Ion Concentration Polarization in Branched Microchannels: Effect of Membrane Thickness and Applied Voltage

Ion concentration polarization (ICP) is a promising electrokinetic technique for the concentration and separation of nanoparticles in microfluidic systems. In this study, we investigated how key parameters, including Nafion membrane thickness, applied voltage, and sample flow rate, influence the size of the ion depletion zone (IDZ), which is a critical factor governing ICP efficiency. Nafion membranes were fabricated via solution casting and patterning, producing non-uniform profiles with thinner centers and thicker edges. We found that thinner membranes (formed from 0.5 to 0.75 wt% solutions) led to IDZ widths 2-5 times greater than those of thicker membranes, likely due to nanogap formation at membrane-channel interfaces that enhanced ion transport. Additionally, higher applied voltages consistently enlarged the IDZ, consistent with the Nernst-Planck model, while increasing the flow rates reduced it. Notably, the combination of thin Nafion membranes and high voltage enabled stable IDZ formation, even at high flow rates. These findings offer important design insights for enhancing the performance and throughput of ICP-based nanoparticle manipulation devices.