Microscale electrodialysis: Concentration profiling and vortex visualization

In this paper, the microscale experimental model system for studying electrodialysis (ED) process is described, which is used for visualizing in situ fluid flow and concentration profile over a wide range of applied voltages (0-100 V). On the transparent silicone rubber, polydimethylsiloxane (PDMS), microscale channels (width similar to 1 mm, height similar to 200 mu m) were fabricated between ion exchange membranes, while allowing microscopic visualization of fluid flow and concentration changes within the ED unit cell. This ED platform was then operated on the commercial microscope with 10 mM NaCl solution. Cationic fluorescence dyes (Rhodamine 6G) were added for flow and salt concentration tracer. Concentration and fluid flow profiles within the ED cell exhibit distinct behaviors in Ohmic, limiting, and overliming regimes, respectively. Although such dynamic behaviors have been predicted previously, to the best of our knowledge, this is the first time that they were clearly visualized in a realistic ED model. Based on visualized fluid flows and concentration profiles, one can elucidate many important factors that affect ion transport (salt removal) efficiency, such as the expansion of boundary layer and vortex instability. This model system would be instrumental to test and optimize ED system parameters, and can be applicable to the study of other desalination processes such as capacitance deionization. (C) 2012 Elsevier B.V. All rights reserved.