In this paper, the variation of permselectivity in the course of concentration polarization is systematically analyzed for a three-layer membrane system consisting of a nonperfectly permselective ion exchange membrane, homogeneous or heterogeneous, flanked by two diffusion layers of a binary univalent electrolyte. For a heterogeneous membrane, an ionic transport model is proposed, which is amenable to analytical treatment. In this model, assuming a constant fixed charge in the membrane and disregarding water splitting, the entire transport problem is reduced to solution of a single algebraic equation for the counterion transport number. It is concluded that for both types of membrane the concentration polarization may significantly affect the permselectivity of the system through the effects of the induced nonuniformity of the coion diffusion flux in the membrane (convexity of the coion concentration profile) and varying membrane-solution interface concentration. While the former is significant for low membrane fixed charge density, for a heterogeneous membrane, the latter might be considerably affected by the flux focusing effect at the permeable membrane segments.

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Herein, we demonstrate digital microfluidics-like manipulations of preconcentrated biomolecule plugs within a continuous flow that is different from the commonly known digital microfluidics involving discrete (i.e. droplets) media. This is...

Ionic concentration-polarization (CP)-based biomolecule preconcentration is an established method for enhancing the detection sensitivity of target biomolecules. However, the formed preconcentrated biomolecule plug rapidly sweeps over the surface-immobilized antibodies, resulting...

Ionic current in a binary electrolyte passing through a charge-selective interface (electrode, ion exchange membrane, micro-nano-channel junction) is a basic element of many electrochemical engineering and micro-fluidic processes, such as electrodeposition, electrodialysis and protein preconcentration. Such current passage is diffusion-limited in the sense that it induces a decrease of electrolyte concentration towards the interface (concentration polarization) whose expression is the saturation of current upon increasing voltage at some value -the limiting current. Upon a further increase of voltage, this saturation is followed by a relatively rapid current increase -over-limiting conductance regime. It is commonly accepted that in open systems over-limiting conductance is mediated by a micro-scale vortical flow which spontaneously develops as a result of electro-convective instability of quiescent concentration polarization near the limiting current. Electro-convection is a flow driven by the electric force acting either upon the space charge of the interfacial electric double layer (electro-osmosis) or the residual space charge of the quasi-electroneutral bulk (bulk electroconvection). There are two types of electro-osmosis, the equilibrium and the non-equilibrium one, the former relating to the action of the tangential electric field upon the space charge of the electric double layer, and the latter pertaining to the similar action upon the extended space charge which forms next to the electric double layer near the limiting current. For a perfectly charge-selective interface, concentration polarization under the equilibrium electro-osmotic slip condition is stable, and so it is with respect to bulk electro-convection, as opposed to non-equilibrium electro-osmosis which may cause instability. For this reason until recently, the electro-convective instability in concentration polarization was attributed to this latter mechanism. Lately, it was shown that imperfect charge-selectivity of the interface makes equilibrium instability possible, driven by either equilibrium electro-osmosis or bulk electro-convection, or both. In this paper we identify and analyze the major surface and bulk factors affecting the electro-convective instability. These factors, some known previously under the names of diffusio-osmosis, electro-osmosis or bulk electroconvection, and some newly identified in this paper, are manifestations of the electric force and pressure gradient, balanced by the viscous force acting in various locations in solution. The contribution of these factors to hydrodynamic stability in concentration polarization is analyzed for a varying perm-selectivity of the interface.