Electrochemically-generated density-gradient induced natural convection in microfluidic systems

by Sahore, Vishal; Kreidemacher, Adam; Nash, Christena K.; Fritsch, Ingrid


Exptl. results will be presented that quantify the effects of natural convection induced by electrochem.-generated d. gradients at microelectrodes in small, confined vols. This work is important in explaining limiting currents at electrodes that could affect interpretation of chem. analyses, deciphering the uniformity of electrodeposited films, and implementing new mixing approaches in microfluidic systems. Gold microfabricated electrodes in the shape of disks, rings, and bands of varying dimensions were selectively activated in an initially static, small vol. system (12.5 mm wide × 27.5 mm long × 840.µm high) contg. redox species and where the Reynolds no. was less than unity. The redox species consisted of potassium ferricyanide and potassium ferrocyanide in KCl supporting electrolyte and for comparison, conducting-polymer-modified electrodes in supporting electrolyte. Electrochem. reactions at the electrode-soln. interface and their assocd. counter ion movement in the electrode vicinity lead to a d. mismatch with respect to the bulk soln., thus generating convection due to the onset of buoyant forces. In general, oxidn. of ferrocyanide at the anode generates a less dense vol. element than bulk, which rises, pushing fluid upward and outward at the lid of the cell. Surrounding soln. adjacent to the electrode substrate replaces the rising fluid, producing a net circulation. At the cathode, redn. of ferricyanide takes place, circulating fluid in the reverse direction. The convection affects the concn. distribution near the electrode-soln. interface, and thus causes the electrode current or potential to change differently than would be expected by diffusion alone. Natural convection was thus investigated by monitoring current and potential as a function of time, and coinciding those responses with the fluid velocity that was detd. by tracking the movement of microbeads added to the soln. Expts. involved variations in electrode size and geometry, electrode bias, applied current, and concn. of redox species.