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Electrochimica Acta 87 (2013) 270– 276

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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xperimental validation of induced-charge electrokinetic motion of electricallyonducting particles

asaman Daghighia, Irene Sinnb,c, Raoul Kopelmanb,c, Dongqing Lia,∗

Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109, United StatesDepartment of Environmental Health Sciences, The University of Michigan, Ann Arbor, MI 48109, United States

r t i c l e i n f o

rticle history:eceived 23 May 2012eceived in revised form 30 August 2012ccepted 11 September 2012vailable online 17 September 2012

eywords:nduced charge electrokinetics

a b s t r a c t

A key characteristic in the theory of the induced charge electrokinetic flow is the vortices generated bythe interaction of the applied electric field with the induced dipolar electric double layers around an elec-trically conducting surface. The vortices can significantly influence the electrokinetic motion of a particlewith an electrically conducting surface. This paper provides the visual evidence of the vortices in theinduced charge electrokinetic flow in a “DC field” around a metal surface that is “not an active electrode”.According to authors’ knowledge, this is the first experimental proof of induced-charge electrokineticphenomena for DC electric field while all the literatures employed the AC field as the acting force during

orticesanus particle

the experiments. Moreover, former studies had mostly reported the vortices around active electrode,used as a conducting surface. Current paper reports the induced vortices around a non-electrode con-ducting surface. In addition, the experimental results of this paper show that the measured velocity of theelectrokinetic motion of particles with a metal hemisphere in a microchannel is much higher than that ofthe non-conducting polymer particles of the same size under the electric field. These experimental dataagree well with the theoretical prediction.

. Introduction

Electrokinetics has extensive applications in microfluidics, suchs pumping liquid in microchannels via electroosmosis, and sep-rating molecules or cells by electrophoresis. While most studiesf electrokinetics deal with surfaces of electrically non-conductingaterials, the induced charge electrokinetics (ICEK) studies the

henomena caused by the interaction between the applied electriceld and the induced charge on electrically conducting surfaces. Aey feature of the induced charge electrokinetic flow is the vorticesenerated by the interaction of the applied electric field with thenduced dipolar electric double layers around a conducting surface.

hen an electrically conducting object is placed in an electric field,pposite charges are induced on different parts of the conductingurface which will in turn attract the counter-ions in the liquid toorm dipolar electric double layers around the conducting surface.he interaction of the applied electric field and the net charges inhe dipolar electric double layers will generate vortexes near the

onducting object.

In 1962 Levich [1] was the first one who calculated the perturbedlip profile around a conducting sphere in an external electric

∗ Corresponding author.E-mail address: dongqing@mme.uwaterloo.ca (D. Li).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.09.021

© 2012 Elsevier Ltd. All rights reserved.

field. For this purpose he used the induced-charge double-layerconcept. After almost one decade the structure of the induced dou-ble layer was theoretically analyzed by Simonov and Dukhin [2].The first report on hydrodynamic interaction between two con-ducting particles was released on 1986 by Gamayunov et al. [3].About ten year later Murtsovkin [4] experimentally verified thestudy of Gamayunov et al. [3]. A general model of calculating theinduced-charge electrophoretic (ICEP) velocities of non-sphericalconducting particles was developed by Yariv [5]. A more system-atic theoretical model of ICEP motion of conducting particles, thetransient alignment of asymmetric conducting particles, and thedynamics of rod-like metal particles [6,7] were reported during thenext years. Saintillan et al. [8] implemented a statistical methodto analyze far-field hydrodynamic interactions in the ICEP of col-loidal rod dispersions. Yossifon et al. [9] studied the rotational ICEPmotion of a conducting dielectric spheroid in the presence of a uni-form arbitrarily oriented external electric field. Wu and Li reporteda numerical method to calculate the induced zeta potential aroundconducting surfaces of arbitrary shape, and studied the effects ofinduced charge electrokinetic flows on flow mixing and flow regu-lation in microchannels [10,11]. Following that, Wu and Li studied

the induced charge electrokinetic motion of fully polarizable par-ticles in microchannels [12] and the interactions of the inducedcharge electrokinetic motion of two fully polarizable particles in abulk solution [13]. More recently, the induced charge electrokinetic

himica Acta 87 (2013) 270– 276 271

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otion of heterogeneous particles (Janus particles) in microchan-els was investigated by Daghighi and Li [14,15].

However the previous researches of ICEK phenomena weressentially all limited to the numerical and theoretical studies andhere are only few experimental results available. Green et al. [16]xperimentally observed the streamlines of the liquid flow whichas induced by non-uniform AC field around microelectrodes, indi-

ating the existence of the induced charge electroosmosis aroundctive electrodes. In 2005, Levitan et al. [17] reported an exper-mental study of the induced-charge electroosmotic flow around

metal wire in a microchannel in an applied AC field. Gangwalt al. [18] experimentally studied the motion of heterogeneous par-icles (half conducting and half non-conducting materials) under

uniform low frequency AC field. They demonstrated the elec-rophoretic motion of heterogeneous particles is influenced bytrength and direction of AC electric field. Since some section ofhe heterogeneous particles is conducting surface, vortices formlose to that sections and push the heterogeneous particle with theon-conducting face forward. Later, Kim et al. [19] showed inducedharge electroosmotic flow around three electrodes and used it as

pumping method. Overall, there is lack of experimental evidencef the induced charge electrokinetic flow phenomena around “non-lectrode” conducting surfaces under the DC electric field. There iso reported experimental study of induced charge electrokinetichenomena of electrically conducting particles with DC electriceld in microchannels.

In the present paper we will first report the experimentallybserved flow patterns around a conducting particle in an elec-rolyte solution and under an external DC electric field. If theonducting particle has a symmetric geometry, e.g. sphere, itas been theoretically predicted that there will be four vorticesround the particle. In this paper, these four vortices are first timexperimentally visualized around a fully conducting non-electrodephere for a constant DC field. Effects of the DC electric field strengthn the induced vortices were examined. We also investigated thenduced charge electrokinetic motion of heterogeneous conduct-ng particles under an external DC electric field in a microchannel.he heterogeneous particle in this study is a polymer sphere halff which is coated with a metal layer. In an aqueous solution andnder a DC electric field, the vortices induced around the conduct-

ng hemisphere of the particle will affect the particle movement16,17]. Also it is proven the strength of induced vortices aroundhe conducting surface is a function of electric field. In this work,e experimentally investigated the velocity of the non-conductingarticle and heterogeneous particle under different applied DClectric fields. The velocity of the heterogeneous conducting par-icles is compared with the similar size non-conducting particlesnder the same experimental conditions. According to our exper-

mental results the heterogeneous particle always rotates to aligntself with the applied electric field axis. However the currentesults demonstrate that the type of the applied electric field deter-ines the direction of motion for a heterogeneous particle. Motion

f a heterogeneous particle is perpendicular to the AC eclectic fieldxis [20]; while this particle moves parallel to the direction of DClectric field axis.

. Materials and methods

.1. Microfluidic chip fabrication

The microfluidic chips were made of polydimethylsiloxane

PDMS) plate bonded on a glass substrate by using the soft lithog-aphy method. Details can be found elsewhere [20]. Briefly, theaster was prepared by first spin coating a film of SU-8 25 negative

hoto-resist onto a silicon wafer. After pre-baking, a photo-mask

Fig. 1. Illustration of the experimental system.

bearing the microchannel geometry, i.e. a straight channel, wasplaced on the top of the film. The photo-resist film covered withthe mask was then exposed to UV light. After post-baking anddeveloping, a master was obtained. Then, a 10:1 (w/w) mixtureof PDMS polymer and the curing agent was poured over the masterand cured at 75 ◦C for 3 h after being degassed under low vacuum.The PDMS layer with the microchannel structure was then peeledoff and put in an oxygen plasma discharger with a glass slide for30 s. Finally the PDMS layer was placed on top of the glass slide toform permanent bonding, The microfluidic chip is now ready foruse. For the purpose of this study, the microfluidic chip has a singlestraight channel connecting two wells, as illustrated in Fig. 1. Thechannel has a width 200 �m, depth 20 �m, and length 1.5 mm. Thediameter of both wells is 5 mm.

2.2. Materials and experimental setup

In order to visualize the vortices around a conducting surface,1.90 � dragon green fluorescent particles (Bangs Laboratories Inc.)were used to show the flow pattern. The excitation and emis-sion light wavelengths of the fluorescence particles are 480 nmand 520 nm, respectively. The experimental system consists of thefollowing major units: the microfluidic chip, a fluorescence micro-scope and image system, a power supply, and a data acquisitionsystem, as shown in Fig. 1. The microfluidic chip was fixed on theobservation platform of the microscope (TE2000-E, Nikon). A DC-regulated power supply (CSI12001X, Circuit Specialists Inc., USA)was used to control the voltages to the electrodes. The fluorescentmicroscope with a high-intensity illumination system and a CCDcamera (DS-QilMc, Nikon) was used to monitor the alignment andvisualize the florescent particle’s motion inside the microchannel.The images were captured by the digital camera and digitized bythe computer software (NIS-Elements BR 3.0).

In the studies of vortices around a fully conducting particle,spherical carbon-steel particles of 1.2 mm in diameter were usedin the experiments. To avoid the effects of the particle motion andallow a long period of observation of the vortices, the relativelylarge and heavy carbon-steel particles were chosen so that, due thegravity, the particle remained stationary under the applied electricfield in the experiments. For the studies of particles’ electrokineticmotion, 9.82 �m polymer particles (fluorescent carboxyl polymersmicrospheres – Bangs Laboratories Inc.) were used a homogeneousnon-conducting particles, and 10 �m polystyrene particles with aNi film of 100 nm thickness on one half of the particle surface wereused as the heterogeneous conducting particles. The heterogeneous

particles used for this study were prepared at Kopelman’s labo-ratory in the University of Michigan. Briefly, 10 �m polystyreneNIST particle standards (Polysciences, Inc., Warrington, PA) werere-suspended in methanol at 1% (w/v) first. 500 �L of the solution

272 Y. Daghighi et al. / Electrochimica Acta 87 (2013) 270– 276

Fig. 2. Vortices – the induced charge electrokinetic flow around a carbon–steel sphere, as visualized by 2 �m fluorescent particles. The direction of the electric field is froml ates th

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eft to right, E = 40 V/cm. Particle diameter is equal to 1.2 mm. The dashed line indic

as then spin-coated onto a 4′′ glass wafer at 600 rpm for 10 s. Nioating was introduced by vapor deposition onto the wafer sur-ace. Finally, the particles were removed from the glass wafer byrushing the wafer surface with a damp acrylic paintbrush ande-suspended into DI water. More details about the preparation ofhese heterogeneous particles can be found elsewhere [21].

.3. Experimental operation

For visualize the vortices around a conducting surface, a spheri-al carbon–steel particle was employed as the conducting particle.he particle was placed in the middle of a chamber filled withe-ionised (DI) water. Since the density of carbon steel is muchigher than water the particle will not move after the electric field

s applied. The rectangular chamber is made on a piece of PDMSlate and opens to air. The chamber used in this study has a widthf 2.5 cm, a length of 3.5 cm, and a depth of 2.5 mm. Two platinumires were placed at the two ends of the chamber as the electrodes;

nd connected to the power supply to produce the desired electriceld. The dragon green florescent particles were mixed in DI waternd dropped into the chamber to show the flow pattern around theonducting particle once the voltage is applied to the electrodes.

In the experimental investigations of the induced charge elec-rokinetic motion of particles, both the heterogeneous conductingarticles and the non-conducting polymer particles were used. Theicrofluidic chip has one straight microchannel with two wells at

he ends. Two platinum wires were placed in the two wells respec-ively as the electrodes and were connected to the DC power supplyo produce the desired electric field. To start the experiment, 20 �Lf the DI water is loaded to the left well; and another 20 �L of DIater was added to the right well, in order to maintain the same

evel of liquid in both wells. A small droplet of the diluted particleolution was released into the left well and a desired voltage differ-nce was then applied via the electrodes. The motion of the particless recorded for a period of time by using the microscope imagingystem. By determining the traveling distance and the associatedime for an individual particle by the image analysis software, thepeed of that particle can be obtained. For each experiment, the

article speed will be calculated for several particles. Such an exper-

ment was then repeated in ten microfluidic chips under the sameondition. Finally the average speed of the particles was determinedrom these results for a specific value of the applied electric field.

e particle surface. Image was taken at t = 2 s after applying the electric field.

3. Results and discussion

3.1. Visualization of vortices around a fully conducting sphericalparticle

Consider an electrically conducting object in an aqueous solu-tion. Once an electric field is applied, the opposite electric chargesare induced on different parts of the surface of the conductingobject. Consequently the counter-ions in the liquid will be attractedto the surface and EDL (electric double layer) forms around thesurface. Generally the induced charges on the surface facing theapplied electric field and the induced charges on the surface back tothe applied electric field have opposite signs. Therefore the electricdouble layers on these two sides have opposite electrical polarity;consequently the induced EDL on these two sides also oppositepolarity. The interaction between the applied electric field andthe net charges in the dipolar EDL will generate opposite electro-osmotic flow and hence vortexes near the conducting object. Theexistence of these vortices is predicted theoretically [12–15]. How-ever, very few experimental evidences are reported, especially forconducting objects that do not used as working electrodes underDC field.

In this study, a spherical carbon steel particle of 1.2 mm in diam-eter was placed in the middle of an open chamber (3.5 cm in length,2.5 cm in width, and 2.5 mm in height) in a PDMS plate. The cham-ber was filled with DI water. A DC electric field of 40 V/cm from leftto right was applied via two electrodes placed at the ends of thechamber to induce electric charge on the surface of the conductingparticle. Fig. 2 shows four vortices around the conducting particle.The dashed line represents the particle’s surface. Since particle sizeand the surrounding area with the vortices are large, the particularmicroscope (TE2000-E Nikon) has a limited field of view, the wholepicture in Fig. 2 was obtained in two steps. First we focused at theleft hand side of the particle which is faced to the positive elec-trode, and captured the flow pattern around the left hemisphereof conducting particle at t = 2 s after applying the DC electric field.Then keeping all the conditions unchanged, we repeated the exper-iment for the right hemisphere and captured the flow pattern onthat side at t = 2 s after applying the same DC electric field. Fig. 2demonstrates the induced vortices around the spherical carbon

steel particle at t = 2 s and E = 40 V/cm. At t = 2 s, the positions ofthe vortices are not stabilized yet, and will move outwards to thefinal steady state positions. However, the final positions of the fourvortices are far out of the field view of the microscope image system.

Y. Daghighi et al. / Electrochimica Acta 87 (2013) 270– 276 273

F e carbon steel particle facing to the positive electrode. The diameter of the carbon steelp

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ig. 3. Variation of the vortex pattern with the applied electric field in front of tharticle is 1.2 mm. Images were taken at t = 10 s after applying the electric field.

To visualize the effects of the applied electric field on inducedortex, several different values of the DC electric field were appliedo the carbon–steel particle in the middle of the chamber filled withI water. It is clearly demonstrated in Fig. 3 that increasing the elec-

ric field has a direct effect on the strength and size of the inducedortices around the conducting particle. Due to the limited field ofiew, Fig. 3 shows only one fully developed vortex in the front ofhe particle facing to the positive electrode. Obviously, the vortexrows with the external electric field. The stronger vortex (Fig. 3c)roduces a large “sink hole” in its center and circulates the fluidaster. These pictures are captured by TE2000-E Nikon florescent

icroscope at t = 10 s.

.2. Motion of heterogeneous conducting particles

The vortices around a particle with an electrically conductingurface will affect the motion of the particle immersed in an aque-us solution in a microchannel. Let’s consider a heterogeneousarticle, i.e. a particle with a conducting surface on one hemi-phere and a non-conducting surface on the other hemisphere.f the heterogeneous particle is aligned with the applied electriceld with the conducting surface on the upstream side and fac-

ng the applied electric field, there will be two vortices on thepstream side of the particle. These two vortices will push thearticle moving towards the downstream. In comparison with

non-conducting (e.g. polymer) particle under the same condi-ions, the heterogeneous particle will move fast. The numericalimulation has predicted this behavior [15]. In order to verifyhis prediction, 10 �m heterogeneous particles were used in ourxperimental investigation. These particles are polystyrene parti-les coated with a 100 nm thick Ni film on one hemisphere. TheEM photographs of these particles are shown in Fig. 4. As describedreviously, a microfluidic chip with one straight microchannel con-

ecting two wells was used in the experiments. The particles wereixed in DI water. A drop of the diluted particle solution was

eposited into the left well. Then an electric field was applied viahe electrodes inserted in the two wells. As an example, Fig. 5 shows

Fig. 4. The SEM pictures of the heterogeneous particles. 10 � spherical polystyreneparticles coated with 100 nm nickel film on one hemisphere. The dark side is thecoated section.

274 Y. Daghighi et al. / Electrochimica Acta 87 (2013) 270– 276

Fig. 5. Comparison of the electrokinetic motion of the heterogeneous particles with that of the polymer particles under the same external DC electric field of 80 V/cm. DCelectric field direction is from left to right. The particle diameter is 10 �.

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ig. 6. The velocity of the electrokinetic motion of the heterogeneous particlesnd that of the polymer particles as functions of the external DC electric field. Theiameter of both particles is 10 �.

he positions of a heterogeneous particle at different time after anlectric field was applied. For comparison the positions of a car-oxyl polymer particle under the same condition are also reported

n this figure. Clearly this figure shows that the heterogeneous par-icle moves much faster than the polymer particle, as predicted byhe numerical simulation [15].

Imaging analysis method was used to calculate the velocity ofhe particles. The motion of the particles under the applied DC elec-ric field was recorded over a period of time. By tracing the positionf each particle during that period, both the distance that the par-

icle traveled and the time period can be obtained and hence thearticle’s velocity can be calculated. Fig. 6 shows the velocity of theeterogeneous particles and polymer particles in the microchan-el as functions of the applied DC electric field. Each data point

in this figure is the average value of more than 40 particles overa period from 10s to 35s after applying DC electric field. Clearly,the velocity of the heterogeneous particles is much higher than thevelocity of the polymer particles under the same DC electric field.In addition, the results reveal that the velocity of the polymer parti-cles follows a linear relationship with the applied DC electric field.This is exactly the prediction of the classical theory of electrophore-sis. However, as shown in Fig. 6, the correlation of the velocity ofthe heterogeneous particles with the applied DC electric field isclearly non-linear, and may be fitted by a quadratic curve with a fit-ness confidence interval of 95% for the data obtained in this study.Moreover, the slope of the velocity curve of the heterogeneous par-ticles decreases with the DC electric field. This might be due to thefact that the conducting particle has a maximum polarization limit.Generally increasing the electric field will increase the polarizationof the conducting surface till the surface charge reaches a limit andbecome saturated. Approaching this point the strength of inducedvortices will be maximum, and hence the velocity of the particletends to reach its maximum.

By applying a theoretical model and numerical simulationmethod developed by us previously [14,15], the experimentallymeasured velocity of the Janus particle as reported in Fig. 6 wascompared with the velocity predicted by the numerical simula-tion. Fig. 7 shows the comparison of the measured velocity withthe predicted velocity under three different values of applied DCelectric field. The numerical results are in good agreement withthe experimental data. The small difference between the numericalsimulation and experimental data might be the result of simplify-ing assumptions used in the numerical model and simulation, suchas neglecting the gravity effect.

When the normal vector at the interface of two hemispheresof a Janus particle has a non-zero angle with the direction of theexternal DC electric field, the Janus particle is said to be unalignedwith the applied electric field. The Janus particle in such a positionis unstable and will rotates to align itself with the direction of DCelectric field due to the electric field force and induced vortices

forces (for more details please refer Ref. [14]). If the heteroge-neous particle is aligned with the applied DC electric field, thatis, its conducting surface is on the upstream side and facing theapplied electric field, there will be two vortices on the upstream

Y. Daghighi et al. / Electrochimic

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ig. 7. Comparison of the velocity of the Janus particle between the numericalredictions and the experimental data.

ide of the particle. These two vortices will push the particle mov-ng towards the downstream. However, if a heterogeneous particleligned in the opposite direction to the applied DC electric field,hat is, its conducting surface is on the downstream side, there wille two vortices on the downstream side of the particle. These twoortices will push the particle moving towards the upstream andgainst the electro-osmotic flow of the liquid. This was predicted in

heoretical simulation [14,15] and indeed observed in the experi-

ents. As shown in Fig. 7, some heterogeneous particles movedackwards while other heterogeneous particles moved along the

ig. 8. The opposite orientations of the heterogeneous particles lead to particles’otion in opposite directions. The vortices on the conducting hemisphere side apply

dominant force on the particle and determined the direction of the particle move-ent. DC electric field is 10 V/cm. The graphical circles represent the heterogeneous

articles which the gray side represents the conducting surface, while the brightection is non-conducting part.

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electric field direction. However, the heterogeneous particles movein a way to line up itself with the DC electric field axis. The interfaceof non-conducting and conducting hemispheres of a heteroge-neous particle is perpendicular to the DC electric field to balancethe forces generated (and applied to the particle) by inducedvortices around the conducting section. Though, the heteroge-neous particle under the AC electric field moves with the interfaceplane parallel to the AC electric field axis [18]. This is, accordingto the authors’ knowledge, the first experimentally proven evi-dence of heterogeneous particle motion under the DC electric field(Fig. 8).

4. Conclusion

This experimental study proved the existence of vortices closeto a non-electrode conducting surface submerged in an electrolytesolution under DC electric field. This confirms a key prediction ofthe theory of induced charge electrokinetics flows while DC elec-tric field is involved. We showed four vortices formed around aspherical carbon-steel particle. The size and the strength of thesevortices is a function of the external DC electric field. Furthermore,the induced charge electrokinetic motion of heterogeneous parti-cles, polymer particles with nickel film coated on one hemisphere,in a microchannel was investigated experimentally. The velocity ofthe heterogeneous particles is significantly higher than the poly-mer particles of the same size under the same applied electric field.The quantitative results verified the theoretical predication that thevortices on the conducting surface side push the particle movingto downstream. The direction of heterogeneous particle’s motionwhen DC electric field is involved is totally different from using ACelectric field in the same system. The heterogeneous particle movesin the same direction as the DC electric field axis is. The formerstudies on moving heterogeneous particle by applying AC fieldreported the particle moves perpendicular to the AC electric fieldaxis [18]. Proven vortices can be used for mixing in micro-devicesof biochemical, medicine, and many other useful and criticalaspects.

Acknowledgments

The authors wish to thank the financial support of the CanadaResearch Chair program and the Natural Sciences and EngineeringResearch Council (NSERC) of Canada through a research grant to D.Li.

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