Magnetic Fields for Fluid Motion

Melissa C. Weston, Matthew D. Gerner, and Ingrid Fritsch

University of Arkansas Fayetteville

Three forces induced by magnetic fields offer uniquecontrol of fluid motion and new opportunities in micro-fluidics. This article describes magnetoconvective phe-nomena in terms of the theory and controversy, tuningby redox processes at electrodes, early-stage applicationsin analytical chemistry, mature applications in disciplinesfar afield, and future directions for micro total analysissystems. (To listen to a podcast about this article, pleasego to the Analytical Chemistry multimedia page atpubs.acs.org/page/ancham/audio/index.html.)

Miniaturization drives much of the research and development inanalytical chemistry. Portability, smaller volume requirements, lesswaste and material, and reduced power consumption are advan-tages of downsizing chemical analyses. Smaller scales can offerimprovements in sensitivity and new approaches in solvinganalytical chemistry problems. For example, micro total analysissystems (µTAS) aim to combine multiple laboratory proceduresin single hand-held devices. Such systems could revolutionizechemical analyses in genomics, environmental monitoring, medicaldiagnostics, and drug discovery. New challenges arise, however,in handling fluids at small dimensions in an automated fashion:small volumes can evaporate quickly unless enclosed, laminar flowconditions inhibit mixing and stirring, and a pump must matchthe system’s parameters and flow requirements (stopping, starting,speed, and direction). Traditional approaches in microfluidicsattempt to address these needs, but the gaps in capabilities drivefurther development of existing methods and introduction of newones.

The intention of this article is to expand awareness in theanalytical chemistry community about non-mechanical, fluid-pumping phenomena involving magnetic fields, collectively knownas magnetoconvection, with an emphasis on magnetohydrody-namics (MHD) in solutions and their application to µTAS. Thereader is referred to Pamme1 for a broader description of growinguses of magnetism in small chemical systems for pumping,trapping and transporting, mixing, detection, patterning, sorting,and separation. A recent review by Qian and Bau2 discussespublications specific to MHD in microfluidics. In contrast, this

Feature is more fundamental, elaborating on redox processes thatcontribute not only toward MHD, but also toward magnetocon-vective phenomena that require the presence of paramagneticspecies. This article also presents theories of magnetoconvectionand their controversies, reviews recent reports in analyticalchemistry, brings together for inspiration magnetoconvection fromdisciplines far afield from chemistry, and offers an outlook onfuture directions.

FLUID FLOW FROM MAGNETIC FIELDS: HOW ITWORKS

The three central forces of magnetoconvection are the magneticforce (FB, also known as the magnetohydrodynamic force),magnetic gradient force (F3Β),3 and paramagnetic concentrationgradient force (F3C).4,5 Figure 1 illustrates the relationshipsbetween each force and different variables that can be used todesign devices incorporating magnetoconvective microfluidics.

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The MHD force has received the most attention of the three.Figure 1a illustrates representations of both the body force FB

(newton/cubic meter) and particle force FL,B (newton). Whena species (ion or electron) with charge, q (coulomb), moveswith velocity, v (meter/second), at a right angle to a magneticfield, B (tesla), the force, FL,B, that is one of two terms in theLorentz equation, acts on it in a direction sensitive to the signof the charge and perpendicular to both v and B, with a crossproduct relationship, FL,B ) qv × B. Momentum transferbetween this species and solvent causes the localized fluid toflow in the direction of the force. When considering all ions ina unit volume, their net movement can be described by theflux j (coulomb/[second square meter])sthe sum of qv overall species. Positive j is defined in the direction that positiveions move; ions of opposite sign moving in opposing directionshave an additive effect on j. Thus, the body force obeys thesimple right hand rule, FB ) j × B.

Both F3B and F3C can be thought of as arising from agradient in “magnetic energy”. F3B acts on paramagneticspecies (ions and neutrals) in the direction of the magneticfield gradient.3 Figure 1b shows a scaled illustration for realexperimental conditions near a permanent magnet, where thelargest gradients occur at the edges. The reader is cautioned topay attention to the vector calculus notation for the F3B equation

because multiple mathematically nonequivalent forms arepresent in the literature.6-8

F3C derives its magnitude and direction in a magnetic field(Figure 1c) from a gradient in the magnetic susceptibility suchas a concentration gradient of a paramagnetic species (3CP).Whether F3C plays a measurable role compared to the othertwo forces is an issue under active debate. Leventis and Dass5

give an excellent review of the debate’s evolution, as well asevidence supporting the force’s role. Coey and coworkers4

respond with an argument against the force’s role.How and where forces are applied by magnetic fields in fluids

depend on the properties of the fluid. For example, in goodconductors such as liquid metals, in which the charge carriersare electrons, flow resulting from FB along a rectangular conduitis accomplished by passing a current crosswise to the conduitin the presence of a magnetic field oriented perpendicular tothis electron flux j. In aqueous or nonaqueous solutions, thesituation is quite different: the charge carriers are ions, andthe conductivity is at least five orders of magnitude smallerthan in liquid metals. Electrochemistry can be used to ac-complish an ion flux, j (to induce FB) and/or create or depleteparamagnetic species to change CP and 3CP and effect F3B

and F3C, respectively. Therefore, a brief primer on electro-chemical processes is presented here before elaborating further

Figure 1. Illustration of body forces that cause magnetoconvection in solution. (a) The magnetohydrodynamic, or magnetic force (FB), acts onfluid containing ions with a flux (j) having a perpendicular component to a magnetic field (B). Here, negative ions (purple) are generated at thenegative electrode and positive ions (green) are generated at the positive electrode by redox reactions. The expanded view in the lower half ofthe figure shows the force acting on individual ions (FL,B) moving with a velocity, v. FL,B is the magnetic portion of the Lorentz force (FL); anelectric field component, FL,E (migration; q is particle charge and E is electric field), makes up the other part of FL. (b) The magnetic fieldgradient force (F3B) acts on fluid containing paramagnetic species with uniform concentration, CP. The magnitude of B is scaled by color fordifferent field lines in the y-z plane for a NdFeB disk magnet (gray) of common dimensions; lines were generated with AMPERES V64 (IntegratedEngineering Software, Winnipeg, Manitoba). Relative magnitudes of F3B vectors are shown to scale by length and with multiplier where indicated.(c) The concentration gradient force (F3C) acts on fluid containing a paramagnetic species concentration gradient (3CP) and is independent ofmagnetic field direction. Relative magnitudes of F3C vectors at different locations along the concentration gradient are drawn to scale for auniform B field (not shown). (NA is Avogadro’s number, m* is related to magnetic susceptibility, and k is the Boltzmann constant. Vectors arein boldface.)

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on magnetoconvective applications. More thorough discussionson electrochemistry can be found in textbooks such as Bardand Faulkner.9

In an electrolyte system, controlling the potential or currentbetween two (or more) electrodes can affect the chemical speciesin the solution, producing a current or potential, respectively, thatcan be monitored. The current involves two processes: charging(Figure 2a) and faradaic (Figure 2b). Charging at the electrode-solution interface occurs when ions migrate to accommodate anelectrode potential that differs from that of the solution (thepotential of zero charge). This leads to a double layer at theelectrode-solution interface, often represented electrically as acapacitor. The distance away from the electrode across which theelectric field can mobilize ions (diffuse layer, Figure 2a) can bevery short (approximately three to hundreds of angstroms) insolutions containing electrolyte because of screening of the fieldby other ions in the double layer.

A faradaic process at the electrode-solution interface (Figure2b) occurs when a chemical species undergoes an electrontransfer, gaining or losing electrons. This process leads to a net

ion flux, j, from diffusion of electroactive ions and counter ionmovement. If a one-electron process is involved, then theparamagnetic species concentration also changes, contributing to3CP and CP.10 In contrast to charging processes, faradaicprocesses can affect solution quite far from the electrode. Inthe diffusion layer, where the concentration gradients reside,thickness (hundreds of micrometers) evolves with time andcan be further influenced by the convection.

Early work with faradaic processes in electrochemistry in thepresence of magnetic fields has been reviewed.11-14 More recentfundamental studies on this topic include those by Leventis andcoworkers at millimeter-sized electrodes3,5,10,15 and by White andcoworkers at microelectrodes16-19 and investigations in electro-deposition.8,11,12 Figure 2 illustrates examples of redox-MHD(RMHD) (2c and 2d) and gradient forces (2e) that lead to controland containment of flow through changes in solution composition,placement and dimensions of electrodes, and orientation relativeto the magnetic field. Note that density, viscosity, and temperaturegradients also have an effect on fluid motion (e.g., upper image,Figure 2e).

Figure 2. Electrochemistry can change concentrations of ions and paramagnetic species and their gradients in a redox-containing solution,thereby manipulating fluid motion in the presence of a magnetic field. (a) Double layer formation that results in charging current consists of theinner Helmholtz plane (IHP), outer Helmholtz plane (OHP), and diffuse layer. (b) Faradaic current involves electron transfer due to redox processes.Counter ion movement can also be involved. Diffusion of the transformed species can affect solution composition to several hundreds ofmicrometers from the electrode as shown in (a); D is the diffusion coefficient and t is the time. (c) RMHD with FB around a single microbandelectrode held at an oxidizing potential;21 flow is tracked with 10-µm beads. Ion gradients parallel to the electrode from radial diffusion form anonzero cross product with B to produce circular flow. (d) RMHD-generated flow tubes resulting from circular flow at microdisk electrodes forthe same reasons as those in (c).19 (e) Gradient forces confine purple cation radical generated at a NdFeB disk electrode held at oxidizingpotentials (lower image), offsetting natural density gradients that cause convection in the absence of a magnetic field (upper image, Au diskelectrode).10 Component of j perpendicular to B is not sufficient to produce MHD flow at these millimeter-sized electrodes.

3413Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

To determine flow velocities and profiles, colored chemicalspecies and beads (Figure 2c-e) can be tracked. Measuringchanges in faradaic current is more convenient and has been usedextensively to monitor changes of magnetoconvection.5,10,13,14,16-20

However, this approach is insufficient for very small electrodes17

and in small, confined volumes21 in which convection might notbe able to perturb the diffusion layer enough at the electrode tocause quantifiable changes in current.

APPLICATIONS TO ANALYTICAL CHEMISTRYNon-redox MHD Pumping in Channels. Several reports on

the development of direct current (dc) MHD pumps for aqueoussolutions have appeared in the literature. The first report of dcMHD pumping of saline solution appeared in 1832.22 One of thefirst micropumps, however, was developed by Jang and Lee in2000.23 They demonstrated pumping of a saline solution in siliconmicrochannels using Al electrodes with a NdFeB permanentmagnet. Bau and coworkers have also reported on dc MHD inmicrochannels using low temperature co-fired ceramic (LTCC)as a substrate with electrodes patterned along the channelwalls.24,25 These studies demonstrated pumping of mercury slugs,water, and saline solution. With devices using side-wall verticalelectrodes embedded in SU-8 microchannels, Lee and coworkers26

used the bidirectional pumping capability of dc MHD to pumpand sort biological cells in phosphate buffer solution. However,in all of these reports of dc MHD micropumps, bubble generationdue to electrolysis of water caused interferences in flow and, insome cases, electrode dissolution occurred. In order to alleviatesome of the problems caused by dc micropumps, devices whichisolate the pumping channel from bubbles generated by theelectrolysis have been developed, thereby reducing interferencesto the flow.27,28

Using alternating current (ac) (achieved by applying currentor potential) can maximize the contribution from charging of thedouble layer and limit faradaic contributions such as electrolysis.The ac approach is also better suited to the smaller range ofchannel dimensions for microfluidics (tens of micrometers com-pared to hundreds of micrometers for dc systems). However, thecurrent must be synchronous with a corresponding alternatingmagnetic field, making the devices more complex than dcsystems.

There have been several reports of ac MHD micropumps. In2000, Lemoff and Lee demonstrated a micropump that produceda continuous flow of several electrolyte solutions using anelectromagnet.29 Later, in 2003, they reported an ac MHDmicrofluidic switch, which combined two ac MHD pumps toswitch flow from one channel to another.30 In these systems, thepeak-to-peak voltage is still large enough to induce faradaiccurrent, and bubble formation remains a problem. Higher fre-quencies should resolve this issue because a larger fraction ofthe potential drops across the double layer, but inductive heatingis a drawback.31

Nuclear Magnetic Resonance. There is interest in usingMHD as a pump for integrated microfluidic/NMR systems. Inone study,32 a dc MHD pump generated fluid velocities of 2.8mm/s in a 7 T superconducting NMR magnet. Bubble generationwas a problem only when the system was used for >30 minutes,and Joule heating became an issue only when the applied voltagewas >∼20 V.

Separations. Eijkel and coworkers proposed a circular acMHD micropump for chromatographic applications.31 The device(Figure 3a) consisted of a 30-µm-high, 200-µm-wide circularchannel with gold walls. Five ferrite electromagnets generated a

Figure 3. AC MHD micropumps for analytical applications. (a) Circular channel structure for chromatographic applications. (b) Device insertedinto the gap of semicircle arrangement of electromagnets with ferrite cores provides a magnetic field perpendicular to the chip. (a and b reprintedfrom ref. 31 with permission from Elsevier.) (c) Microreactor for continuous flow chemistry and PCR amplification. (d) A particle moves at 342µm/s in 0.500 M KCl. (c and d from ref. 33; reproduced by permission of The Royal Society of Chemistry.)

3414 Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

B-field of 0.1 T (Figure 3b). The maximum flow velocity was 40µm/s, as measured by tracking fluorescent beads in 1 M KNO3.This theoretically corresponds to a chromatographic efficiencyof 0.2 plates/s, which is much lower than conventionalseparation methods. Again, bubble formation and inductiveheating were problems.

Polymerase Chain Reaction. AC MHD actuation has alsobeen used for DNA amplification via PCR.33 MHD devices usedin this work consisted of a ring-shaped channel with copper/platinum electrodes on the walls (Figure 3c). Video microscopyof 6-µm particles monitored fluid flow (e.g., 342 µm/s, Figure 3d).MHD-induced convection was observed for solutions containingKCl and PCR reagents. Electrolysis, however, became a severeproblem at the elevated temperatures required for PCR becausebubbles blocked channels and byproducts were formed that couldinterfere with the PCR chemistry.

MHD Mixing. Traditionally, magnetic stir plates and mechan-ical shakers have been used for large-scale solution mixing.Stirring in microdevices, however, is not trivial because lowReynolds numbers (Re , 1) prevent turbulent flow, which isbeneficial for mixing.34 Mixing is critical for carrying out chemicalreactions (e.g., synthesis, DNA sequencing, and immunoassays).To date, only a few reports in the literature describe microscalemixing using MHD.35,36

Qian and Bau developed a MHD device for stirring andpumping (Figure 4).36 A Y-shaped microchannel fabricated fromPDMS with copper electrodes installed along opposite walls ofthe channel was placed on a NdFeB permanent magnet. Apotential was applied in different time and space variationsbetween electrodes to achieve mixing. Initially (t ) 0), well-separated red and green dyes were introduced into the conduit.A potential difference of ∆V ) 2.5 V was alternately applied at 4second intervals between electrode pairs, blending the red and

green dyes. A solution containing a redox species (0.5 M CuSO4)helped avoid bubble formation.

RMHD for Pumping and Trace Metal Analysis. Addingredox species alleviates the problems of electrode degradationand water electrolysis because the resulting faradaic processesgenerate high currents while maintaining low applied voltages.RMHD has been demonstrated for pumping in LTCC channelswith gold electrodes on opposing sidewalls.20,37 The redox speciesnitrobenzene (NB, 0.5 M) in an electrolyte in acetonitrile was usedto generate a faradaic current that produced a velocity of 5.0 mm/sin the presence of a NdFeB permanent magnet.20 This exampledemonstrates the use of RMHD for microfluidics with nonaqueoussolvents. Figure 5a shows how simply switching the polarity ofthe electrodes reverses flow direction. Also, in RMHD, flowvelocity is easily controlled with applied potential and a changein redox species concentration (Figure 5b). An equimolar mixtureof oxidized and reduced forms of a redox couple (e.g., Fe(CN)6

3-

and Fe(CN)64-) offers greater protection than a single form

because both the anode and the cathode have electrochemicalalternatives to electrode dissolution and electrolysis.20,37

These RMHD micropumps apply potential (or current) togenerate ion gradients, which cause ion flux (j) in a thermody-namically uphill (electrolytic) process. An alternative is to choosechemical species that spontaneously react (in a galvanic process)to avoid the need for an outside energy source and its associatedinstrumentation. Leventis and Gao3 reported such a self-propelledRMHD micropump to move fluids in channels.

RMHD has also been investigated to increase sensitivity andachieve lower detection limits in anodic stripping voltammetry(ASV) for trace metal analysis.38,39 High concentrations of “pump-ing” redox species (Fe3+) are added to the solution and a lowreducing voltage is applied. This generates FB (where reductionof Fe3+ to Fe2+ dominates j) to accelerate co-deposition of a

Figure 4. Demonstration of mixing via MHD-induced convection. (Reprinted from ref. 36 with permission from Elsevier.) (a) Schematicrepresentation of a MHD stirrer. (b) Cu electrodes each with length LE and gap c between adjacent ones are placed along opposite walls(designated as Ci

+ and Ci-) in a stirrer’s channel-like cavity. (c) Mixing process of red and green dyes in a stirrer over time. T corresponds to

a 4-s period during which 2.5 V was applied between electrode pairs, alternating between C0- and C1

+ and C1-and C0

+. The underlying NdFeBmagnet provided the magnetic field (0.4 T).

3415Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

Hg film with analytes Cd, Pb, and Cu (at ppb levels) onto anelectrode. The enhanced deposition step was followed by ASVto quantify the deposited analytes. Disposable electrodes(screen-printed carbon [SPC] on a LTCC substrate) were usedwith small volumes (150 µL) and permanent magnets with theultimate objective of making portable devices (Figure 5c). ASVpeak areas for analytes increased by 75% in the presence of amagnetic field when compared to those in the absence of amagnetic field (Figure 5d).

More recently, a method was developed for investigatingtrajectories and velocities of RMHD fluid flow at microelectrodearrays in confined volumes using microbeads (Figure 2c).21 Afairly flat flow profile between pumping electrodes without theneed of nearby channel walls indicates possible use for separationsand reconceptualization of wall placement in microfluidic devices.

A drawback of RMHD is the need for high concentrations ofredox species to provide a large enough ion flux that a sufficientmagnitude of FB can be generated in the low magnetic fieldsof permanent magnets. High concentrations could cause

interferences in analysis and detection methods. By trackingmicrobeads, velocities have been measured recently for con-centrations in the low tens of millimolar range.21 To furtherdecrease the necessary concentration of redox species, newelectrode patterns that take advantage of reinforcing flowsandtherefore provide higher fluid velocitiessare being evaluated.Creating new channel designs that separate pumping solutioncontaining redox species from sample/analyte solution isanother approach that addresses this challenge.

LESSONS LEARNED OUTSIDE OF ANALYTICALCHEMISTRY

Electrodeposition and Electrodissolution. There are prob-ably more theoretical and experimental studies of magnetocon-vection in solution in the field of electrodeposition than reportedin any other field. This body of work largely concerns varyingthe properties of deposited pure metals and alloyssboth ferro-magnetic and non-ferromagneticsby controlling convection atelectrode surfaces during reduction in magnetic fields.8,11,12 These

Figure 5. RMHD magnetoconvection for analytical applications. (a) LTCC device with screen-printed gold electrodes on opposing sidewalls.The device, showing magnet placement, controls fluid flow direction by switching electrode polarity: (i) no magnet, orange NB•- emerges fromboth reservoirs, (ii) magnetic field (0.55 T) at 90° induces flow to left, and (iii) magnetic field (0.55 T) at 270° induces flow to right. (b) Dependenceof fluid speed on applied potential for 0.1 M NB (closed circles) and 0.25 M NB (open triangles) in the presence of a NdFeB magnet (0.41 T).(a and b modified and reprinted with permission from ref. 20 Copyright 2006, reproduced by permission by ESC—The Electrochemical Society.)(c) Setup for RMHD-enhancement of ASV. (d) ASV responses for a 150-µL sample of 2-µM Cu2+, Pb2+, and Cd2+. (c and d from ref. 39sreproducedby permission of The Royal Society of Chemistry.)

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MHD systems naturally involve redox species and faradaicchemistry. Contribution to j by reduction of H+ and the changein pH that accompanies electrodeposition has been consid-ered.40 The influence on convection of concentration gradientsof paramagnetic species in uniform fields and magnetic fieldgradients caused by deposits of ferromagnetic materials in auniform external magnetic field have been discussed.8,12 Thelessons learned from these diverse ion-gradient and generation/depletion studies could be transferred to magnetoconvection-basedµTAS.

Coey and coworkers8 give an interesting comparison of therelative magnitude of mass transport forces for a set of copperelectrodeposition conditions at a 1 T field and gradient of 1 T/m.For this case, diffusion and migration forces are the strongestand paramagnetic gradient and natural convection forces are sevenorders of magnitude lower. The roles that these different forcesplay on a microfluidic scale will change depending on theconditions.

In addition, the electrodeposition literature contains significantcontributions toward modeling and simulations of magneto-convection.41,42 However, system dimensions are millimeters orlarger rather than the micrometer scale suitable for µTAS, inwhich laminar flow dominates.34 Particle image velocimetry(PIV)42 has shown complex flows in a closed cylindrical elec-trodeposition cell. Microconvection has been demonstrated insidethe diffusion layer and attributed to gradient forces.43,44 The rolethat complex microconvective flows would play in smaller confinedµTAS devices has yet to be determined.

The process known as “electrodissolution”, “anodic dissolu-tion”, or “corrosion” has also been addressed.12,42,45 The “disad-vantage” of electrode corrosion claimed by some investigators ofMHD microfluidics is predicted by and explained through thisliterature. A better understanding of the process could lead towardintentional sacrificing or recycling of electrodes in futureapplications.

Metallurgy. The history of MHD in metallurgy is longstand-ing, with origins over a century ago, but the past three decadeshave brought the greatest attention to metallurgical MHD and amaturing of its understanding and application.46 However, dis-coveries made in metallurgical MHD are not always directlyapplicable to analytical chemistry systems because the typicalconditions are quite different (higher electrical conductivities,boundary layers between solid and liquid metal, and high tem-peratures). Thus, care should be taken when translating lessonslearned from this maturing discipline.

Ferrofluids. Applications of ferrofluids in microfluidics arelimited. A “magnetocaloric pump” that has no moving mechanicalparts was recently reported.47 A ferrofluid within a channel isattracted into a region containing a magnetic field where subse-quent heating weakens this attraction and cooler ferrofluiddisplaces the warm one. This process can then push other non-ferrofluids through the microsystem. A challenge has been to findsuitable channel materials that are easy to clean and compatiblewith both oil- and water-based ferrofluids.

WHAT’S ON THE HORIZON?The use of magnetoconvection for manipulating fluid flow in smallsystems such as µTAS is still in its infancy. Magnetoconvectioncould offer unique control of fluid motion that is currently not

possible with existing microfluidic pumping methods. Stirring,pumping in a loop, and changing flow direction are possible withMHD. Reversing the polarity of the electrodes can switch the levelof containment, release, or motion of paramagnetic species whengradient forces are involved, which has no equivalent in othermicrofluidic approaches. Because the focus so far has been onusing FB for microfluidics, we will likely see growing investiga-tions on the horizon in using the gradient forces F3C and F3B

for µTAS applications as well.Channel dimensions such as those associated with electroki-

netic pumping are achievable with ac MHD, but more attentionneeds to be focused on the relative contributions of charging andfaradaic current. Future work must focus on simplifying thefabrication and implementation of ac electromagnets. Stirringappears to be the most practical application for ac MHD at thistime because it requires only permanent magnets.

Large dimensions of hundreds of micrometers are needed toachieve practical microfluidic pumping in a channel with dc MHDand permanent magnets with low B fields (for simplicity andportability), which fill a gap between narrow channels of electro-kinetic pumping and wider ones of mechanical methods. Linearvelocities (tens of micrometers to millimeters per second) for dcMHD are on the low end of those for electrokinetic pumping(millimeters to centimeters per second). But because channelscan be larger, the volume flow rate can be much higher withMHD. In fact, magnetoconvection in general opens a newdimension of µTAS devices because channel walls are not requiredto direct flow patterns.

With a better understanding of the electrochemistry at elec-trodes and in solution, problems that were previously cited in theliterature are likely to be overcome. For example, there is alreadyevidence that the addition of redox species to the solution lowersthe voltages applied and avoids bubble formation and electrodecorrosion, thus suggesting that RMHD will invigorate microfluidicapplications in analytical chemistry in both analysis and separa-tions. Efforts to commercialize RMHD as a unique microfluidictechnology in the form of pumping in a loop are underway.48

More fundamental studies of magnetoconvection in solutionare needed to achieve a better understanding of the influence ofthe forces on fluid flow and optimization of parameters for µTAS.Not only is a better account of the charging and faradaic processesdesirable, but better integration of the fluid dynamics and thephysics of magnetic fields is needed. Investigators must bridgetraditional boundaries of scientific and engineering disciplines toaccomplish this. Computer simulations that begin to apply thesedifferent phenomena to µTAS dimensions are in progress.49,50

Development of future µTAS might combine the most suc-cessful features of magnetoconvection, such as RMHD for pump-ing and enhancement of analysis signal with ac MHD for stirring,or unite the benefits of magnetoconvection with other moreestablished microfluidic technologies. Finally, we anticipate seeingactivities in upcoming years that tap into existing knowledge onmagnetoconvection in fields adjacent to and far from analyticalchemistry for small-scale, magnetoconvective analytical devices.

ACKNOWLEDGMENTOur work in this area is supported by grants from the National

Science Foundation (research grants CHE-0096780 and CHE-0719097 and a REU grant 0243978 that partially supported M. C.

3417Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

W) and the Arkansas Biosciences Institute. We acknowledge Drs.Emily C. Anderson and Prabhu U. Arumugam for their earlyRMHD studies and Dr. Christine Evans of SFC Fluidics, LLC foruseful discussions about commercial applications.

Melissa C. Weston is currently a graduate student in the Chemistry andBiochemistry program at the University of Arkansas Fayetteville (UAF).Her research interests involve analytical chemistry applications of RMHDfluidics. Matthew D. Gerner recently received a M.S. degree from theMicroelectronics/Photonics graduate program at UAF. His researchfocuses on fundamental relationships among fluid flow, electrochemistry,and magnetic fields. Ingrid Fritsch is a professor at UAF in theDepartment of Chemistry and Biochemistry. Her research interests arein the development of multifunctional, miniaturized analytical devicesand sensors with integrated components on a single substrate, includingprotein and DNA-hybridization microarrays interfaced to electrochemicaldetection, novel microelectrochemical strategies for detection of smallmolecules, and microfluidics. Address correspondence to Ingrid Fritsch,Department of Chemistry and Biochemistry, University of Arkansas,Fayetteville, AR 72701 (ifritsch@uark.edu).

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