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.     

AC dielectrophoretic deformable particle-particle interactions and their relative motions

This paper develops a numerical simulation model to research the deformable particle-particle interactions caused by dielectrophoresis (DEP) under AC electric fields. The DEP force is calculated by using Maxwell stress tensor method, and the hydrodynamic force is obtained by calculating the hydrodynamic stress tensor. Simulation results show that the DEP interactive motion will facilitate the particles forming particle chains that are parallel to the electric field, and the particles with low shear modulus present a lower x-component velocity. Also, the electric field intensity and particles radius have some effects on the DEP motions, and for different particles, smaller particles with larger electric field intensity easily reach a larger velocity. The numerical research may provide universal guidance for biological cells manipulation and assembly.     

DC-dielectrophoretic separation of microparticles using an oil droplet obstacle

A new dielectrophoretic particle separation method is demonstrated and examined in the following experimental study. Current electrodeless dielectrophoretic (DEP) separation techniques utilize insulating solid obstacles in a DC or low-frequency AC field, while this novel method employs an oil droplet acting as an insulating hurdle between two electrodes. When particles move in a non-uniform DC field locally formed by the droplet, they are exposed to a negative DEP force linearly dependent on their volume, which allows the particle separation by size. Since the size of the droplet can be dynamically changed, the electric field gradient, and hence DEP force, becomes easily controllable and adjustable to various separation parameters. By adjusting the droplet size, particles of three different diameter sizes, 1 microm, 5.7 microm and 15.7 microm, were successfully separated in a PDMS microfluidic chip, under applied field strength in the range from 80 V cm-1 to 240 V cm-1. A very effective separation was realized at the low field strength, since the electric field gradient was proved to be a more significant parameter for particle discrimination than the applied voltage. By utilizing low strength fields and adaptable field gradient, this method can also be applied to the separation of biological samples that are generally very sensitive to high electric potential.     

Dielectrophoretic force on a sphere near a planar boundary

The small gap distance separating a spherical colloidal particle in electrophoretic motion from a planar nonconducting surface is a required parameter for calculating its electrophoretic mobility. In the presence of an externally applied electric field, this gap distance is determined by balancing the van der Waals, electrical double layer interaction, and gravitational forces with a dielectrophoretic (DEP) force. Here, the DEP force was determined analytically by integration of the Maxwell stress over the surface of the particle. The account of this force showed that its previous omission from the analysis always resulted in underpredicted gap distances. Furthermore, the DEP force dominated under conditions of low particle density or high electric field strength and led to much higher gap distances on the order of a few microns. In one particular case, a combination of low particle density and small particle size produced two possible equilibrium gap distances for the particle. However, the particle was unstable in the second equilibrium position when subjected to small perturbations. In general, larger particles had smaller gap sizes. The effects of four other parameters on gap distance were studied, and gap distances were found to increase with lower particle density, higher electric field strength, higher particle and wall zeta potentials, and lower Hamaker constants. Retardation effects on van der Waals attraction were considered.     

Dielectric force and relative motion between two spherical particles in electrophoresis

When two particles close to each other are in electrophoretic motion, each particle is under the influence of the nonuniform electric field generated by the other particle. Two particles may attract or repel each other due to the dielectric force, depending on their positions in the nonuniform electric field. In this work, the dielectric interaction and the subsequent relative motion of the two arbitrarily oriented spherical particles are analyzed. The dielectric force is obtained by integrating the Maxwell stress. The result is valid for arbitrary orientations of the particles under the thin electrical-double-layer assumption. The magnitude of the dielectric force is compared to the so-called inertia-induced force, which shows that the dielectric force is normally much greater than the inertia-induced force. The relative velocity of particles is determined by the force balance between the dielectric force and the Stokes drag. The regions of attraction and repulsion are defined. It is shown that a pair of particles eventually aligns parallel to the externally applied electric field, except in the case where the two particles are initially oriented perpendicular to the electric field. A closed-form analytical solution is obtained for the particle trajectory by using the approximate expression for the dielectric force valid for not-too-closely located particles.     

Theory for hierarchical assembly with dielectrophoresis and the role of particle anisotropy

Nonuniform electric fields cause polarizable particles to move through an effect known as dielectrophoresis (DEP). Additionally, the particles themselves create nonuniform fields due to their induced dipoles. When the nonuniform field of one particle causes another to move, it represents a path to hierarchical assembly termed mutual DEP (mDEP). Anisotropic particles potentially provide further opportunities for assembly through intense and intricate local field profiles. Here, we construct a theoretical framework for describing anisotropic particles as templates for assembly through mDEP by considering the motion of small nanoparticles near larger anisotropic nanoparticles. Using finite element analysis, we study eight particle shapes and compute their field enhancement and polarizability in an orientation-specific manner. Strikingly, we find a more than tenfold enhancement in the field near certain particle shapes, potentially promoting mDEP. To more directly relate the field intensity to the anticipated assembly outcome, we compute the volume experiencing each field enhancement versus particle shape and orientation. Finally, we provide a framework for predicting how mixtures of two distinct particle species will begin to assemble in a manner that allows for the identification of conditions that kinetically bias assembly toward specific hierarchical outcomes.     

Continuous dielectrophoretic separation of particles in a spiral microchannel

Particle separation is a fundamental operation in the areas of biology and physical chemistry. A variety of force fields have been used to separate particles in microfluidic devices, among which electric field may be the most popular one due to its general applicability and adaptability. So far, however, electrophoresis‐based separations have been limited primarily to batchwise processes. Dielectrophoresis (DEP)‐based separations require in‐channel micro‐electrodes or micro‐insulators to produce electric field gradients. This article introduces a novel particle separation technique in DC electrokinetic flow through a planar double‐spiral microchannel. The continuous separation arises from the cross‐stream dielectrophoretic motion of particles induced by the non‐uniform electric field inherent to curved channels. Specifically, particles are focused by DEP to one sidewall of the first spiral, and then dielectrophoretically deflected toward the other sidewall of the second spiral at a particle‐dependent rate, leading to focused particle streams along different flow paths. This DEP‐based particle separation technique is demonstrated in an asymmetric double‐spiral microchannel by continuously separating a mixture of 5/10 μm particles and 3/5 μm particles.     

Front Cover: Enhancement of dielectrophoresis-based particle collection from high conducting fluids due to partial electrode insulation

Dielectrophoresis (DEP) is a successful method to recover nanoparticles from different types of fluid. The DEP force acting on these particles is created by an electrode microarray that produces a nonuniform electric field. To apply DEP to a highly conducting biological fluid, a protective hydrogel coating over the metal electrodes is required to create a barrier between the electrode and the fluid. This protects the electrodes, reduces the electrolysis of water, and allows the electric field to penetrate into the fluid sample. We observed that the protective hydrogel layer can separate from the electrode and form a closed domed structure and that collection of 100 nm polystyrene beads increased when this occurred. To better understand this collection increase, we used COMSOL Multiphysics software to model the electric field in the presence of the dome filled with different materials ranging from low-conducting gas to high conducting phosphate-buffered saline fluids. The results suggest that as the electrical conductivity of the material inside the dome is reduced, the whole dome acts as an insulator which increases electric field intensity at the electrode edge. This increased intensity widens the high-intensity electric field factor zone resulting in increased collection. This informs how dome formation results in increased particle collection and provides insight into how the electric field can be intensified to the increase collection of particles. These results have important applications for increasing the recovery of biologically-derived nanoparticles from undiluted physiological fluids that have high conductance, including the collection of cancer-derived extracellular vesicles from plasma for liquid biopsy applications.     

Dielectrophoresis and AC-induced assembly in binary colloidal suspensions

Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.     

Performance characterization of an insulator-based dielectrophoretic microdevice

Dielectrophoresis (DEP), the motion of particles in nonuniform electric fields, is a nondestructive electrokinetic (EK) transport mechanism can be used to concentrate and separate bioparticles. Traditionally, DEP has been performed employing microelectrodes, an approach that is expensive due to the cost of microelectrode fabrication. An alternative is insulator-based DEP (iDEP), an inexpensive method where nonuniform electric fields are created with arrays of insulating structures. This study presents the effects of operating conditions on the dielectrophoretic behavior of polystyrene microparticles under iDEP. Experiments were performed employing microchannels containing insulating structures that worked as insulators. The parameters varied were pH (8-9) and conductivity (25-100 microS/cm) of the bulk medium, and the magnitude of the applied field (200-850 V/cm). Optimal operating conditions in terms of pH and conductivity were obtained, and the microdevice performance was characterized in terms of concentration factor and minimum electric field required (minimum energy consumption). This is the first report on improving iDEP processes when EOF is present. DEP and EOF have been studied extensively, however, this study integrates the effect of suspending medium characteristics on both EK phenomena. These findings will allow improving the performance of iDEP microdevices achieving the highest concentration fold with the lowest energy consumption.     

Vertical motion of a charged colloidal particle near an AC polarized electrode with a nonuniform potential distribution: theory and experimental evidence

Electroosmotic flow in the vicinity of a colloidal particle suspended over an electrode accounts for observed changes in the average height of the particle when the electrode passes alternating current at 100 Hz. The main findings are (1) electroosmotic flow provides sufficient force to move the particle and (2) a phase shift between the purely electrical force on the particle and the particle's motion provides evidence of an E2 force acting on the particle. The electroosmotic force in this case arises from the boundary condition applied when faradaic reactions occur on the electrode. The presence of a potential-dependent electrode reaction moves the likely distribution of electrical current at the electrode surface toward uniform current density around the particle. In the presence of a particle the uniform current density is associated with a nonuniform potential; thus, the electric field around the particle has a nonzero radial component along the electrode surface, which interacts with unbalanced charge in the diffuse double layer on the electrode to create a flow pattern and impose an electroosmotic-flow-based force on the particle. Numerical solutions are presented for these additional height-dependent forces on the particle as a function of the current distribution on the electrode and for the time-dependent probability density of a charged colloidal particle near a planar electrode with a nonuniform electrical potential boundary condition. The electrical potential distribution on the electrode, combined with a phase difference between the electric field in solution and the electrode potential, can account for the experimentally observed motion of particles in ac electric fields in the frequency range from approximately 10 to 200 Hz.     

Dielectrophoretic choking phenomenon of a deformable particle in a converging‐diverging microchannel

The translational motion of small particles in an electrokinetic fluid flow through a constriction can be enhanced by an increase of the applied electric potential. Beyond a critical potential, however, the negative dielectrophoresis (DEP) can overpower other forces to prevent particles that are even smaller than the constriction from passing through the constriction. This DEP choking phenomenon was studied previously for rigid particles. Here, the DEP choking phenomenon is revisited for deformable particles, which are ubiquitous in many biomedical applications. Particle deformability is measured by the particle shear modulus, and the choking conditions are reported through a parametric study that includes the channel geometry, external electric potential, and particle zeta potential. The study was carried out using a numerical model based on an arbitrary Lagrangian‐Eulerican (ALE) finite‐element method.     

DC dielectrophoresis separation of marine algae and particles in a microfluidic chip

This paper reports a microfluidic method of continuous separation of marine algae and particles by DC dielectrophoresis. The locally non-uniform electric field is generated by an insulating PDMS triangle hurdle fabricated within a PDMS microchannel. Both the particles and algae are subject to negative DEP forces at the hurdle where the gradient of local electric-field strength is the strongest. The DEP force acting on the particle or the algae depends on particles’ or algae’s volume, shape and dielectric properties. Thus the moving particles and algae will be repelled to different streamlines when passing the hurdle. In this way, combined with the electroosmotic flow, continuous separation of algae of two different sizes, and continuous separation of polystyrene particles and algae with similar volume but different shape were achieved. This first demonstration of DC DEP separation of polystyrene particles and algae with similar sizes illustrates the great influence of dielectric properties on particle separation and potentials for sample pretreatment.     

Insulator-based dielectrophoresis of microorganisms: theoretical and experimental results

Dielectrophoresis (DEP) is the motion of particles due to polarization effects in nonuniform electric fields. DEP has great potential for handling cells and is a non-destructive phenomenon. It has been utilized for different cell analysis, from viability assessments to concentration enrichment and separation. Insulator-based DEP (iDEP) provides an attractive alternative to conventional electrode-based systems; in iDEP, insulating structures are used to generate nonuniform electric fields, resulting in simpler and more robust devices. Despite the rapid development of iDEP microdevices for applications with cells, the fundamentals behind the dielectrophoretic behavior of cells has not been fully elucidated. Understanding the theory behind iDEP is necessary to continue the progress in this field. This work presents the manipulation and separation of bacterial and yeast cells with iDEP. A computational model in COMSOL Multiphysics was employed to predict the effect of direct current-iDEP on cells suspended in a microchannel containing an array of insulating structures. The model allowed predicting particle behavior, pathlines and the regions where dielectrophoretic immobilization should occur. Experimental work was performed at the same operating conditions employed with the model and results were compared, obtaining good agreement. This is the first report on the mathematical modeling of the dielectrophoretic response of yeast and bacterial cells in a DC-iDEP microdevice.     

Dielectrophoresis of nanoparticles

A numerical scheme based on the distributed Lagrange multiplier method (DLM) is used to study the motion of nano-sized particles of dielectric suspensions subjected to uniform and nonuniform electric fields. Particles are subjected to both electrostatic and hydrodynamic forces, as well as Brownian motion. The results of the simulations presented in this paper show that uniform electric fields the evolution of the particle structures depends on the ratio of electrostatic particle-particle interactions and Brownian forces. When this ratio is of the order of 100 or greater, particles form stable chains and columns, whereas when it is of the order of 10 or smaller the particle distribution is random. For the nonuniform electric field cases considered in this paper, the relative magnitude of Brownian forces is in the range such that it does not influence the eventual collection of particles by dielectrophoresis and the particular locations where the particles are collected. However, Brownian motion is observed to influence the transient particle trajectories. The deviation of the particle trajectories compared to those determined by the electrostatic and hydrodynamic forces alone is characterized by the ratio of Brownian and dielectrophoretic forces.     

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.     

Experimental study of dielectrophoresis and liquid dielectrophoresis mechanisms for particle capture in a droplet

This work presents a microfluidic system that can transport, concentrate, and capture particles in a controllable droplet. Dielectrophoresis (DEP), a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field, is used to manipulate particles. Liquid dielectrophoresis (LDEP), a phenomenon in which a liquid moves toward regions of high electric field strength under a non-uniform electric field, is used to manipulate the fluid. In this study, a mechanism of droplet creation presented in a previous work that uses DEP and LDEP is improved. A driving electrode with a DEP gap is used to prevent beads from getting stuck at the interface between air and liquid, which is actuated with an AC signal of 200 V(pp) at a frequency of 100 kHz. DEP theory is used to calculate the DEP force in the liquid, and LDEP theory is used to analyze the influence of the DEP gap. The increment of the actuation voltage due to the electrode with a DEP gap is calculated. A set of microwell electrodes is used to capture a bead using DEP force, which is actuated with an AC signal of 20 V(pp) at a frequency of 5 MHz. A simulation is carried out to investigate the dimensions of the DEP gap and microwell electrodes. Experiments are performed to demonstrate the creation of a 100-nL droplet and the capture of individual 10-μm polystyrene latex beads in the droplet.     

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 tube's 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.     

Rapid microparticle patterning by enhanced dielectrophoresis effect on a double-layer electrode substrate

We present a feasible dielectrophoresis (DEP) approach for rapid patterning of microparticles on a reusable double-layer electrode substrate in microfluidics. Simulation analysis demonstrated that the DEP force was dramatically enhanced by the induced electric field on top interdigitated electrodes. By adjusting electric field intensity through the bottom electrodes on thin glass substrate (100 μm), polystyrene particles (10 μm) were effectively patterned by top electrodes within several seconds (<5 s). The particle average velocity can reach a maximum value of about 20.0±3.0 μm/s at 1 MHz with the strongest DEP force of 1.68 pN. This approach implements integration of functional electrodes into one substrate and avoids direct electrical connection to biological objects, providing a potential lab-on-chip system for biological applications.     

Dielectrophoresis for manipulation of micro/nano particles in microfluidic systems

Dielectrophoretic (DEP) force is exerted when a neutral particle is polarized in a non-uniform electric field, and depends on the dielectric properties of the particle and the suspending medium. The integration of DEP and microfluidic systems offers numerous applications for the separation, trapping, assembling, transportation, and characterization of micro/nano particles. This article reviews the applications of DEP forces in microfluidic systems. It presents the theory of dielectrophoresis, different configurations, and the applications of such systems for particle manipulation and device fabrication.