Dielectrophoretic Effect Research Articles

Dielectrophoretic Trapping and Polarizability of DNA: The Role of Spatial Conformation

Dielectrophoresis is a convenient tool for controlled manipulation of DNA with numerous applications, including DNA trapping, stretching, and separation. However, the mechanisms behind the dielectrophoretic properties of DNA are still under debate, and the role of conformation has not been addressed yet. Here, we quantify dielectrophoretic effects on DNA by determining its polarizability from microfluidic single molecule trapping experiments. We systematically study different DNA configurations (linear and supercoiled, 6-164 kbp) and demonstrate that the polarizability strongly depends on the specific conformation and size of the DNA molecules. The connection to its spatial extension is established by measuring diffusion coefficients and from that the radii of gyration; details about the spatial DNA structure are obtained from atomic force microscopy images. For linear and supercoiled DNA fragments, we found a power-law scaling for the polarizabilities and the diffusion coefficients. Our results imply a scaling of the polarizability with the radius of gyration, alpha approximately Rg0.9+/-0.1 and alpha approximately Rg1.6+/-0.2 for linear and supercoiled DNA, respectively. As an application, we demonstrate the separation of DNA topoisomers based on their dielectrophoretic properties, achieving baseline resolution within 210 s. Purified DNA samples of specific configuration may be of great importance for DNA nanoassembly or future DNA vaccines.

Light induced patterning of poly(dimethylsiloxane) microstructures

A new method for direct patterning of Poly(dimethylsiloxane) (PDMS) microstructures is developed by taking advantage of photorefractive effect in a functionalized substrate. Here we show that when a x-cut Iron doped Lithium Niobate (LN) crystal is exposed to appropriate structured laser light, a charge density pattern builds-up in the crystal and a space charge field arise that is able to induce self-patterning of the PDMS liquid film deposited on its surface via the dielectrophoretic effects. Proper heating treatment allows to achieve polymeric linking process creating a solid and stable PDMS microstructures. The self-patterned structures replicate the illuminating light pattern. We show that 1D and 2D patterning of PDMS gratings can be achieved. This new soft-lithographic approach can pave the way for realizing PDMS micro-structures with high degree of flexibility that avoids the need of moulds fabrication.

Electrode-rail dielectrophoretic assembly effect: formation of single curvilinear particle-chains on spiral microelectrodes

An electrode-rail dielectrophoretic (DEP) assembly effect for microparticle pearl-chain formation is reported. Under this assembly effect, single curvilinear particle chains are formed along spiral strip electrodes in a dielectrophoretic microfluidic device actuated by alternating signals. Moreover, the snaking traveling movement of the entire individual curving pearl chain along the spiral electrode is also observed. The formation of single curvilinear pearl chain could be mainly interpreted by the specific distribution of the electric field above the spiral microelectrode array. Further, the inter-attracting DEP force between neighboring particles in pearl chains is generated when the originally uniform electric field is strongly perturbed due to the existence of the particles themselves. The entire individual pearl chain’s movement along the spiral strip electrode could be rationalized by the non-ignorable ohmic resistance distribution along the strip electrodes, which leads to the nonuniformity of field strength and DEP force along the spiral elongation direction.

Electrically reconfigurable optical metamaterial based on colloidal dispersion of metal nanorods in dielectric fluid

Optical metamaterials capture the imagination with breathtaking promises of nanoscale resolution in imaging and invisibility cloaking. We demonstrate an approach to construct a metamaterial in which metallic nanorods, of dimension much smaller than the wavelength of light, are suspended in a fluid and placed in a nonuniform electric field. The field controls the spatial distribution and orientation of nanorods because of the dielectrophoretic effect. The field-controlled placement of nanorods causes optical effects such as varying refractive index, optical anisotropy (birefringence), and reduced visibility of an object enclosed by the metamaterial.

A new method for fabricating high density and large aperture ratio liquid microlens array

Conventional liquid microlens arrays are facing bottlenecks in controlling droplet size and shape, and limited aperture ratio. We report a new liquid/solvent approach to overcome these obstacles for making uniform droplet array and achieving approximately 90% aperture ratio. The droplets are very stable due to the pinning effect of the polymer walls and substrate surfaces. Using the fabricated droplet array, we demonstrate a tunable-focus microlens array based on dielectrophoretic effect. The microlens array exhibits a large dynamic range and fast response time (tens of milliseconds). Besides liquid microlens arrays, this fabrication method also opens a new door for making other tunable photonic devices.

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.

Capillarity and dielectrophoresis of liquid deuterium

The ponderomotive force, exerted upon all dielectric liquids by a non-uniform electric field, can be used for the remote, voltage-controlled manipulation of 10 to 100 µL volumes of cryogenic liquids. This liquid dielectrophoretic (DEP) effect, imposed by specially designed electrodes, combines with capillarity to influence the hydrostatic equilibria of liquid deuterium. A simple, one-dimensional model accurately predicts the measured meniscus rise of D2 against gravity for sufficiently wide, parallel electrodes. For narrow electrodes, where the sidewalls influence the equilibrium, a finite element solution using the Surface Evolver software correctly describes the shape of the meniscus. A bifurcation phenomenon previously observed for room temperature dielectrics is also observed in liquid deuterium. Cryo-DEP actuation has potential applications ranging from metering precise volumes of liquid deuterium for laser target fuelling to manipulation of liquid nitrogen for cryo-preservation of biological samples.

DC Electrokinetic Particle Transport in an L-Shaped Microchannel

Electrokinetic transport of particles through an L-shaped microchannel under DC electric fields is theoretically and experimentally investigated. The emphasis is placed on the direct current (DC) dielectrophoretic (DEP) effect arising from the interactions between the induced spatially nonuniform electric field around the corner and the dielectric particles. A transient multiphysics model is developed in an arbitrary Lagrangian-Eulerian (ALE) framework, which comprises the Navier-Stokes equations for the fluid flow and the Laplace equation for the electrical potential. The predictions of the DEP-induced particle trajectory shift in the L-shaped microchannel are in quantitative agreement with the obtained experimental results. Numerical studies also show that the DEP effect can alter the angular velocity and even the direction of the particle's rotation. Further parametric studies suggest that the L-shaped microfluidic channel may be utilized to focus and separate particles by size via the induced DEP effect.

Nonlinear alternating electric field dipolophoresis of spherical nanoparticles

We consider the nonlinear electrokinetic problem of a freely suspended conducting (infinitely polarized) spherical micro- or nanosize particle surrounded by an unbounded electrolyte solution. The uncharged particle is exposed to an alternating (ac), nonuniform, and axisymmetric ambient electric field. As a result, the particle acquires a dipolophoretic (DIP) mobility of magnitude, which is quadratic in the amplitude of the applied electric field. The resulting phoretic velocity is driven by two independent nonlinear mechanisms. One is the common dielectrophoretic effect, whereby the nonuniform field exerts an electrostatic force on the image multipole singularity system within the particle. The other is the so-called “induced-charge electrophoresis” resulting from the action of the electric field on the excess charge around the particle induced in the diffused layer by the field itself. Both effects are quadratic in the amplitudes of the electric field and depend on the forcing frequency and on the dimensionless Debye screening length scale. It is demonstrated in the sequel that the two generally act in opposite directions which may result in mutual cancellation. Under the assumptions of a “weak” electric field and the neglect of surface conductance, we present a concise analysis of the resulting nonlinear streaming (dc) velocity (averaged over a period) for a spherical metalic particle that is exposed to a time-harmonic oscillating (ac) electric field. The analysis of this fundamental nonlinear DIP problem is provided for arbitrary forcing frequencies and for any Debye thickness. Numerical simulations are given for the case of a “two-mode” interaction consisting of a uniform-gradient electric field combined with a uniform field, where the two modes are either “in” or “out” of phase.

Single-cell trapping utilizing negative dielectrophoretic quadrupole and microwell electrodes

The handling of individual cells, which has attracted increasing attention, is a key technique in cell engineering such as gene introduction, drug injection, and cloning technology. Alternating current (AC) electrokinetics has shown great potential for microfluidic functions such as pumping, mixing, and concentrating particles. The non-uniform electric field gives rise to Joule heating and dielectrophoresis (DEP). The motion of particles suspended in the medium can be influenced directly, by means of dielectrophoretic effects, and indirectly, via fluid flow through a viscous drag force that affects the particles. Thus alternating current electrothermal effect (ACET) induced flow and DEP force can be combined to manipulate and trap single particles and cells. This study presents a microfluidic device which is capable of specifically guiding and capturing single particles and cells by ACET fluid flow and the negative dielectrophoretic (nDEP) trap, respectively. The experiment was operated at high frequencies (5–12 MHz) and in a culture medium whose high conductivity ( σ = 1.25 S/m) is of interest to biochemical analysis and environmental monitoring, which are both prone to producing ACET and nDEP. Manipulation of particle motion using ACET-induced fluid flow to the target trap is modeled numerically and is in good agreement with the experimental results.

Bacterial handling under the influence of non-uniform electric fields: dielectrophoretic and electrohydrodynamic effects

This paper describes the modeling and experimental verification of a castellated microelectrode array intended to handle biocells, based on common dielectrophoresis. The proposed microsystem was developed employing platinum electrodes deposited by lift-off, silicon micromachining, and photoresin patterning techniques. Having fabricated the microdevice it was tested employing Escherichia coli as bioparticle model. Positive dielectrophoresis could be verified with the selected cells for frequencies above 100 kHz, and electrohydrodynamic effects were observed as the dominant phenomena when working at lower frequencies. As a result, negative dielectrophoresis could not be observed because its occurrence overlaps with electrohydrodynamic effects; i.e. the viscous drag force acting on the particles is greater than the dielectrophoretic force at frequencies where negative dielectrophoresis should occur. The experiments illustrate the convenience of this kind of microdevices to micro handling biological objects, opening the possibility for using these microarrays with other bioparticles. Additionally, liquid motion as a result of electrohydrodynamic effects must be taken into account when designing bioparticle micromanipulators, and could be used as mechanism to clean the electrode surfaces, that is one of the most important problems related to this kind of devices.

Adaptive dielectric liquid lens

A tunable-focus liquid lens using dielectrophoretic effect is demonstrated. When a voltage is applied to a dielectric liquid droplet, the generated electric field inside the droplet is inhomogeneous. As a result, the liquid bears a dielectric force and its surface profile can be reshaped which causes the focal length to change. Adaptive lenses with different apertures are fabricated and their performances evaluated. In comparison to the patterned-electrode liquid lenses, our lens uses continuous electrode which is much simpler for fabrication.

Dielectrophoretic and Electrothermal Effects at Alternating Current Heated Disk Microelectrodes

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for "jet boiling" observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.

Microfluidic on chip viscometers

We present the design and the process of fabrication of micromachined capillary on chip rheometers which have performed wall shear stress and shear rate measurements on silicon oil and ethanol-based nanofluids. The originality of these devices comes from the fact that local pressure drop measurements are performed inside the microchannels. Thus, the advantage over existing microviscometers is that they can be used with the fluid under test alone; no reference fluid nor posttreatment of the data are needed. Each on chip viscometer consists of anodically bonded silicon-Pyrex derivative microchannels equipped with local probes. The anodic bonding allows to reach relatively high pressure levels (up to approximately 10 bars) in the channels, and a broad range of shear stress and shear rate values is attainable. Dielectrophoretic and electrorheological effects can be highlighted by employing alternate microstripe electrodes patterned onto the inner side of the Pyrex wall.

Manipulation of ZnO nanostructures using dielectrophoretic effect

Abstract In this paper, the dielectrophoretic manipulation of the nanostructured zinc oxide (ZnO) with microfabricated electrodes and electrode arrays had been studied. The nanorod-like ZnO prepared by the chemical solution growth, with the length of ∼10 μm, was used as the manipulation target. The electrodes and electrode arrays were prepared by standard IC process. The SEM pictures have been used to examine and evaluate the manipulation results. The influences of the pattern of electrodes, the applied frequency, the concentration and the applied voltage on the dielectrophoretic manipulation effect have been investigated to research the manipulation of particles by dielectrophoresis. We succeeded in manipulating ZnO particles along the electric field and depositing them across the gaps between two electrodes by modulating different factors. It is concluded that the nanostructured ZnO can be manipulated by dielectrophoresis and both the positive dielectrophoretic effect and the negative dielectrophoretic effect can be observed. This manipulation technique is potential for lots of application such as the construction of micro/nano sensors and the nanoelectronic devices.

Tunable-focus liquid microlens array using dielectrophoretic effect

A tunable-focus liquid microlens array based on dielectrophoretic effect was demonstrated. In a lens cell, two immiscible dielectric liquids but with different dielectric constants are sandwiched between electrodes. One electrode has a holed pattern while the other electrode is continuous. The applied voltage generates an inhomogeneous electric field near the hole regions. Owing to such an electric field, the generated dielectric force would separate the low dielectric liquid into many pieces and each piece is pushed to its neighboring hole. After balance, the droplet array is formed in the contracting state surrounded by the high dielectric constant liquid. Each droplet exhibits a lens character. When the voltage is removed, the droplets relax slightly but still keep a certain contact angle. Reactivating the cell will again enforce the droplets to reshape from relaxing to contracting states and, therefore, changing the focal length. Such a lens cell can be fabricated easily and its response time is reasonably fast. Its potential applications in image processing and zoom lens are emphasized.

미소채널 내 전기역학 및 유전영동 현상 해석을 위한 수치 프로그램 개발 및 검증

Electrokinesis and dielectrophoresis are important transport phenomena produced by external electric field applied to a microchannel containing a conductive fluid. We developed a CFD code to predict electrokinetic and dielectrophoretic flows in a microchannel with a uniform circular post array. Using the code, we calculated particle velocities driven by electrokinesis and dielectrophoresis, and conducted Monte Carlo simulations to visualize the particle motions. The code was validated by comparing the results with those from previous studies in literature. At a low electric field, electrokinesis and diffusion is the dominant transport mechanism. At a moderate electric field, dielectrophoresis is balanced with electrokinesis and diffusion, resulting in flowing filaments of particles in the microchannels. However, dielectrophoresis overwhelms the flow at a high electric field and traps particles locally. These results provide useful insight for optimizing design parameters of a microfluidic chip for biochemical analysis, especially for development of on-chip sample pretreatment techniques using electrokinetic and dielectrophoretic effects.

An analysis of interdigitated electrode geometry for dielectrophoretic particle transport in micro-fluidics

As researchers work to develop the “Lab-on-a-Chip” system, dielectrophoresis (DEP) is being examined as a mechanism for the micro-fluidic transport and separation of small biological samples such as cells, proteins, and DNA. Interdigitated electrodes are commonly used within such devices to generate the non-uniform electric fields that induce movement. Among other parameters, the magnitude of the DEP force depends upon the gradient of the square of the electric field that is generated by such electrodes. By understanding the effect that the dimensions of the electrodes have on this quantity, micro-fluidic devices can be designed to produce the most effective dielectrophoretic effect on the biological samples. This article examines the relationship between the geometry of the interdigitated electrodes and the magnitude of the DEP force. This is done by obtaining and analyzing an equation for the gradient of the square of the electric field. The equation is generated by fitting the results of extensive numerical simulations. Although the complete equation introduced in this work is based upon theoretical results, many fundamental portions of the present work agree with accepted experimental results. These verifications are also addressed.

GEOFLOW: simulation of convection in a spherical shell under central force field

Abstract. Time-dependent dynamical simulations related to convective motion in a spherical gap under a central force field due to the dielectrophoretic effect are discussed. This work is part of the preparation of the GEOFLOW-experiment which is planned to run in a microgravity environment. The goal of this experiment is the simulation of large-scale convective motion in a geophysical or astrophysical framework. This problem is new because of, on the one hand, the nature of the force field (dielectrophoretic effect) and, on another hand, the high degree of symmetries of the system, e.g. the top-bottom reflection. Thus, the validation of this simulation with well-known results is not possible. The questions concerning the influence of the dielectrophoretic force and the possibility to reproduce the theoretically expected motions in the astrophysical framework, are open. In the first part, we study the system in terrestrial conditions: the unidirectional Earth's force is superimposed on the central dielectrophoretic force field to compare with the laboratory experiments during the development of the equipment. In the second part, the GEOFLOW-experiment simulations in weightless conditions are compared with theoretical studies in the astrophysical framework's, in the first instance a fluid under a self-gravitating force field. We present complex time-dependent dynamics, where the dielectrophoretic force field causes significant differences in the flow compared to the case that does not involve this force field.

The aerosol penetration through an electret fibrous filter

The aim of this paper is to present a theoretical study of the aerosol penetration through an electret fibrous filter, using a numerical approach. The aerosol sizes considered in this study were in the submicron range, and in the numerical model, the conventional mechanical mechanisms (impaction, interception, diffusion and gravitationally settling) were taken into consideration along with the electrostatic mechanisms, including the Coulombic and dielectrophoretic effects. The aerosol penetration through an electret fibrous filter is heavily dependent on the aerosol penetration of a single fibre. The aerosol penetration through a single electret fibre under various filtration conditions was calculated. The effects of aerosol diameter, aerosol and fibre charge state, face velocity, packing density and aerosol dielectric constant on the aerosol penetration were investigated.