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The surface charge property of nanofluidic devices plays an essential role in electrokinetic transport of ions, fluids, and particles in them. The nanofluidic field effect transistor (FET), referring to a nanochannel embedded with an electrically controllable gate electrode, provides a simple way to rapidly regulate its surface charge property, which in turn controls the electrokinetic transport phenomena within the nanochannel. In this study, approximate analytical expressions are derived for the first time to estimate the surface-charge property and electroosmotic flow (EOF) in charge-regulated nanochannels tuned by the nanofluidic FET and are validated by comparing their predictions to the existing experimental data available from the literature. The control of the surface charge property as well as the EOF by the nanofluidic FET depends on the pH and ionic concentration of the aqueous solution.

The boundary effect on the diffusiophoretic behavior of a particle is analyzed theoretically by considering the diffusiophoresis of a charged sphere under arbitrary surface potential and double-layer thickness at an arbitrary position in an uncharged spherical cavity. We show that the phenomenon under consideration is governed by double-layer relaxation, chemiosmotic/diffusioosmotic flow, and two types of competing double-layer polarization. The presence of the cavity has a profound influence on the diffusiophoretic behavior of the particle, especially when the surface potential is high. For instance, the scaled diffusiophoretic velocity of the particle has a local maximum as the position of the particle varies; it may have a local maximum and local minimum as the thickness of the double-layer varies. The significance of the effect of double-layer relaxation depends upon the level of surface potential and magnitude of the electric Peclet number.

Nanopores functionalized with synthetic or biological polyelectrolyte (PE) brushes have significant potentials to rectify ionic current and probe single biomacromolecules. In this work, electric-field-induced ion transport and the resulting conductance in a PE-modified nanopore are theoretically studied using a continuum-based model, composed of coupled Poisson−Nernst− Planck (PNP) equations for the ionic mass transport, and Stokes and Brinkman equations for the hydrodynamic fields in the exterior and interior of the PE layer, respectively. Because of the competition between the transport of counterions and co-ions in the nanopore, two distinct types of ion concentration polarization (CP) occur at either opening of the PE-modified nanopore. These distinct CP behaviors, which significantly affect the nanopore conductance, can be easily manipulated by adjusting the bulk salt concentration and the imposed potential bias. The induced CP in the PEmodified nanopore is more appreciable than that in the corresponding bare solid-state nanopore.

Nanopores have emerged as promising next-generation devices for DNA sequencing technology. The two major challenges in such devices are: (i) find an efficient way to raise the DNA capture rate prior to funnelling a nanopore, and (ii) reduce the translocation velocity inside it so that single base resolution can be attained efficiently. To achieve these, a novel soft nanopore comprising a solid-state nanopore and a functionalized soft layer is proposed to regulate the DNA electrokinetic translocation. We show that, in addition to the presence of an electroosmotic flow (EOF), which reduces the DNA translocation velocity, counterion concentration polarization (CP) occurs near the entrance of the nanopore. The latter establishes an enrichment of the counterion concentration field, thereby electrostatically enhancing the capture rate. The dependence of the ionic current on the bulk salt concentration, the soft layer properties, and the length of the nanopore are investigated. We show that if the salt concentration is low, the ionic current depends largely upon the length of the nanopore, and the density of the fixed charge of the soft layer, but not upon its degree of softness. On the other hand, if it is high, ionic current blockade always occurs, regardless of the levels of the other parameters. The proposed soft nanopore is capable of enhancing the performance of DNA translocation while maintaining its basic signature of the ionic current at high salt concentration. The results gathered provide the necessary information for designing devices used in DNA sequencing.

The diffusiophoresis of a concentrated spherical dispersion of colloidal particles subject to a small electrolyte gradient is analyzed theoretically for an arbitrary zeta potential and double layer thickness. In particular, the influence of the difference in the diffusivities of cations and anions is discussed. A unit cell model is used to simulate a spherical dispersion, and a pseudospectral method is adopted to solve the equations governing the phenomenon under consideration. We show that, as in the case of an infinitely dilute dispersion, when the diffusivities of cations and anions are different, the diffusiophoretic mobility is no longer an even function of the zeta potential or double layer thickness. In contrast to the case of identical diffusivity of cations and anions, a local electric field is induced in the present case due to an unbalanced charge distribution between higher and lower concentration regions. Depending upon the direction of this induced electric field, the diffusiophoretic mobility can be larger or smaller than that for the case of identical diffusivity. The diffusiophoretic mobility is influenced mainly by the induced electric field arising from the difference in the ionic diffusivities, the concentration gradient, and the effect of double layer polarization.

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