1. Introduction

  Ion concentration polarization is the fundamental transport phenomenon that occurs near ion-selective membranes, but this important membrane phenomenon has been poorly understood due to theoretical and experimental challenges. Here, we report the first direct measurements of detailed flow and electric potential profiles within and near the depletion region. This work is an important step towards a full characterization of this coupled transport problem. Using microfabricated electrodes integrated with the microfluidic device, we measured and confirmed that the electric field inside an ion depletion region is amplified more than 30 fold compared to outside of the depletion zone due to the highly non-uniform ion concentration distribution along the microchannel. As a result, the electrokinetic motion of both fluid (electroosmosis) and particle (electrophoresis) was significantly amplified. The detailed flow profile within the depletion zone was also measured for the first time by optically tracking photobleached neutral dye molecules. We further showed that the amplified electrokinetic flows generated in this device may be used as a field-controlled, microfluidic fluid pump and switch.

2. Device and System

Figure 1. Schematic diagram of (a) micro/nanofluidic hybrid channel system and (b) electrokinetic configurations under dc bias in the system.

3. Amplified Electric Fields inside the depletion zone

Figure 2. (a) In situ measurement of local electric fields inside and outside the ion depletion using microelectrodes integrated along the microchannel. (b) Schematic plot of electric potential and electrical field along the main microchannel (EPH: electrophoresis). The electrokinetic migration of charged particles in (c) SG and (d) DG device. Estimated velocity of pointed particles was approximately (c) 140mm/sec at VH = 10V and VL = 5V and (d) 500mm/sec at VH = 20V and VL = 15V.

3. Amplified Particle Motions

Figure 3. The electrokinetic migration of charged particles in SG (left) and DG device (right). Estimated velocity of pointed particles was approximately (left) 140mm/sec at VH = 10V and VL = 5V and (right) 500mm/sec at VH = 20V and VL = 15V.

4. Amplified Fluid Motions

Figure 4. Details of AEK flow field inside/outside the ion depletion zone by photobleaching technique. (b) Schematics of AEK flow as a function of ET.

5. Conclusions

We have measured both the electric field and flow profile within the strongly depleted ICP regions near perm-selective nanojunctions and determined the exact flow mechanism in this coupled electrokinetic flow system. This study has several important implications in understanding the amplified electrokinetic response due to ICP. Once ion depletion is triggered, the electric field distribution in the system becomes highly non-uniform, generating extremely high electric fields within the ion depletion zone. As a result, electrokinetic responses are significantly amplified within that region and significantly affect the overall motion of both fluids and particles. Based on current results, efficient concentration of peptides and proteins previously reported may be explained by AEK flow motions. Fast convection within the microchannel seems to prevent any significant development of space charge layer, as demonstrated by our concentration estimation within the depletion zone (via field measurement). However, the role of nonlinear electroosmotic slip, which is expected at the nanojunction interface, demands further investigation, possibly with higher spatial resolution in flow measurements. The AEK mechanism presented here is also an attractive candidate for microfluidic flow pumping and switching, as it can generate much higher flow rate at lower driving potentials than needed for equilibrium EOF and can be independent of the fluid’s ion concentration. These systems can potentially replace pneumatic pumping actuation with field-driven, high throughput microfludic pumping and switching, with wide applicability to the field of microfluidics.

References

1. S. J. Kim, Y. C. Wang, J. H. Lee, H. Jang, and J. Han, Physical Review Letters, 99, 045501 (2007).
2. Y. C. Wang, A. Stevens, and J. Han, Analytical Chemistry, 77, 4293 (2005).
3. S. J. Kim and J. Han, Analytical Chemistry, 80, 3507 (2008).