Magnetic field driven convective transport at inlaid disk microelectrodes: The dependence of flow patterns on electrode radius

Magnetic field driven convective transport at inlaid-disk Pt microelectrodes was investigated as a function of the electrode radius (6≤a≤250 μm) using video imaging and steady-state voltammetry. A uniform magnetic field (B=1.0 T), oriented orthogonal to the microelectrode surface in order to induce rotational flow, is shown to cause either an increase or decrease in voltammetric limiting currents (−37 to +119%) depending solely on the size of the electrode. For inlaid disk electrodes with radii less than ∼100 μm, the magnetic field driven flow results in a decrease in transport limited current, a consequence of rotational solution flow adjacent to the surface preventing gravity-driven natural convection. The magnetic force at electrodes with radii larger than ∼100 μm generates a vortex flow pattern, resulting in convective transport of redox reactant from the bulk solution inward towards the electrode surface. The difference in flow patterns is shown to result from a transition from planar to radial diffusion as the electrode size is reduced.