Jie Li

Soft nanochannels have attracted considerable attention in micro-/nanofluidic applications due to their unique capability for regulating ion transport. However, the coupling mechanism between the rheological behavior of non-Newtonian fluids and electrokinetic transport, particularly under active field-effect transistor regulation, remains insufficiently understood. In this study, a coupled Poisson–Nernst–Planck and Navier–Stokes numerical model was developed for polyelectrolyte layer (PEL)-functionalized soft nanochannels to systematically investigate the electrokinetic transport characteristics of power-law fluids under field-effect control. The results demonstrate that negative gate voltages significantly enhance electro-osmotic flow (EOF), ionic current, and cation selectivity compared with positive gate voltages. As the PEL charge density increases, cation selectivity is continuously strengthened, while both EOF velocity and ionic current under negative gate voltage significantly increase. For instance, at a power-law index of 0.8, a PEL thickness of 5 nm, and a gate voltage of −50 V, increasing the PEL charge density from 10 to 100 mol/m3 enhances the EOF velocity by approximately 476%, the ionic current by 144%, and the cation selectivity coefficient by 10%. In contrast, the influence of PEL thickness exhibits a more complex behavior. Increasing the PEL thickness generally promotes ionic current while simultaneously reducing ion selectivity, whereas the EOF velocity displays a non-monotonic dependence on PEL thickness. These findings reveal that the interplay between gate-induced charge regulation and fluid rheology governs ion transport in soft nanochannels, providing fundamental insights for the precise and tunable manipulation of electrokinetic transport in nanofluidic systems.

This study investigates the induced-charge electro-osmotic flow generated by an eccentric cylindrical electrode within a rotating (ROT) electric field. Using a bipolar coordinate system, we derive an analytical solution for the induced zeta potential and slip velocity on the electrode surface under the excitation of a ROT electric field, with theoretical predictions rigorously validated by numerical simulations. Analysis reveals that geometric eccentricity fundamentally breaks the flow symmetry, inducing a novel asymmetric nested vortex structure and resulting in a non-monotonic dependence of the peak flow velocity on the offset distance. While increasing the electric field amplitude or electrode size enhances the flow intensity, higher excitation frequencies induce flow attenuation due to incomplete double-layer relaxation. Furthermore, by integrating an alternating current flow field-effect transistor, dynamic flow reconfiguration was achieved: the amplitude of the gate voltage linearly scales the flow velocity, while its initial phase provides a distinct control dimension, transforming classical symmetric quadrupolar vortices into tunable asymmetric modes via constructive and destructive potential interference. This work provides a quantitative analytical framework for active, programable flow reconfiguration. By coupling geometric asymmetry with dynamic electric field modulation, it enables precise, on-demand fluid manipulation in advanced microfluidic devices.

The exploration of ion transport properties in nanochannels with non-uniform surface charge density is imperative to enhance comprehension of the ionic current rectification phenomenon and to broaden the implementation of the rectification effect. However, the majority of research in this domain is confined to Newtonian fluids. In this study, the Poisson–Nernst–Planck equation is employed to address the electro-osmotic flow of power-law fluids in a nanochannel. The impact of the charging conditions of the nanochannel wall and the fluid properties on the ion concentration, ionic current, and velocity is investigated through numerical simulations. It is observed that when the length of the high surface charge density wall is increased from 10 to 40 nm, the ratio of the maximum to minimum concentrations on the central axis increases from 5.5 to 9.1. An increase in the power-law index, from 0.9 to 0.95, results in a 45% rise in the maximum total ion concentration along the central axis. In addition, both decreasing the low surface charge density and increasing the power-law index can effectively improve the rectification ratio of the nanochannel with non-uniform surface charge density. Specifically, a 38% enhancement in the ionic current rectification ratio (CRR) is observed when the low surface charge density is reduced from −5 to −1.7 mC/m2. Furthermore, a 42% increase in the CRR is attained by increasing the power-law index from 0.9 to 1.0.

We investigated the phenomenon of induced charge electro-osmotic flow (ICEOF) driven by a cylindrical electrode in a cylindrical container, focusing on the flow characteristics under different electric field conditions. Through analytical derivation, we provided the approximate expressions for the zeta potential and slip velocity induced around the surface of the electrode under direct current and rotating electric fields (ROT), as well as in the presence of an alternating current flow field effect transistor. The study primarily analyzes the flow velocity distribution under the influence of the ROT electric field, and the validity of the derived formulas was verified through numerical simulations. The results show that increasing the electric field amplitude and electrode size enhances the intensity of the ICEOF, while the frequency of the ROT electric field causes a phase lag in the flow velocity, characterized by a decrease in the peak flow velocity and a delay in the flow phase. Furthermore, applying a gate voltage creates a sinusoidal flow velocity band, where both the peak and width of the velocity band are modulated by the gate voltage amplitude, and the phase of the velocity band is controlled by the initial phase of the gate voltage.

Efficient manipulation and mixing of viscoelastic fluids at the microscopic scale is essential for advancing various applications in biomedical and chemical analysis. Chaos-induced charge electro-osmosis (CICEO), which utilizes irregular vortex flow fields generated by electric field-induced charges to improve mixing efficiency, is a promising solution. Herein, a two-dimensional cavity filled with an electrolyte solution with a polarizable conductive cylinder in the center of the cavity is considered. The simplified Phan-Thien-Tanner constitutive model is used to accurately describe the viscoelastic behavior of the fluid inside the microfluidic mixer. The fully coupled Navier–Stokes (NS) and Poisson-Nernst-Planck equations are solved using numerical simulations. The results show that both the thin electric double layer and high electric field strength promote CICEO behavior. The near-wall charge density at κ̃ = 1000 is 32 154, which is about 1763 times higher than that at κ̃ = 10. The peak velocity is about 1 for Ṽ = 1, while it is about 130 for Ṽ = 30.

The study of the complex rheological properties of viscoelastic fluids in nanochannels will facilitate the application of nanofluidics in biomedical and other fields. However, the flow of viscoelastic fluids in nanochannels has significant instabilities, and numerical simulation failures are prone to occur at high Weissenberg numbers (Wi). In this study, the simplified Phan-Thien-Tanner viscoelastic fluid model is solved using the log-conformation tensor approach, and the effects of rheological parameters of the viscoelastic fluid, such as the Weissenberg number (Wi), extensibility parameter (ε), and viscosity ratio (β), on the flow characteristics and flow instability within the nanochannel are investigated. The results indicate that the variation of rheological parameters of viscoelastic fluids has a significant effect on the flow state and flow instability of fluids in nanochannels. When the rheological parameters are in a specific range, the flow velocity and outlet current in the nanochannel exhibit relatively regular periodic fluctuations. As the flow transitions from an up-and-down moving single-vortex state to a symmetric double-vortex state, the average velocity of the central axis in the nanochannel is increased by about 15%. Furthermore, when Wi increases from 150 to 400, the length and height of the vortex increase by 50% and 100%, respectively.