AC Electrokinetics Research Articles

Ac electrokinetic phenomena over semiconductive surfaces: Effective electric boundary conditions and their applications

Electrokinetic boundary conditions are derived for ac electrokinetic phenomena over leaky dielectric (i.e., semiconducting) surfaces. Such boundary conditions correlate the electric potentials across a semiconductor-electrolyte interface (consisting of an electric double layer inside the electrolyte solution and a space charge layer inside the semiconductor) in an ac electric field with arbitrary wave forms. The presented electrokinetic boundary conditions allow for evaluation of the induced ζ potential contributed by both bond charges (due to electric polarization) and free charges (due to electric conduction) from the leaky dielectric materials. Two well-known limiting cases, (i) the conventional insulating boundary condition and (ii) the perfectly polarizable boundary condition, can be recovered from the generalized electrokinetic boundary conditions derived in the present paper. Subsequently, we demonstrate the implementation of the derived boundary conditions for analyzing the ac induced-charge electrokinetic flow around a semiconducting cylinder. The results show that the flow circulations exist around the semiconducting cylinder and become stronger in the ac field with a lower frequency and around the semiconducting cylinder with a higher conductivity.

Dc electrokinetics for spherical particles in salt-free concentrated suspensions including ion size effects

We study the electrophoretic mobility of spherical particles and the electrical conductivity in salt-free concentrated suspensions including finite ion size effects. An ideal salt-free suspension is composed of just charged colloidal particles and the added counterions that counterbalance their surface charge. In a very recent paper [Roa et al., Phys. Chem. Chem. Phys., 2011, 13, 3960-3968] we presented a model for the equilibrium electric double layer for this kind of suspensions considering the size of the counterions, and now we extend this work to analyze the response of the suspension under a static external electric field. The numerical results show the high importance of such corrections for moderate to high particle charges, especially when a region of closest approach of the counterions to the particle surface is considered. The present work sets the basis for further theoretical models with finite ion size corrections, concerning particularly the ac electrokinetics and rheology of such systems.

Applications of electrohydrodynamics and Joule heating effects in microfluidic chips: A review

This review article presents an overview on the application of electrohydrodynamics and Joule heating effects in microfluidic chips. A brief introduction of microfluidic chips and a classification of electrohydrodynamics as well as the applications in microfluidic devices are first given. Then basic theories and governing equations of classical electromagnetics are summarized and electroviscous effects in pressure driven flows in a microchannel are presented. Principles and applications of DC electrokinetics, including DC electroosmotic flow, DC electrophoresis, as well as principles of AC electrokinetics, including AC electroosmotic flow and dielectrophoresis are also reviewed. Finally, Joule heating effects in both DC and AC electrokinetics, especially the newly discovered electrothermal flow, are summaried.

Dynamic Light-Activated Control of Local Chemical Concentration in a Fluid

This article reports a method for controlling the chemical concentration in a localized region of a fluid using optoelectrofluidic mechanisms. Optoelectrofluidic fluorescence microscopy (OFM), in which an optoelectrofluidic device is integrated into a conventional fluorescence microscope, allows both modulation and detection of local chemical concentration in an easy and simple way. Here, we present the first experimental investigation of the concentration change of polysaccharides, proteins, and fluorophores, due to the frequency-dependent ac electrokinetics and electrostatic interactions in an optoelectrofluidic device. The dynamic modulation of the local concentration of biomolecules such as dextran and serum albumin was demonstrated in a temporal and spatial manner. This OFM can be a useful tool for controlling the local chemical concentration in several chemical and biological applications.

DC-Biased AC-Electrokinetic Mixing: A Mechanistic Investigation

This paper presents an experimental investigation on the DC-biased AC-electrokinetic (ACEK) flow vortex in a rectangular microchannel. This is a new flow phenomenon in AC-electrokinetics and had been successfully utilized for microfluidic mixing as reported in our previous work. The microchannel is aligned parallel to a pair of energized coplanar microelectrodes such that the generated flow vortex is perpendicular to the pressure-driven flowing samples. With the application of a DC bias AC voltage, incipient Faradaic reactions occur above the electrodes and thus producing ions and creating a net dominant flow vortex. Further investigation to understand the generation of the DC-biased ACEK flow was conducted. This is performed by direct visualization of such flow vortex formation from the cross-section of a rectangular microchannel.

Effects of Water Dissociation and CO2 Contamination on the Electrophoretic Mobility of a Spherical Particle in Aqueous Salt-Free Concentrated Suspensions

In a very recent paper ( Ruiz-Reina , E. ; Carrique , F. J. Phys. Chem. B 2008 , 112 , 11960. ) we studied the effect of water dissociation and CO(2) contamination on the equilibrium electrical double layer of spherical particles in salt-free concentrated suspensions in aqueous solutions. It was shown that in most cases (dilute to moderately concentrated suspensions), the neglecting of those effects would lead to a very poor description of common salt-free suspensions, especially if the suspensions have been in contact with air. In the present contribution we explore the influence of the latter effects on the dc electrophoresis in realistic salt-free suspensions. This kind of system consists of aqueous suspensions without any electrolyte added during the preparation. The ionic species in solution can solely be (i) the "added counterions" stemming from the particles that counterbalance their surface charge, (ii) the H(+) and OH(-) ions from water dissociation, and (iii) the ions produced by the atmospheric CO(2) contamination. Our model follows the classical Poisson-Boltzmann approach, a spherical cell model and the appropriate local chemical reactions. We have applied it to the study of the electrophoretic mobility of a spherical particle for different particle volume fractions varphi and surface charge densities. The numerical results have shown the quite large influence that water dissociation ions and/or CO(2) contamination have on the electrophoretic mobility at low-moderate particle volume fractions. In those situations the role of the added counterions is screened by the other ionic species. These effects yield the mobility to reach plateau values instead of further increasing as volume fraction decreases. It is concluded that it is necessary to take into account the water dissociation influence for varphi lower than approximately 10(-2), whereas the atmospheric contamination, if the suspensions have been exposed to the atmosphere, is not negligible if varphi < 10(-1). The present work sets the basis for further theoretical models concerning particularly the ac electrokinetics and dielectric response of such systems.

Flows in Microchannels and Cavities – Visco-Elasticity, Faradaic Charging and Surface Roughness Effects

Two phenomena for flows in microchannels, namely visco-elastic instability and Faradaic charging effects, are discussed. Aqueous solutions, which do not exhibit elastic behaviors, will behave visco-elastically with the addition of a small amount of polymers. Visco-elastic flow instability which promote mixing has been observed in the flow of these visco-elastic solutions through a constriction in microchannels. In addition, we recently show experimentally the existance of Faradaic charging in AC-electrokinetic (ACEK) flow. Lastly, surface roughness effects in micro- plastic injection molding are highlighted. The micro-injection molding process involves the flow of polymer melt in micro-cavities. Our recent investigations demonstrate that the flow model for macro-cavities could be employed satisfactorily to simulate the flow in micro-cavities if roughness effects are considered in an appropriate manner.

Experimental verification of Faradaic charging in ac electrokinetics.

This paper investigates the phenomenon of Faradaic charging in ac electrokinetics. Faradaic reactions were suggested as a key effect responsible for the reversal of pumping direction in ac micropumps. However, this hypothesis has yet to be proven convincingly and directly. Here we present an ion detection strategy to determine the production of ions through Faradaic hydrolytic reactions originating from direct application of voltage to electrolytic solutions during ac electrokinetics. Experiments were performed with symmetrical planar electrodes aligned along a microfluidic channel. Fluorescein, a pH-dependent dye, was employed as the pH indicator for the detection of ion production. Images were captured for analysis at various voltage levels. From analyzing the fluorescence intensity and its distribution, it can be concluded that the production of ions from hydrolytic reactions takes place and increases with the ac voltage. The coefficient of deviation indicates a significant enhancement at ac voltage above 11 V(pp). Lastly, we demonstrate a strategy using dc-biased ac electrokinetics to achieve controllability in direction and magnitude of the net fluid flow in pumping application.

AC Electrokinetics of Concentrated Suspensions of Soft Particles

In this work, we show how the cell model traditionally used for the evaluation of the electrokinetic properties of concentrated suspensions can be modified to include the case of soft particles, that is, particles consisting of a rigid core and a polyelectrolyte membrane. The Navier-Stokes and Poisson's equations have been modified to account for the presence of extra friction and a volume-distributed charge in the membrane. In addition to the boundary conditions on the particle and the cell boundary, it is necessary to define conditions on the polymer-electrolyte solution interface. The frequency dependence of the dynamic mobility and electric permittivity of suspensions of soft particles with arbitrary solids concentration is computed. It is shown that the dynamic mobility of these systems is larger than that corresponding to hard particles with the same charge. For the permittivity, the same trends are observed: the R-relaxation amplitude increases upon coating. It is found that friction plays an important role in determining the mobility, while the permittivity is more affected by the concentration of solids. The model also predicts that the charges on the core and in the membrane are very important parameters, although their effects differ on the mobility and the permittivity. While the former depends mainly on the membrane charge, the latter is responsive to both charges at comparable extents.

Comprehensive analysis of particle motion under non-uniform AC electric fields in a microchannel

AC electrokinetics is rapidly becoming a foundational tool for lab-on-a-chip systems due to its versatility and the simplicity of the components capable of generating them. Predicting the behavior of fluids and particles under non-uniform AC electric fields is important for the design of next generation devices. Though there are several important phenomena that contribute to the overall behavior of particles and fluids, current predictive techniques consider special conditions where only a single phenomenon may be considered. We report a 2D numerical simulation, using COMSOL Multiphysics, which incorporates the three major AC electrokinetic phenomena (dielectrophoresis, AC electroosmosis and electrothermal effect) and is valid for a wide range of operational conditions. Corroboration has been performed using experimental conditions that mimic those of the simulation and shows good qualitative agreement. Furthermore, a broad range of experiments has been performed using four of the most widely reported devices under varying conditions in order to show their behavior as it relates to the simulation. The large number of experimental conditions reported, together with the comprehensive numerical simulation, will help provide guidelines for scientists and engineers interested in incorporating AC electrokinetics into their lab-on-a-chip systems.

Rapid and selective concentration of microparticles in an optoelectrofluidic platform

We demonstrate rapid manipulation and selective concentration of microparticles using AC electrokinetics such as dielectrophoresis (DEP) and AC electro-osmosis (ACEO) in an optoelectrofluidic platform based on a liquid crystal display (LCD). When 10 V bias at 10 kHz was applied to the optoelectrofluidic device, only the 1 microm-diameter polystyrene particles were concentrated into the projected LCD image patterns and closely packed, forming the crystalline structure by ACEO flow, while the 6 microm-diameter particles were repelled by negative DEP forces. We have characterized this frequency-dependency of the optoelectrofluidic particle behavior according to the particle diameter. On the basis of these results, we can rapidly concentrate the 1 microm-diameter particles and separate them from the 6 microm particles, by applying an AC signal of 10 kHz frequency. This novel technique can be applied to rapidly concentrate, separate and pattern micro-/nanoparticles in many biological and chemical applications.

Particle-localized AC and DC manipulation and electrokinetics

Colloidal particles suspended in water respond to direct (DC) or alternating current (AC) fields in a variety of ways, including directional motion along or across the field direction, field-gradient dependent response and induced particle–particle interaction. We review here some of these effects and their applications in new techniques for particle manipulation and assembly, making of novel biomaterials and designing of new self-propelling microdevices. The coupling of the counterionic layer mobility, fluid flows and the resulting particle motion are the basis not only of the classic electrophoretic effects, but also of the recent developments in AC electrohydrodynamics and induced charge electrophoresis of asymmetric particles. We also discuss how dielectrophoresis (particle interaction with external AC field gradients), could be used to manipulate and assemble objects on any size scale. We discuss the interactions leading to the assembly of such structures, ways to simulate the dynamics of the process and the effect of particle size and conductivity on the type of structure obtained. Finally, we demonstrate how an additional level of complexity can be engineered to turn miniature semiconductor diodes into prototypes of self-propelling micromachines and micropumps. The diodes suspended in water propel themselves electro-osmotically when a uniform alternating electric field is applied across the container. Semiconductor diodes embedded in channel walls could serve as distributed microfluidic pumps and mixers powered by a global external field.

Biological Applications of Programmable Optoelectrofluidic Manipulation

AbstractThis paper presents a new programmable particle manipulation using a lab-on-a-display platform, which is a kind of optoelectrofluidic platform applying a liquid crystal display (LCD) as a display device for generating virtual electrodes in optoelectronic tweezers. The reconfigurable virtual electrodes in the lab-on-a-display are more advantageous than other devices which apply the micro-patterned electrodes, because we can freely control the size and position of electrodes as well as the voltage conditions, which affect the particle movements such as concentration and separation of particles. Due to its simple structures, cheap manufacturing costs, and high performances, this new LCD-based optoelectrofluidic platform can be applied to the interactive manipulation of polystyrene microspheres and blood cells. In addition, a method to discriminate normal oocytes for in vitro fertilization is demonstrated by combining the gravity effect with the optically induced positive dielectrophoresis (DEP). The discrimination performance can be enhanced due to the reduction of friction forces acting on the oocytes which are relatively large and heavy cells being affected by the gravity field. With the same device, we also demonstrate the size-dependent microparticle separation as well as the local concentration and assembly of microparticles originated from the image-driven AC electrokinetics such as DEP and AC electroosmosis. The particle movements result from the frequency-dependent behavior according to the particle diameter. This novel technique can be applied to rapidly concentrate, separate and pattern micro-/nanoparticles and biomolecules in many biological and chemical applications.

1509 高周波交流動電現象を利用したマイクロチャネル内粒子操作に関する研究(OS15-2 ナノ・マイクロ流体のダイナミクス,オーガナイズドセッション)

In a microfluidic device, utilization of electrokinetics is effective for particle control. Under high frequency alternating-current (AC) electric field, the control of particles can be achieved in the condition with low voltage. In this study, we focus on electrothermal flow subjected to AC electric field. A lateral observation system with a minute prism has been developed in order to measure the velocity field normal to the substrate. In this system, clear particle image can be obtained by using the PDMS (polydimethylsiloxane) channel with the side wall thickness of 125μm. It is observed that the particle velocity in the vicinity of electrode edge with 20 V_<P-P> at 300 kHz is 75 μm/s. This result indicates that the electrothermal effect is valid for controlling the fluid with high electric conductivity.

Analytical solutions of the dielectrophoretic and travelling wave forces generated by interdigitated electrode arrays

In AC electrokinetics, the application of an AC electric field to a suspension of particles results in the manipulation and separation of the particles also the movement of the fluid. One application is dielectrophoresis (DEP). The second effect is travelling wave dielectrophoresis (twDEP). This paper presents the analytical solutions of the dielectrophoretic and travelling wave forces for the interdigitated electrode arrays energised with either a two- or four-phase signal, respectively. The torque that rotates the particle in the four-phase travelling wave arrays is also analytically solved.

Electric field analysis using Schwarz-Christoffel mapping

Electrical techniques based on AC electrokinetics and impedance spectroscopy are widely used to manipulate and characterize biological particles in the microfluidic systems. This paper presents the application of the Schwarz-Christoffel mapping method to analytically solve the electric field distributions in different microfluidic systems, which are composed of different microelectrode patterns and boundary conditions. The derived results can be further utilized to analyze the movement and electrical response of the biological particles in each system.

Advances of LOC-Based Particle Manipulation by AC Electrical Fields

This article presents recent patents and advances on electrical methods for particle manipulation in a microfluidic environment. At microscale, electric fields have unique advantages in performing analytical functions such as manipulation of biological molecules and cells. The advent of AC electrokinetics in recent years further promotes the development of laboratory on a chip, providing versatility and flexibility to interface with many current methods and technologies in multiple biological, chemical and physical disciplines. This article gives an overview of AC electrical principles and their applications, with an emphasis on particle manipulation by electric fields.

Microtubule Alignment and Manipulation Using AC Electrokinetics

The kinesin-microtubule system plays an important role in intracellular transport and is a model system for integrating biomotor-driven transport into microengineered devices. AC electrokinetics provides a novel tool for manipulating and organizing microtubules in solution, enabling new experimental geometries for investigating and controlling the interactions of microtubules and microtubule motors in vitro. By fabricating microelectrodes on glass substrates and generating AC electric fields across solutions of microtubules in low-ionic-strength buffers, bundles of microtubules are collected and aligned and the electrical properties of microtubules in solution are measured. The AC electric fields result in electro-osmotic flow, electrothermal flow, and dielectrophoresis of microtubules, which can be controlled by varying the solution conductivity, AC frequency, and electrode geometry. By mapping the solution conductivity and AC frequency over which positive dielectrophoresis occurs, the apparent conductivity of taxol-stabilized bovine-brain microtubules in PIPES buffer is measured to be 250 mS m(-1). By maximizing dielectrophoretic forces and minimizing electro-osmotic and electrothermal flow, microtubules are assembled into opposed asters. These experiments demonstrate that AC electrokinetics provides a powerful new tool for kinesin-driven transport applications and for investigating the role of microtubule motors in development and maintenance of the mitotic spindle.

AC Electrokinetic Phenomena Generated by Microelectrode Structures

The field of AC electrokinetics is rapidly growing due to its ability to perform dynamic fluid and particle manipulation on the micro- and nano-scale, which is essential for Lab-on-a-Chip applications. AC electrokinetic phenomena use electric fields to generate forces that act on fluids or suspended particles (including those made of dielectric or biological material) and cause them to move in astonishing ways1, 2. Within a single channel, AC electrokinetics can accomplish many essential on-chip operations such as active micro-mixing, particle separation, particle positioning and micro-pattering. A single device may accomplish several of those operations by simply adjusting operating parameters such as frequency or amplitude of the applied voltage. Suitable electric fields can be readily created by micro-electrodes integrated into microchannels. It is clear from the tremendous growth in this field that AC electrokinetics will likely have a profound effect on healthcare diagnostics3-5, environmental monitoring6 and homeland security7. In general, there are three AC Electrokinetic phenomena (AC electroosmosis, dielectrophoresis and AC electrothermal effect) each with unique dependencies on the operating parameters. A change in these operating parameters can cause one phenomena to become dominant over another, thus changing the particle or fluid behavior. It is difficult to predict the behavior of particles and fluids due to the complicated physics that underlie AC electrokinetics. It is the goal of this publication to explain the physics and elucidate particle and fluid behavior. Our analysis also covers how to fabricate the electrode structures that generate them, and how to interpret a wide number of experimental observations using several popular device designs. This video article will help scientists and engineers understand these phenomena and may encourage them to start using AC Electrokinetics in their research.

AC Electrokinetic Phenomena Generated by Microelectrode Structures

The field of AC electrokinetics is rapidly growing due to its ability to perform dynamic fluid and particle manipulation on the micro- and nano-scale, which is essential for Lab-on-a-Chip applications. AC electrokinetic phenomena use electric fields to generate forces that act on fluids or suspended particles (including those made of dielectric or biological material) and cause them to move in astonishing ways. Within a single channel, AC electrokinetics can accomplish many essential on-chip operations such as active micro-mixing, particle separation, particle positioning and micro-pattering. A single device may accomplish several of those operations by simply adjusting operating parameters such as frequency or amplitude of the applied voltage. Suitable electric fields can be readily created by micro-electrodes integrated into microchannels. It is clear from the tremendous growth in this field that AC electrokinetics will likely have a profound effect on healthcare diagnostics, environmental monitoring and homeland security. In general, there are three AC Electrokinetic phenomena (AC electroosmosis, dielectrophoresis and AC electrothermal effect) each with unique dependencies on the operating parameters. A change in these operating parameters can cause one phenomena to become dominant over another, thus changing the particle or fluid behavior. It is difficult to predict the behavior of particles and fluids due to the complicated physics that underlie AC electrokinetics. It is the goal of this publication to explain the physics and elucidate particle and fluid behavior. Our analysis also covers how to fabricate the electrode structures that generate them, and how to interpret a wide number of experimental observations using several popular device designs. This video article will help scientists and engineers understand these phenomena and may encourage them to start using AC Electrokinetics in their research.