Visualization and Measurement of Natural Convection from Electrochemically-Generated Density Gradients at Concentric Microdisk and Ring Electrodes in a Microfluidic System
by Sahore, V.; Kreidermacher, A.; Khan, F. Z.; Fritsch, I.
Natural convection actuated by electrochemically-generated density gradients at microelectrodes was investigated under different conditions by simultaneously visualizing fluid flow with the electrochemical response. The studies elucidate deviations of electrochemical behavior from theoretical expectations and parameters that control natural convection, which can be exploited in electroanalysis, microfluidics, and electrodeposition. Experiments involved an enclosed, small volume containing 0.00475-0.095 M each of K3Fe(CN)(6) and K4Fe(CN)(6) in 0.095 M KCl, over concentric gold disk (radius: 16-156 mu m) and ring (inner radius: 2001600 mu m, outer radius: 250-2000 mu m) microelectrodes. Fluid velocities were obtained with video microscopy by tracking 10-mu m beads added to the solution. Flow radiates near the disk either inwardly or outwardly at the bottom of the cell and reverses direction at the top, producing a vertical circulation. Maximum velocities of similar to 10 mu m/s were measured for the 156-mu m disk in 0.095 M. After application of potential or current, the onset of natural convection occurred at shorter times (6 s) than measurable affects in electrochemical current/voltage responses (tens of seconds). Convection from density gradients occurred without corresponding changes in electrochemical responses for the 78-mu m disk at the lowest concentration (0.00475 M) and for the smallest, 16-mu m disk at the highest concentration (0.095 M).