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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 realized using one- and two-dimensional arrays of individually addressable ion-permselective membranes with interconnecting microfluidic channels. The location of powered electrodes, dictates which of the membranes are active and generates either enrichment/depletion diffusion layers, which, in turn, control the location of the preconcentrated plug. An array of such powered membranes enables formation of multiple preconcentrated plugs of the same biosample as well as of preconcentrated plugs of multiple biosample types introduced via different inlets in a selective manner. Moreover, digital-microfluidics operations such as up-down and left–right translation, merging, and splitting, can be realized, but on preconcentrated biomolecule plugs instead of on discrete droplets. This technology, based on nanoscale electrokinetics of ion transport through permselective medium, opens future opportunities for smart and programmable digital-like manipulations of preconcentrated biological particle plugs for various on-chip biological applications. [ABSTRACT FROM AUTHOR]

For a long time, based on the analysis pertaining to a perfectly charge selective interface, electro-convective instability in concentration polarization was attributed to a nonequilibrium mechanism related to the extended space charge which forms next to that of the electric double layer near the limiting current. More recently, 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 that study, addressing stability of a quiescent binary electrolyte, equal ionic diffusivities were assumed. Here we study the effect of non-equal ionic diffusivities and imposed longitudinal flow upon the onset and further nonlinear development of the equilibrium electro-convective instability at a non-perfectly permselective interface. It is observed through a suitable analytical and numerical study that the imposed flow along the perm-selective interface does not affect fundamentally the equilibrium electro-convective instability in concentration polarization either in terms of the temporal instability threshold or the resulting nonlinear flow. For the former, the critical voltage is practically identical with that in quiescent concentration polarization. For the latter, with non-slip interface conditions, the resulting nonlinear flow, with high accuracy, may be represented as a superposition of the imposed Poiseuille flow and the vortices of the quiescent instability. Differing ionic diffusivities may have a considerable effect upon the onset of the electro-convective instability. In particular, co-ionic diffusivity appreciably lower than the counter-ionic one may yield an appreciable increase of the critical voltage. This is explained by the stabilizing effect of the diffusion potential's contribution to the electric potential fluctuations. [ABSTRACT FROM AUTHOR]

We investigate the development of electro-osmotic (Teorell) oscillations at a weakly charged microporous membrane without a preimposed transmembrane electrolyte concentration drop. This drop, necessary for the occurrence of oscillations, develops spontaneously as a result of concentration polarization in the solution layers adjacent to the membrane. A three-layer model comprising a membrane flanked by two diffusion layers is proposed and analyzed for galvano- and potentiostatic regimes of operation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Presented is a novel flow sensor based on electrochemical sensing of the ionic concentration-polarization (CP) layer developed within a microchannel-ion permselective membrane device. To demonstrate the working principle of the electrochemical flow sensor, the effect of advection on the transient and steady-state ionic concentration-polarization (CP) phenomenon in microchannel-Nafion membrane systems is studied. In particular, we focused on the local impedance, measured using an array of electrode pairs embedded at the bottom of the microchannel, as well as the total current across the permselective medium, as two approaches for estimating the flow. We examined both a stepwise application of CP under steady-state flow and a stepwise application of flow under steady-state CP.