DC electrokinetic particle transport in an L-shaped microchannel

Electrokinetic Nanorod Translocation through a Dual-Nanopipette

Glass nanopipettes, as important sensing tools, have attracted great interest due to their wide range of applications in detecting single molecules, nanoparticles, and cells. In this study, we investigated the translocation behavior of nanorod particles through dual-nanopipettes using a transient continuum-based model based on an arbitrary Lagrangian-Eulerian approach. Our findings indicate that the translocation of nanorods is slowed down in the dual-nanopipette system, especially in the dual-nanopipette system with a nanobridge. These results are in qualitative agreement with previous experimental findings reported in the literature. Additionally, the translocation of nanorods is influenced by factors such as bulk concentration, initial location of the nanorod, and surface charge of the nanopipette. Notably, when the surface charge density of the nanopipette is relatively high and the initial location of the nanorod is in the reservoir, the nanorod can hardly enter the nanopipette, resulting in a relatively low translocation efficiency. However, the translocation efficiency can be improved by initially positioning the nanorod in one of the barrels. The resulting dual-blockade current signal can be used to correlate the characteristics of the nanorod.

Dielectrophoretic Colloidal Levitation by Electrode Polarization in Oscillating Electric Fields

Controlled colloidal levitation is key to many applications. Recently, it was discovered that polymer microspheres were levitated to a few micrometers in aqueous solutions in alternating current (AC) electric fields. A few mechanisms have been proposed to explain this AC levitation such as electrohydrodynamic flows, asymmetric rectified electric fields, and aperiodic electrodiffusiophoresis. Here, we propose an alternative mechanism based on dielectrophoresis in a spatially inhomogeneous electric field gradient extending from the electrode surface micrometers into the bulk. This field gradient is derived from electrode polarization, where counterions accumulate near electrode surfaces. A dielectric microparticle is then levitated from the electrode surface to a height where the dielectrophoretic lift balances gravity. The dielectrophoretic levitation mechanism is supported by two numerical models. One model assumes point dipoles and solves for the Poisson-Nernst-Planck equations, while the second model incorporates a dielectric sphere of a realistic size and permittivity and uses the Maxwell-stress tensor formulation to solve for the electrical body force. In addition to proposing a plausible levitation mechanism, we further demonstrate that AC colloidal levitation can be used to move synthetic microswimmers to controlled heights. This study sheds light on understanding the dynamics of colloidal particles near an electrode and paves the way to using AC levitation to manipulate colloidal particles, active or passive.

Deep-Learning Based Estimation of Dielectrophoretic Force

The ability to accurately quantify dielectrophoretic (DEP) force is critical in the development of high-efficiency microfluidic systems. This is the first reported work that combines a textile electrode-based DEP sensing system with deep learning in order to estimate the DEP forces invoked on microparticles. We demonstrate how our deep learning model can process micrographs of pearl chains of polystyrene (PS) microbeads to estimate the DEP forces experienced. Numerous images obtained from our experiments at varying input voltages were preprocessed and used to train three deep convolutional neural networks, namely AlexNet, MobileNetV2, and VGG19. The performances of all the models was tested for their validation accuracies. Models were also tested with adversarial images to evaluate performance in terms of classification accuracy and resilience as a result of noise, image blur, and contrast changes. The results indicated that our method is robust under unfavorable real-world settings, demonstrating that it can be used for the direct estimation of dielectrophoretic force in point-of-care settings.

Controlled protein adsorption on a silica microparticle

In the present study, controlled protein adsorption on a rigid silica microparticle is investigated numerically using classical Langmuir and two-state models under electrokinetic flow conditions. The instantaneous particle locations are simulated along a straight microchannel using an arbitrary Lagrangian-Eulerian framework in the finite element method for the electrophoretic motion of the charged particle. Within the scope of the parametric study, the strength of the external electric field (E), particle diameter (D_p ), the zeta potential of the particle (ζ_p ), and the location of the microparticle away from the channel wall (H) are systematically varied. The results are also compared to the data of pressure-driven flow having a parabolic flow profile at the inlet whose maximum magnitude is set to the particle's electrophoretic velocity magnitude. The validation studies reveal that the code developed for the particle motion in the present simulations agrees well with the experimental results. It is observed that protein adsorption can be controlled using electrokinetic phenomena. The plug-like flow profile in electrokinetics is beneficial for a microparticle at every spatial location in the microchannel, whereas it is not valid for the pressure-driven flow. The electric field strength and the zeta potential of the particle accelerate the protein adsorption. The wall shear stress and shear rate are good indicators to predict the adsorption process for electrokinetic flow.

Simulation of BNNSs Dielectrophoretic Motion under a Nanosecond Pulsed Electric Field

Using a nanosecond pulsed electric field to induce orientation and arrangement of insulating flake particles is a novel efficient strategy, but the specific mechanism remains unclear. In this study, the dielectrophoretic motion of boron nitride nanosheets (BNNSs) in ultrapure water under a nanosecond pulsed electric field is simulated for the first time. First, the simulation theory is proposed. When the relaxation polarization time of the dielectric is much shorter than the pulse voltage width, the pulse voltage high level can be considered a short-term DC voltage. On this basis, the Arbitrary Lagrangian–Euler (ALE) method is used in the model, considering the mutual ultrapure water–BNNS particles-nanosecond pulsed electric field dielectrophoretic interaction, to study the influence of different BNNSs self-angle α and relative angle β on local orientation and global arrangement. The particles are moved by the dielectrophoretic force during the pulse voltage high level and move with the ultrapure water flow at the zero level, without their movement direction changing during this period, so the orientation angle and distance changes show step-like and wave-like curves, respectively. The model explains the basic mechanism of dielectrophoretic motion of BNNSs under a pulsed electric field and summarizes the motion law of BNNSs, providing a reference for subsequent research.

Light-Induced Dynamic Control of Particle Motion in Fluid-Filled Microchannels

The design of remotely programmable microfluidic systems with controlled fluid flow and particle transport is a significant challenge. Herein, we describe a system that harnesses the intrinsic thermal response of a fluid to spontaneously pump solutions and regulate the transport of immersed microparticles. Irradiating a silver-coated channel with ultraviolet (UV) light generates local convective vortexes, which, in addition to the externally imposed flow, can be used to guide particles along specific trajectories or to arrest their motion. The method provides the distinct advantage that the flow and the associated convective patterns can be dynamically altered by relocating the source of UV light. Moreover, the flow can be initiated and terminated "on-demand" by turning the light on or off.

A new droplet breakup phenomenon in electrokinetic flow through a microchannel constriction

A completely new droplet breakup phenomenon is reported for droplets passing through a constriction in an electrokinetic flow. The breakup occurs during the droplet shape recovery process past the constriction throat by the interplay of the dielectrophoretic stress release and the interface energy for droplets with smaller permittivity than that of the ambient fluid. There are conditions for constriction ratios and droplet size that the droplet breakup occurs. The numerical predictions provided here require experimental verification, and then can give rise to a novel microfluidic device design with novel droplet manipulations.

Dielectrophoretic choking phenomenon in a converging-diverging microchannel for Janus particles

The dielectrophoretic (DEP) choking phenomenon is revisited for Janus particles that are transported electrokinetically through a microchannel constriction by a direct-current (DC) electric field. The negative DEP force that would block a particle with a diameter significantly smaller than that of the constriction at its inlet is seen to be relaxed by the rotation of the Janus particle in a direction that minimizes the magnitude of the DEP force. This allows the particle to pass through the constriction completely. An arbitrary Lagrangian-Eulerian (ALE) numerical method is used to solve the nonlinearly coupled electric field, flow field, and moving particle, and the DEP force is calculated by the Maxwell stress tensor (MST) method. The results show how Janus particles with non-uniform surface potentials overcome the DEP force and present new conditions for the DEP choking by a parametric study. Particle transportation through microchannel constrictions is ubiquitous, and particle surface properties are more likely to be non-uniform than not in practical applications. This study provides new insights of importance for non-uniform particles transported electrokinetically in a microdevice.

Numerical Investigation of DC Dielectrophoretic Deformable Particle⁻Particle Interactions and Assembly

In a non-uniform electric field, the surface charge of the deformable particle is polarized, resulting in the dielectrophoretic force acting on the surface of the particle, which causes the electrophoresis. Due to dielectrophoretic force, the two deformable particles approach each other, and distort the flow field between them, which cause the hydrodynamic force correspondingly. The dielectrophoresis (DEP) force and the hydrodynamic force together form the net force acting on the particles. In this paper, based on a thin electric double layer (EDL) assumption, we developed a mathematical model under the arbitrary Lagrangian⁻Eulerian (ALE) numerical approach method to simulate the flow field, electric field, and deformable particles simultaneously. Simulation results show that, when two deformable particles' distances are in a certain range, no matter the initial position of the two particles immersed in the fluid field, the particles will eventually form a particle⁻particle chain parallel to the direction of the electric field. In actual experiments, the biological cells used are deformable. Compared with the previous study on the DEP motion of the rigid particles, the research conclusion of this paper provides a more rigorous reference for the design of microfluidics.

Deformability-Based Electrokinetic Particle Separation

Deformability is an effective property that can be used in the separation of colloidal particles and cells. In this study, a microfluidic device is proposed and tested numerically for the sorting of deformable particles of various degrees. The separation process is numerically investigated by a direct numerical simulation of the fluid–particle–electric field interactions with an arbitrary Lagrangian–Eulerian finite-element method. The separation performance is investigated with the shear modulus of particles, the strength of the applied electric field, and the design of the contracted microfluidic devices as the main parameters. The results show that the particles with different shear moduli take different shapes and trajectories when passing through a microchannel contraction, enabling the separation of particles based on their difference in deformability.

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.

Dynamic drag force based on iterative density mapping: A new numerical tool for three-dimensional analysis of particle trajectories in a dielectrophoretic system

Dielectrophoresis is a widely used means of manipulating suspended particles within microfluidic systems. In order to efficiently design such systems for a desired application, various numerical methods exist that enable particle trajectory plotting in two or three dimensions based on the interplay of hydrodynamic and dielectrophoretic forces. While various models are described in the literature, few are capable of modeling interactions between particles as well as their surrounding environment as these interactions are complex, multifaceted, and computationally expensive to the point of being prohibitive when considering a large number of particles. In this paper, we present a numerical model designed to enable spatial analysis of the physical effects exerted upon particles within microfluidic systems employing dielectrophoresis. The model presents a means of approximating the effects of the presence of large numbers of particles through dynamically adjusting hydrodynamic drag force based on particle density, thereby introducing a measure of emulated particle-particle and particle-liquid interactions. This model is referred to as "dynamic drag force based on iterative density mapping." The resultant numerical model is used to simulate and predict particle trajectory and velocity profiles within a microfluidic system incorporating curved dielectrophoretic microelectrodes. The simulated data are compared favorably with experimental data gathered using microparticle image velocimetry, and is contrasted against simulated data generated using traditional "effective moment Stokes-drag method," showing more accurate particle velocity profiles for areas of high particle density.

Numerical and experimental characterization of solid-state micropore-based cytometer for detection and enumeration of biological cells

Portable diagnostic devices have emerged as important tools in various biomedical applications since they can provide an effective solution for low-cost and rapid clinical diagnosis. In this paper, we present a micropore-based resistive cytometer for the detection and enumeration of biological cells. The proposed device was fabricated on a silicon wafer by a standard microelectromechanical system processing technology, which enables a mass production of the proposed chip. The working principle of this cytometer is based upon a bias potential modulated pulse, originating from the biological particle's physical blockage of the micropore. Polystyrene particles of different sizes (7, 10, and 16 μm) were used to test and calibrate the proposed device. A finite element simulation was developed to predict the bias potential modulated pulse (peak amplitude vs. pulse bandwidth), which can provide critical insight into the design of this microfluidic flow cytometer. Furthermore, HeLa cells (a type of tumor cell lines) spiked in a suspension of blood cells, including red blood cells and white blood cells, were used to assess the performance for detecting and counting tumor cells. The proposed microfluidic flow cytometer is able to provide a promising platform to address the current unmet need for point-of-care clinical diagnosis.

Exploiting the wall-induced non-inertial lift in electrokinetic flow for a continuous particle separation by size

Separating particles from a heterogeneous mixture is important and necessary in many engineering and biomedical applications. Electrokinetic flow-based continuous particle separation has thus far been realized primarily by the use of particle dielectrophoresis induced in constricted and/or curved microchannels. We develop in this work a new electrokinetic method that exploits the wall-induced non-inertial lift in a straight uniform microchannel to continuously separate particles by intrinsic properties (e.g., size and surface charge). Such an electrically originated lift force arises from the asymmetric electric field distribution around a particle nearby a planar dielectric wall. We demonstrate this method through separating both a binary and ternary mixture of dispersed polystyrene microspheres by size in a T-shaped microchannel. A semi-analytical model is also developed to simulate and understand the particle separation process. The predicted particle trajectories in the entire microchannel agree reasonably well with the experimental measurements.

Simultaneous determination of electrophoretic and dielectrophoretic mobilities of human red blood cells

Electrophoresis and dielectrophoresis of cells can reveal many distinct cellular properties but are often conducted separately. Herein a simultaneous strategy was proposed, and a simple method was established by making cells migrate through a cross channel under a micro video for real-time observation. The experiment can be performed within 0.044-1 s. In combination with digital calculation based on electromagnetic theory, the method was validated to be applicable to the determination of electrophoretic and dielectrophoretic mobilities, μEP and μDEP , of human blood erythrocytes, giving μEP = -(0.87 ± 0.16)× 10(-4) cm(2) ·V(-1) · s(-1) and μDEP = -(4.5 ± 1.3) × 10(-8) cm(4) ·V(-2) ·s(-1) by vector decomposition, or μEP = -(0.89 ± 0.14) × 10(-4) cm(2) ·V(-1) · s(-1) and μDEP = -(4.6 ±1.2) × 10(-8) cm(4) ·V(-2) · s(-1) by least squares fitting, all agreeing with published data. Hydrodynamic and EOFs were eliminated for better measurement. It was found that the location of cells had a serious impact on the measurement precision, and the upstream of the cross channel along the electric field was chosen for precise measurement. The method is also extendable to the study of other cells and particles.

Microfluidic electrical sorting of particles based on shape in a spiral microchannel

Shape is an intrinsic marker of cell cycle, an important factor for identifying a bioparticle, and also a useful indicator of cell state for disease diagnostics. Therefore, shape can be a specific marker in label-free particle and cell separation for various chemical and biological applications. We demonstrate in this work a continuous-flow electrical sorting of spherical and peanut-shaped particles of similar volumes in an asymmetric double-spiral microchannel. It exploits curvature-induced dielectrophoresis to focus particles to a tight stream in the first spiral without any sheath flow and subsequently displace them to shape-dependent flow paths in the second spiral without any external force. We also develop a numerical model to simulate and understand this shape-based particle sorting in spiral microchannels. The predicted particle trajectories agree qualitatively with the experimental observation.

Electrokinetic particle entry into microchannels

The fundamental understanding of particle electrokinetics in microchannels is relevant to many applications. To date, however, the majority of previous studies have been limited to particle motion within the area of microchannels. This work presents the first experimental and numerical investigation of electrokinetic particle entry into a microchannel. We find that the particle entry motion can be significantly deviated from the fluid streamline by particle dielectrophoresis at the reservoir-microchannel junction. This negative dielectrophoretic motion is induced by the inherent non-uniform electric field at the junction and is insensitive to the microchannel length. It slows down the entering particles and pushes them toward the center of the microchannel. The consequence is the demonstrated particle deflection, focusing, and trapping phenomena at the reservoir-microchannel junction. Such rich phenomena are studied by tuning the AC component of a DC-biased AC electric field. They are also utilized to implement a selective concentration and continuous separation of particles by size inside the entry reservoir.

Effect of direct current dielectrophoresis on the trajectory of a non-conducting colloidal sphere in a bent pore

Dielectrophoresis has shown a wide range of applications in microfluidic devices. Force approximations utilizing the point-dipole method in dielectrophoresis have provided convenient predictions for particle motion by neglecting interactions between the particle and its surrounding electric and flow fields. The validity of this approach, however, is unclear when the particle size is comparable to the characteristic length of the channel and when the particle is in close proximity to the channel wall. To address this issue, we apply an accurate numerical approach based on the boundary-element method (BEM) to solve the coupled electric field, flow, and particle motion. This method can handle much closer particle-wall distances than the other numerical approaches such as the finite-element method. Using the BEM and integrating the Maxwell stress tensor, we simulate an electrokinetic, spherical particle moving within a bent cylindrical pore to investigate how the dielectrophoretic force affects the particle's trajectory. In the simulation, both the particle and the channel wall are non-conducting, and the electric double layers adjacent to the solid surfaces are assumed to be thin with respect to the particle radius and particle-wall gap. The results show that as the particle comes close to the wall, its finite size has an increasingly important effect on its own transient motion and the point-dipole approximation may lead to significant error.

Dielectrophoresis in microfluidics technology

Dielectrophoresis (DEP) is the movement of a particle in a non-uniform electric field due to the interaction of the particle's dipole and spatial gradient of the electric field. DEP is a subtle solution to manipulate particles and cells at microscale due to its favorable scaling for the reduced size of the system. DEP has been utilized for many applications in microfluidic systems. In this review, a detailed analysis of the modeling of DEP-based manipulation of the particles is provided, and the recent applications regarding the particle manipulation in microfluidic systems (mainly the published works between 2007 and 2010) are presented.

Curvature-induced dielectrophoresis for continuous separation of particles by charge in spiral microchannels

The separation of particles from a heterogeneous mixture is critical in chemical and biological analyses. Many methods have been developed to separate particles in microfluidic devices. However, the majority of these separations have been limited to be size based and binary. We demonstrate herein a continuous dc electric field driven separation of carboxyl-coated and noncoated 10 μm polystyrene beads by charge in a double-spiral microchannel. This method exploits the inherent electric field gradients formed within the channel turns to manipulate particles by dielectrophoresis and is thus termed curvature-induced dielectrophoresis. The spiral microchannel is also demonstrated to continuously sort noncoated 5 μm beads, noncoated 10 μm beads, and carboxyl-coated 10 μm beads into different collecting wells by charge and size simultaneously. The observed particle separation processes in different situations are all predicted with reasonable agreements by a numerical model. This curvature-induced dielectrophoresis technique eliminates the in-channel microelectrodes and obstacles that are required in traditional electrode- and insulator-based dielectrophoresis devices. It may potentially be used to separate multiple particle targets by intrinsic properties for lab-on-a-chip applications.

Electrokinetic particle translocation through a nanopore

Nanoparticle electrophoretic translocation through a single nanopore induces a detectable change in the ionic current, which enables the nanopore-based sensing for various bio-analytical applications. In this study, a transient continuum-based model is developed for the first time to investigate the electrokinetic particle translocation through a nanopore by solving the Nernst-Planck equations for the ionic concentrations, the Poisson equation for the electric potential and the Navier-Stokes equations for the flow field using an arbitrary Lagrangian-Eulerian (ALE) method. When the applied electric field is relatively low, a current blockade is expected. In addition, the particle could be trapped at the entrance of the nanopore when the electrical double layer (EDL) adjacent to the charged particle is relatively thick. When the electric field imposed is relatively high, the particle can always pass through the nanopore by electrophoresis. However, a current enhancement is predicted if the EDL of the particle is relatively thick. The obtained numerical results qualitatively agree with the existing experimental results. It is also found that the initial orientation of the particle could significantly affect the particle translocation and the ionic current through a nanopore. Furthermore, a relatively high electric field tends to align the particle with its longest axis parallel to the local electric field. However, the particle's initial lateral offset from the centerline of the nanopore acts as a minor effect.

Electrophoretic motion of a nanorod along the axis of a nanopore under a salt gradient

The phoretic translation of a charged, elongated cylindrical nanoparticle, such as a DNA molecule and nanorod, along the axis of a nanopore driven by simultaneous axial electric field and salt concentration gradient, has been investigated using a continuum model, which consists of the Poisson-Nernst-Planck equations for the ionic concentrations and electric potential, and the Stokes equations for the hydrodynamic field. The induced particle motion includes both electrophoresis, driven by the imposed electric field, and diffusiophoresis, arising from the imposed salt concentration gradient. The particle's phoretic velocity along the axis of a nanopore is computed as functions of the imposed salt concentration gradient, the ratio of the its radius to the double-layer thickness, the nanorod's aspect ratio (length/radius), the ratio of the nanopore size to the particle size, the surface-charge density of the particle, and that of the nanopore in KCl solution. The diffusiophoresis in a nanopore mainly arises from the induced electrophoresis driven by the generated electric field, stemming from the double-layer polarization, and can be used to regulate electrophoretic translocation of a nanorod, such as a DNA molecule, through a nanopore. When both the nanorod and the nanopore wall are charged, the induced electroosmotic flow arising from the interaction of the overall electric field with the double layer adjacent to the nanopore wall has a significant effect on both electrophoresis driven by the imposed electric field and diffusiophoresis driven by the imposed salt gradient.

Electrodiffusiophoretic motion of a charged spherical particle in a nanopore

The electrodiffusiophoretic motion of a charged spherical nanoparticle in a nanopore subjected to an axial electric field and electrolyte concentration gradient has been investigated using a continuum model, composed of the Poisson-Nernst-Planck equations for the ionic mass transport and the Navier-Stokes equations for the flow field. The charged particle experiences electrophoresis in response to the imposed electric field and diffusiophoresis caused solely by the imposed concentration gradient. The diffusiophoretic motion is induced by two different mechanisms, an electrophoresis driven by the generated electric field arising from the difference of ionic diffusivities and the double layer polarization and a chemiphoresis due to the induced osmotic pressure gradient around the charged nanoparticle. The electrodiffusiophoretic motion along the axis of a nanopore is investigated as a function of the ratio of the particle size to the thickness of the electrical double layer, the imposed concentration gradient, the ratio of the surface charge density of the nanopore to that of the particle, and the type of electrolyte. Depending on the magnitude and direction of the imposed concentration gradient, one can accelerate, decelerate, and even reverse the particle's electrophoretic motion in a nanopore by the superimposed diffusiophoresis. The induced electroosmotic flow in the vicinity of the charged nanopore wall driven by both the imposed and the generated electric fields also significantly affects the electrodiffusiophoretic motion.