Experimental results will be presented that quantify the effects of natural convection induced by electrochemically-generated density gradients at microelectrodes in small, confined volumes. This work is important in explaining limiting currents at electrodes that could affect interpretation of chemical 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 volume system (12.5 mm wide � 27.5 mm long � 840. �m high) containing redox species and where the Reynolds number 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. Electrochemical reactions at the electrode-solution interface and their associated counter ion movement in the electrode vicinity lead to a density mismatch with respect to the bulk solution, thus generating convection due to the onset of buoyant forces. In general, oxidation of ferrocyanide at the anode generates a less dense volume element than bulk, which rises, pushing fluid upward and outward at the lid of the cell. Surrounding solution adjacent to the electrode substrate replaces the rising fluid, producing a net circulation. At the cathode, reduction of ferricyanide takes place, circulating fluid in the reverse direction. The convection affects the concentration distribution near the electrode-solution 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 determined by tracking the movement of microbeads added to the solution. Experiments involved variations in electrode size and geometry, electrode bias, applied current, and concentration of redox species.

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

247th National Meeting (2014)

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