnetic flux density, applied voltage, and time scale. Studies show bidirectional capability by changing the direction of the current. Flow velocities of up to 5.0 mm/s were observed in a solution of 0.5 M nitrobenzene (NB) and 0.5 M tetrabutylammonium hexafluorophosphate (TBAPF(6)) in acetonitrile using a 0.41 T NdFeB permanent magnet, at an applied voltage of - 1.3 V vs Ag/AgCl (saturated KCl) (approximately a potential difference of 1.2 - 1.7 V between wall electrodes in the microchannel) and the corresponding current enhancements (caused by the increased convection) were as large as 145%. Flow rates for 0.1 M NB and 0.25 M NB in 0.1 M TBAPF6 in acetonitrile were measured at three different applied voltages near the redox potential of NB. Comparisons were also made to theory. A mixture of oxidized and reduced forms of the same redox couple, instead of a single species like NB, can avoid electrode dissolution at the oxidizing electrode (or bubble generation at either electrode). One example of this approach involves a solution of 0.125 M Fe2+ and 0.125 M Fe3+ in 3.0 M KCl and another involves a solution of 0.125 M Fe(CN)(6)(3-) and 0.125 M Fe(CN)(6)(4-) in 2.0 M KCl, where potential differences between wall electrodes in the microchannel can be less than 0.4 V. Findings from these studies should be useful in development of sealed microanalytical devices using redox MHD pumps with possible lab-on-a-chip applications. (c) 2006 The Electrochemical Society.
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