Concentration Polarization at Microchannel-Nanochannel Interfaces
Principal Investigators: J.G. Santiago and Thomas A. Zangle
Ionic current in nanochannels is dominated by counter-ions to the wall charge (positive ions for a negatively charged wall). When a negatively charged nanochannel is placed in series with a microchannel, this ionic flux imbalance leads to a net neutral increase in ionic strength on the cathode side and a decrease in ionic strength on the anode side (Figure 1). This phenomenon is called concentration polarization (CP). We have performed a detailed analytical, experimental and computational study of CP in a microfluidic system. Figure 2 shows a movie of a sample computational result. Figure 3 shows a simulation result and an experimental result side-by-side. Our simulation shows excellent agreement with experiments using nanochannel wall charge as the only fitting parameter.
Our results show that CP can create shocks of disturbed concentration and electric field which propagate rapidly (~100 s) into several cm long microchannels. These concentration shock waves can prevent analyte ions from reaching a nanofluidic device for sample detection or separation. Our analysis provides insight into the behavior of these shock waves. Figure 4 shows the results of 70+ CP experiments plotted as the enrichment region concentration factor, , versus. the depletion to enrichment region shock velocity ratio, . This result shows excellent agreement to the theoretical prediction that . We are using our CP models to predict the behavior of analyte ions in a microchannel-nanochannel system.
Figure 1. Schematic of microchannel-nanochannel system. Dark grey indicates enrichment. White indicates depletion. When a current is applied to a system with negatively charged walls, an enrichment region is formed on the cathode side of the nanochannel and a depletion region is formed on the anode side.
Figure 2. Movie showing sample computational results for propagating CP enrichment and depletion zones.
Figure 3. Spatiotemporal plots showing (a) simulation result (b) experimental result for 1 mM Alexa Fluor 488 in a 1 mm deep microchannel, 50 nm deep nanochannel system. The nanochannel is 100 mm long, and is visible as a grey region at the center of the image. White indicates high concentration. Black indicates low concentration. These results are for a constant current of 200 pA.
Figure 4. Background electrolyte enrichment factor versus enrichment to depletion shock velocity ratio. The solid line is a least squares best fit line with a fixed y-intercept of 1. We measured shock velocities and enrichment region concentration for 70+ realizations, varying the concentration of Alexa Fluor 488 and the nanochannel height.