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Based on the continuum mechanics theory, we propose herein a universal design of microfluidic ionic diode based on external concentration polarization of a gating ion-selective medium embedded in the microfluidic network with four power terminals. This micro/nanofluidic hybrid chip employs a cation-exchange nanoporous membrane (CEM) coupled with both a control and output microfluidic channel. Under the action of a vertical electric field throughout the CEM, nanoscale surface conduction of excessive counterions within the charged nanopores is converted to the propagation of either enriched or depleted boundary toward the opposing electrode-terminal in phase with the electroconvective flow, thereby making an adjustment in the electrical conductance of output microchannel for achieving high-flux field-effect current control and diode functionality. Three basic working states, including the “on,” “transition,” and “off” statuses, are distinguished in different ranges of source voltage magnitude. The rectification factor of the proposed ionic circuit platform can attain one hundred-fold even at small source and gate voltages. The presented field-effect-tunable microfluidic ion diode is easily scalable, permits appreciable fluid flow due to an intrinsically small hydrodynamic resistance, and holds promise for producing high-flux ion current rectification in next-generation integrated circuits.

The phenomenon of electrothermal (ET) convection has recently captured great attention for transporting fluidic samples in microchannels embedding simple electrode structures. In the classical model of ET‐induced flow, a conductivity gradient of buffer medium is supposed to arise from temperature‐dependent electrophoretic mobility of ionic species under uniform salt concentrations, so it may not work well in the presence of evident concentration perturbation within the background electrolyte. To solve this problem, we develop herein a microscopic physical description of ET streaming by fully coupling a set of Poisson‐Nernst‐Planck‐Navier‐Stokes equations and temperature‐dependent fluid physicochemical properties. A comparative study on a standard electrokinetic micropump exploiting asymmetric electrode arrays indicates that, our microscopic model always predicts a lower ET pump flow rate than the classical macroscopic model even with trivial temperature elevation in the liquid. Considering a continuity of total current density in liquids of inhomogeneous polarizability, a moderate degree of fluctuation in ion concentrations on top of the electrode array is enough to exert a significant influence on the induction of free ionic charges, rendering the enhanced numerical treatment much closer to realistic experimental measurement. Then, by placing a pair of thin‐film resistive heaters on the bottom of an anodic channel interfacing a cation‐exchange medium, we further provide a vivid demonstration of the enhanced model's feasibility in accurately resolving the combined Coulomb force due to the coexistence of an extended space charge layer and smeared interfacial polarizations in an externally‐imposed temperature gradient, while this is impossible with conventional linear approximation. This leads to a reliable method to achieve a flexible regulation on spatial‐temporal evolution of ion‐depletion layer by electroconvective mixing. These results provide useful insights into ET‐based flexible control of micro/nanoscale solid entities in modern micro‐total‐analytical systems.

Induced‐charge electroosmosis (ICEO) has attracted tremendous popularity for driving fluid motion from the microfluidic community since the last decade, while less attention has been paid to ICEO‐based nanoparticle manipulation. We propose herein a unique concept of hybrid electroosmotic kinetics (HEK) in terms of bi‐phase ICEO (BICEO) actuated in a four‐terminal spiral electrode array, for effective electrokinetic enrichment of fluorescent polystyrene nanoparticles on ideally polarizable metal strips. First, by alternating the applied AC voltage waves between consecutive discrete terminals, the flow stagnation lines where the sample nanoparticles aggregate can be switched in time between two different distribution modes. Second, we innovatively introduce the idea of AC field‐effect flow control on BICEO; by altering the combination of gating voltage sequence, not only the number of circulative particle trapping lines is doubled, but the collecting locations can be flexibly reconfigured as well. Third, hydrodynamic streaming of DC‐biased BICEO is tested in our device design, wherein the global linear electroosmosis dominates BICEO contributed from both AC and DC components, resulting in a reduction of particle enrichment area, while with a sharp increase in sample transport speed inside the bulk phase. The flow field associated with HEK is predicted using a linear asymptotic analysis under Debye–Huckel limit, with the simulation results in qualitative agreement with in‐lab observations of nanoparticle trapping by exploiting a series of improved ICEO techniques. This work provides an affordable and field‐deployable platform for real‐time nanoparticle trapping in the context of dilute electrolyte.

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC‐biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC‐biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC‐field induced‐charge electrokinetic (ICEK) phenomenon, which behaves as counter‐rotating micro‐vortices around the ideally polarizable surfaces of FE. Finite‐element simulation is carried out by coupling the dual‐frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500 nm) and BSA molecules (∼10 nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous‐flow analyte manipulation in modern microfluidic systems.