Abstract
Ion concentration polarization (ICP) is a fundamental electrokinetic process that occurs near a perm-selective membrane under dc bias. Overall process highly depends on the current transportation mechanisms such as electro-convection, surface conduction and diffusioosmosis and the fundamental characteristics can be significantly altered by external parameters, once the permselectivity was fixed. In this work, a new ICP device with a bifurcated current path as for the enhancement of the surface conduction was fabricated using a polymeric nanoporous material. It was protruded to the middle of a microchannel, while the material was exactly aligned at the interface between two microchannels in a conventional ICP device. Rigorous experiments revealed out that the propagation of ICP layer was initiated from the different locations of the protruded membrane according to the dominant current path which was determined by a bulk electrolyte concentration. Since the enhancement of surface conduction maintained the stability of ICP process, a strong electrokinetic flow associated with the amplified electric field inside ICP layer was significantly suppressed over the protruded membrane even at condensed limit. As a practical example of utilizing the protruded device, we successfully demonstrated a non-destructive micro/nanofluidic preconcentrator of fragile cellular species (i.e. red blood cells).
Highlights
Rprotrude was fixed by the material property of the membrane and Rover was inversely proportional to the electrolyte concentration inside the microchannel so that the current path highly depended on the electrolyte concentration
The protruded membrane was fabricated in the micro/nanofluidic Ion concentration polarization (ICP) platform to greatly enhance the surface conduction of the ionic current which is the essential key of suppressing the unwanted instability and high shear stress due to the amplified electrokinetic responses
Experimental visualizations revealed that the ICP layer was initiated from either the end of protruded membrane or the end of microchannel at the dilute limit or the condensed limit, respectively
Summary
Recent nanoscale electrokinetic researches have focused on understanding a novel ion transportation through nanoporous membrane[1, 2] or nanochannel system[3,4,5,6,7], since it forms essential basis for developing various engineering applications such as nanofluidic diodes[8,9,10,11,12], energy harvesting device[13, 14], (bio-)molecular preconcentrator[15,16,17,18,19,20] and electro-desalinator[21,22,23,24,25]. The instability and the chaotic motions of the ion depletion zone are caused by the thermal fluctuation of the extended space charge layer and the amplified electric field inside the ion depletion zone with the absence of the surface conduction and the electro-diffusio-convection[37]. Fabricated nanochannels were unable to achieve this surface conduction enhancement because of a lower surface charge of substrates than one of Nafion (e.g. silicon, glass and PDMS ~ 10–50 mC/m2 and Nafion ~ 200–600 mC/m2)[38] Rigorous characterization of this protruded device by both experiments and numerical simulations would be given here and, a non-destructive preconcentration of fragile cells (red blood cell (RBC)) that required a stable ICP layer at high concentration and minimizing the shear stress due to strong electro-convection was successfully demonstrated
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