Self-Diffusiophoresis of Janus Catalytic Micromotors in Confined Geometries

The self-diffusiophoresis of Janus catalytic micromotors (JCMs) in confined environment is studied using direct numerical simulations. The simulations revealed that, on average, the translocation of a JCM through a short pore is moderately slowed down by the confinement. This slowdown is far weaker compared to the transport of particles through similar pores driven by forces induced by external means or passive diffusiophoresis. Pairing of two JCMs facilitates the translocation of the one JCM entering the pore first but slows down the second JCM. Depending on its initial orientation, a JCM near the entrance of a pore can exhibit different rotational motion, which determines whether it can enter the pore. Once a JCM enters a narrow pore, it can execute a self-alignment process after which it becomes fully aligned with the pore axis and moves to the center line of the pore. Analysis of these results showed that, in addition to hydrodynamic effect, the translation and rotation of JCM is also affected by the “chemical effects”, i.e., the modification of the chemical species concentration around a JCM by confining walls and neighboring JCMs. These chemical effects are unique to the self-diffusiophoresis of JCMs and should be considered in design and operations of JCMs in confined environment

Self-Diffusiophoresis of Janus Catalytic Micromotors in Confined Geometries

The self-diffusiophoresis of Janus catalytic micromotors (JCMs) in confined environment is studied using direct numerical simulations. The simulations revealed that, on average, the translocation of a JCM through a short pore is moderately slowed down by the confinement. This slowdown is far weaker compared to the transport of particles through similar pores driven by forces induced by external means or passive diffusiophoresis. Pairing of two JCMs facilitates the translocation of the one JCM entering the pore first but slows down the second JCM. Depending on its initial orientation, a JCM near the entrance of a pore can exhibit different rotational motion, which determines whether it can enter the pore. Once a JCM enters a narrow pore, it can execute a self-alignment process after which it becomes fully aligned with the pore axis and moves to the center line of the pore. Analysis of these results showed that, in addition to hydrodynamic effect, the translation and rotation of JCM is also affected by the “chemical effects”, i.e., the modification of the chemical species concentration around a JCM by confining walls and neighboring JCMs. These chemical effects are unique to the self-diffusiophoresis of JCMs and should be considered in design and operations of JCMs in confined environment

Tunable Streaming Current in a pH-Regulated Nanochannel by a Field Effect Transistor

Many experimental results demonstrated that ion transport phenomena in nanofluidic devices are strongly dependent on the surface charge property of the nanochannel. In this study, active control of the surface charge property and the streaming current, generated by a pressure-driven flow, in a pH-regulated nanochannel using a field effect transistor (FET) are analyzed for the first time. Analytical expressions for the surface charge property and the streaming current/conductance have been derived taking into account multiple ionic species, surface chemistry reactions, and the Stern layer effect. The model is validated by the experimental data of the streaming conductance in the silica nanochannel available in the literature. Results show that the pH-dependent streaming conductance of the gated silica nanochannel is consistent with its modulated zeta potential; however, the salt concentration-dependent streaming conductance might be different from the zeta potential behavior, depending on the solution pH and the gate potential imposed. The performance of the field effect modulation of the zeta potential and the streaming conductance is significant for lower solution pH and salt concentration. The results gathered are informative for the design of the next-generation nanofluidics-based power generation apparatus

pH-Regulated Ionic Conductance in a Nanochannel with Overlapped Electric Double Layers

Accurately and rapidly analyzing the ionic current/conductance in a nanochannel, especially under the condition of overlapped electric double layers (EDLs), is of fundamental significance for the design and development of novel nanofluidic devices. To achieve this, an analytical model for the surface charge properties and ionic current/conductance in a pH-regulated nanochannel is developed for the first time. The developed model takes into account the effects of the EDL overlap, electroosmotic flow, Stern layer, multiple ionic species, and the site dissociation/association reactions on the channel walls. In addition to good agreement with the existing experimental data of nanochannel conductance available from the literature, our analytical model is also validated by the full model with the Poisson–Nernst–Planck and Navier–Stokes equations. The EDL overlap effect is significant at small nanochannel height, low salt concentration, and medium low pH. Neglecting the EDL overlap effect could result in a wrong estimation in the zeta potential and conductance of the nanochannel in a single measurement

Controlling pH-Regulated Bionanoparticles Translocation through Nanopores with Polyelectrolyte Brushes

A novel polyelectrolyte (PE)-modified nanopore, comprising a solid-state nanopore functionalized by a nonregulated PE brush layer connecting two large reservoirs, is proposed to regulate the electrokinetic translocation of a soft nanoparticle (NP), comprising a rigid core covered by a pH-regulated, zwitterionic, soft layer, through it. The type of NP considered mimics bionanoparticles such as proteins and biomolecules. We find that a significant enrichment of H⁺ occurs near the inlet of a charged solid-state nanopore, appreciably reducing the charge density of the NP as it approaches there, thereby lowering the NP translocation velocity and making it harder to thread the nanopore. This difficulty can be resolved by the proposed PE-modified nanopore, which raises effectively both the capture rate and the capture velocity of the soft NP and simultaneously reduces its translocation velocity through the nanopore so that both the sensing efficiency and the resolution are enhanced. The results gathered provide a conceptual framework for the interpretation of relevant experimental data and for the design of nanopore-based devices used in single biomolecules sensing and DNA sequencing

DNA Electrokinetic Translocation through a Nanopore: Local Permittivity Environment Effect

The effect of the local liquid permittivity surrounding the DNA nanoparticle, referred to as the local permittivity environment (LPE) effect, on its electrokinetic translocation through a nanopore is investigated for the first time using a continuum-based model, composed of the coupled Poisson–Nernst–Planck (PNP) equations for the ionic mass transport and the Stokes and Brinkman equations for the hydrodynamic fields in the region outside of the DNA and within the ion-penetrable layer of the DNA nanoparticle, respectively. The nanoparticle translocation velocity and the resulting current deviation are systematically investigated for both uniform and spatially varying permittivities surrounding the DNA nanoparticle under various conditions. The LPE effect in general reduces the particle translocation velocity. The LPE effect on the current deviation is insignificant when the imposed electric field is relatively high. However, when the electric field and the bulk electrolyte concentration are relatively low, both current blockade and enhancement are predicted with the LPE effect incorporated, while only current blockade is predicted with the assumption of constant liquid permittivity. It is thereby shown that regardless of the electric field imposed the predictions on ionic current with considering the LPE effect are in good qualitative agreement with the experimental observations obtained in the literature