| This dissertation presents the fundamental investigation of the magnetic effects on convection called "magnetoconvection" in systems where electrochemical currents involve oxidation and reduction of redox species and how those effects can be harnessed for solution mixing and pumping in portable devices. The increasing demand for small, fast and inexpensive portable analytical systems has motivated research in development of novel microfluidic and small-scale stirring techniques. Magnetoconvective effects may arise due to the Lorentz force, magnetic field gradient force, or paramagnetic gradient force. The dominance of any one force depends on various parameters such as magnetic flux density, magnetic flux distribution, current density, solution viscosity, redox concentration, temperature, and scan rate. In one study, the magnetic effects on electrochemical currents were investigated using bonded permanent magnets and were compared with enhancement in current caused by fields produced with electromagnets and sintered permanent magnets. For the first time, the utility of bonded magnets in magnetoelectrochemistry was investigated, which is of interest in building portable mangetoconvective devices. In another study, directly embedding the electrode in a permanent magnetic material, where strong magnetic field gradient forces are important, yielded enhancements in electrochemical current consistent with convection that is significant enough to mix or stir small volumes. This setup yielded measurable current enhancements resulting from increased convection of the small volume (∼1 muL) near the microelectrode even at 0.13 T fields. This could be of use in portable probe for enhancing analytical signals for on-site applications.; In the third study, pumping fluids with the use of magnetic fields through microchannels (270 mum wide x 640 mum deep x 2 cm long) constructed from low-temperature co-fired ceramics was demonstrated. Also, the dependence of the pumping on magnetic field orientation, bidirectionality, density gradient, magnetic flux density, time scale of the applied potential (e.g. potential scan rate), electrolyte, portability, and flow rate were performed. Among redox species that were studied, nitrobenzene gave the biggest convection effect in a magnetic field. Interestingly, mixtures of a 50:50 mole ratio of ferricyanide and ferrocyanide or iron (II) and iron (III) showed a complex dependence on the redox properties, electrolyte concentration and the magnetic flux distribution. Understanding these results is very important because the existing magnetoconvective theories do not account for the complexity involved in closed spaces such as microchannels. The flow velocity obtained in microchannels was 5 mm/s, the highest reported in MHD literature. The knowledge gained through these investigations is expected to be useful in developing magnetoconvective based technologies for real-world applications. |
