Sang W. Joo

Transient electrophoretic motion of a charged particle through a converging-diverging microchannel: Effect of direct current-dielectrophoretic force

Transient electrophoretic motion of a charged particle through a converging–diverging microchannel is studied by solving the coupled system of the Navier–Stokes equations for fluid flow and the Laplace equation for electrical field with an arbitrary Lagrangian–Eulerian finite‐element method. A spatially non‐uniform electric field is induced in the converging–diverging section, which gives rise to a direct current dielectrophoretic (DEP) force in addition to the electrostatic force acting on the charged particle. As a sequence, the symmetry of the particle velocity and trajectory with respect to the throat is broken. We demonstrate that the predicted particle trajectory shifts due to DEP show quantitative agreements with the existing experimental data. Although converging–diverging microchannels can be used for super fast electrophoresis due to the enhancement of the local electric field, it is shown that large particles may be blocked due to the induced DEP force, which thus must be taken into account in the study of electrophoresis in microfluidic devices where non‐uniform electric fields are present.

dc electrokinetic transport of cylindrical cells in straight microchannels

Electrokinetic transport of cylindrical cells under dc electric fields in a straight microfluidic channel is experimentally and numerically investigated with emphasis on the dielectrophoretic (DEP) effect on their orientation variations. A two-dimensional multiphysics model, composed of the Navier-Stokes equations for the fluid flow and the Laplace equation for the electric potential defined in an arbitrary Lagrangian-Eulerian framework, is employed to capture the transient electrokinetic motion of cylindrical cells. The numerical predictions of the particle transport are in quantitative agreement with the obtained experimental results, suggesting that the DEP effect should be taken into account to study the electrokinetic transport of cylindrical particles even in a straight microchannel with uniform cross-sectional area. A comprehensive parametric study indicates that cylindrical particles would experience an oscillatory motion under low electric fields. However, they are aligned with their longest axis parallel to the imposed electric field under high electric fields due to the induced DEP effect.

Electrophoretic Motion of a Spherical Particle with a Symmetric Nonuniform Surface Charge Distribution in a Nanotube

The electrophoretic motion of a spherical nanoparticle, subject to an axial electric field in a nanotube filled with an electrolyte solution, has been investigated using a continuum theory, which consists of the Nernst-Planck equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equation for the hydrodynamic field. In particular, the effects of nonuniform surface charge distributions around the nanoparticle on its axial electrophoretic motion are examined with changes in the bulk electrolyte concentration and the surface charge of the tubes wall. A particle with a nonuniform charge distribution is shown to induce a corresponding complex ionic concentration field, which in turn influences the electric field and the fluid motion surrounding the particle and thus its electrophoretic velocity. As a result, contrary to the relatively simple dynamics of a particle with a uniform surface charge, dominated by the irradiating electrostatic force, that with a nonuniform surface charge distribution shows various intriguing behaviors due to the additional interplay of the nonuniform electro-osmotic effects.

Electrokinetic ion and fluid transport in nanopores functionalized by polyelectrolyte brushes

Chemically functionalized nanopores in solid-state membranes have recently emerged as versatile tools for regulating ion transport and sensing single biomolecules. This study theoretically investigated the importance of the bulk salt concentration, the geometries of the nanopore, and both the thickness and the grafting density of the polyelectrolyte (PE) brushes on the electrokinetic ion and fluid transport in two types of PE brush functionalized nanopore: PE brushes are end-grafted to the entire membrane surface (system I), and to its inner surface only (nanopore wall) (system II). Due to a more significant ion concentration polarization (CP), the enhanced local electric field inside the nanopore, the conductance, and the electroosmotic flow (EOF) velocity in system II are remarkably smaller than those in system I. In addition to a significantly enhanced EOF inside the nanopore, the direction of the flow field near both nanopore openings in system I is opposite to that of EOF inside the nanopore. This feature can be applied to regulate the electrokinetic translocation of biomolecules through a nanopore in the nanopore-based DNA sequencing platform.

Diffusiophoretic motion of a charged spherical particle in a nanopore.

The diffusiophoretic motion of a charged spherical particle in a nanopore, subjected to an axial electrolyte concentration gradient, is investigated using a continuum theory, which consists of the ionic mass conservation equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equations for the hydrodynamic field. With the concentration gradient imposed, the particle motion is induced by two different mechanisms: an electrophoresis generated by the induced electric field arising from the difference of ionic diffusivities and the double layer polarization (DLP) and a chemiphoresis by the resulting osmotic pressure gradient induced by the solute gradient in the electrical double layer around the particle. The particle diffusiophoretic velocity along the axis of the nanopore is computed as functions of the ratio of the particle size to the thickness of the electrical double layer, the ratio of the nanopore size to the particle size, the particle surface charge density, and the properties of the salt solution. The diffusiophoretic behavior of a particle comparable to the nanopore size is governed predominantly by the induced electrophoresis generated by the DLP-induced electric field, caused by the imposed concentration gradient and the double layer compression due to the presence of the impervious nanopore wall.

Dielectrophoretic choking phenomenon in a converging-diverging microchannel

Experiments show that particles smaller than the throat size of converging-diverging microchannels can sometimes be trapped near the throat. This critical phenomenon is associated with the negative dc dielectrophoresis arising from nonuniform electric fields in the microchannels. A finite-element model, accounting for the particle-fluid-electric field interactions, is employed to investigate the conditions for this dielectrophoretic (DEP) choking in a converging-diverging microchannel for the first time. It is shown quantitatively that the DEP choking occurs for high nonuniformity of electric fields, high ratio of particle size to throat size, and high ratio of particles zeta potential to that of microchannel.

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.

Pressure-Driven Transport of Particles Through a Converging-Diverging Microchannel

Pressure-driven transport of particles through a symmetric converging-diverging microchannel is studied by solving a coupled nonlinear system, which is composed of the Navier-Stokes and continuity equations using the arbitrary Lagrangian-Eulerian finite-element technique. The predicted particle translation is in good agreement with existing experimental observations. The effects of pressure gradient, particle size, channel geometry, and a particles initial location on the particle transport are investigated. The pressure gradient has no effect on the ratio of the translational velocity of particles through a converging-diverging channel to that in the upstream straight channel. Particles are generally accelerated in the converging region and then decelerated in the diverging region, with the maximum translational velocity at the throat. For particles with diameters close to the width of the channel throat, the usual acceleration process is divided into three stages: Acceleration, deceleration, and reacceleration instead of a monotonic acceleration. Moreover, the maximum translational velocity occurs at the end of the first acceleration stage rather than at the throat. Along the centerline of the microchannel, particles do not rotate, and the closer a particle is located near the channel wall, the higher is its rotational velocity. Analysis of the transport of two particles demonstrates the feasibility of using a converging-diverging microchannel for passive (biological and synthetic) particle separation and ordering.

Ultrasensitive detection of mercury (II) ions using electrochemical surface plasmon resonance with magnetohydrodynamic convection.

A high-precision technique that can detect mercury (II) ions down to 1 fM concentration in aqueous solutions is introduced. The technique combines the conventional electrochemical method, surface plasmon resonance (SPR), and magnetohydrodynamic (MHD) convection. Mercury ions are electroplated onto a gold SPR sensing surface, and then detected quantitatively by applying a potential scan with cyclic voltammetry. Both the SPR angular shift and the electrochemical current signal are recorded for identification and quantification of the mercury ions. The detection sensitivity is further enhanced by applying an MHD convection in the presence of a magnetic field, which does not require any moving parts intruding into the aqueous solution. The technique thus has a great advantage for small detection volume. In the presence of supporting electrolytes, 1 mM nitric acid and 10 mM potassium nitrate, Hg(2+) ionic solutions with concentrations ranging from 1 fM to 1 microM are tested under different magnetic flux densities of B=0, 0.27, 0.53, and 0.71 T. The experimental results demonstrate that the stripping signals of the 1 fM to 1 microM Hg(2+) ions are enhanced by 10-60% with the flux density B=0.71 T.

Effect of linear surface-charge non-uniformities on the electrokinetic ionic-current rectification in conical nanopores

The electrokinetic ionic-current rectification in a conical nanopore with linearly varying surface-charge distributions is studied theoretically by using a continuum model composed of a coupled system of the Nernst-Planck equations for the ionic-concentration field and the Poisson equation for the electric potential in the electrolyte solution. The numerical analysis includes the electrochemistry inside reservoirs connected to the nanopore, neglected in previous studies, and more precise accounts of the ionic current are provided. The surface-charge distribution, especially near the tip of the nanopore, significantly affects the ionic enrichment and depletion, which, in turn, influence the resulting ionic current and the rectification. It is shown that non-uniform surface-charge distribution can reverse the direction, or sense, of the rectification. Further insights into the ionic-current rectification are provided by discussing the intriguing details of the electric potential and ionic-concentration fields, leading to the rectification. Rationale for future studies on ionic-current rectification, associated with other non-uniform surface-charge distributions and electroosmotic convection for example, is discussed.