Magnetic Fields for Fluid Motion - ACS Publications - American

Apr 9, 2010 - (39) Weston, M. C.; Anderson, E. C.; Arumugam, P. U.; Yoga Narasimhan, P.;. Fritsch, I. Analyst 2006, 131, 1322–1331. (40) Koza, J. A...
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Anal. Chem. 2010, 82, 3411–3418

Magnetic Fields for Fluid Motion Melissa C. Weston, Matthew D. Gerner, and Ingrid Fritsch University of Arkansas Fayetteville

Miniaturization drives much of the research and development in analytical chemistry. Portability, smaller volume requirements, less waste and material, and reduced power consumption are advantages of downsizing chemical analyses. Smaller scales can offer improvements in sensitivity and new approaches in solving analytical chemistry problems. For example, micro total analysis systems (µTAS) aim to combine multiple laboratory procedures in single hand-held devices. Such systems could revolutionize chemical analyses in genomics, environmental monitoring, medical diagnostics, 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 flow conditions inhibit mixing and stirring, and a pump must match the system’s parameters and flow requirements (stopping, starting, speed, and direction). Traditional approaches in microfluidics attempt to address these needs, but the gaps in capabilities drive further development of existing methods and introduction of new ones. The intention of this article is to expand awareness in the analytical chemistry community about non-mechanical, fluidpumping phenomena involving magnetic fields, collectively known as magnetoconvection, with an emphasis on magnetohydrodynamics (MHD) in solutions and their application to µTAS. The reader is referred to Pamme1 for a broader description of growing uses of magnetism in small chemical systems for pumping, trapping and transporting, mixing, detection, patterning, sorting, and separation. A recent review by Qian and Bau2 discusses publications specific to MHD in microfluidics. In contrast, this 10.1021/ac901783n  2010 American Chemical Society Published on Web 04/09/2010

ROBERT GATES

Three forces induced by magnetic fields offer unique control of fluid motion and new opportunities in microfluidics. This article describes magnetoconvective phenomena in terms of the theory and controversy, tuning by redox processes at electrodes, early-stage applications in analytical chemistry, mature applications in disciplines far afield, and future directions for micro total analysis systems. (To listen to a podcast about this article, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.)

Feature is more fundamental, elaborating on redox processes that contribute not only toward MHD, but also toward magnetoconvective phenomena that require the presence of paramagnetic species. This article also presents theories of magnetoconvection and their controversies, reviews recent reports in analytical chemistry, brings together for inspiration magnetoconvection from disciplines far afield from chemistry, and offers an outlook on future directions.

FLUID FLOW FROM MAGNETIC FIELDS: HOW IT WORKS The three central forces of magnetoconvection are the magnetic force (FB, also known as the magnetohydrodynamic force), magnetic gradient force (F3Β),3 and paramagnetic concentration gradient force (F3C).4,5 Figure 1 illustrates the relationships between each force and different variables that can be used to design devices incorporating magnetoconvective microfluidics. Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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Figure 1. Illustration of body forces that cause magnetoconvection in solution. (a) The magnetohydrodynamic, or magnetic force (FB), acts on fluid containing ions with a flux (j) having a perpendicular component to a magnetic field (B). Here, negative ions (purple) are generated at the negative electrode and positive ions (green) are generated at the positive electrode by redox reactions. The expanded view in the lower half of the 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); an electric field component, FL,E (migration; q is particle charge and E is electric field), makes up the other part of FL. (b) The magnetic field gradient force (F3B) acts on fluid containing paramagnetic species with uniform concentration, CP. The magnitude of B is scaled by color for different field lines in the y-z plane for a NdFeB disk magnet (gray) of common dimensions; lines were generated with AMPERES V64 (Integrated Engineering 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 of magnetic field direction. Relative magnitudes of F3C vectors at different locations along the concentration gradient are drawn to scale for a uniform B field (not shown). (NA is Avogadro’s number, m* is related to magnetic susceptibility, and k is the Boltzmann constant. Vectors are in boldface.)

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). When a species (ion or electron) with charge, q (coulomb), moves with velocity, v (meter/second), at a right angle to a magnetic field, B (tesla), the force, FL,B, that is one of two terms in the Lorentz equation, acts on it in a direction sensitive to the sign of the charge and perpendicular to both v and B, with a cross product relationship, FL,B ) qv × B. Momentum transfer between this species and solvent causes the localized fluid to flow in the direction of the force. When considering all ions in a unit volume, their net movement can be described by the flux j (coulomb/[second square meter])sthe sum of qv over all species. Positive j is defined in the direction that positive ions move; ions of opposite sign moving in opposing directions have an additive effect on j. Thus, the body force obeys the simple right hand rule, FB ) j × B. Both F3B and F3C can be thought of as arising from a gradient in “magnetic energy”. F3B acts on paramagnetic species (ions and neutrals) in the direction of the magnetic field gradient.3 Figure 1b shows a scaled illustration for real experimental conditions near a permanent magnet, where the largest gradients occur at the edges. The reader is cautioned to pay attention to the vector calculus notation for the F3B equation 3412

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because multiple mathematically nonequivalent forms are present in the literature.6-8 F3C derives its magnitude and direction in a magnetic field (Figure 1c) from a gradient in the magnetic susceptibility such as a concentration gradient of a paramagnetic species (3CP). Whether F3C plays a measurable role compared to the other two forces is an issue under active debate. Leventis and Dass5 give an excellent review of the debate’s evolution, as well as evidence 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 good conductors such as liquid metals, in which the charge carriers are electrons, flow resulting from FB along a rectangular conduit is accomplished by passing a current crosswise to the conduit in the presence of a magnetic field oriented perpendicular to this electron flux j. In aqueous or nonaqueous solutions, the situation is quite different: the charge carriers are ions, and the conductivity is at least five orders of magnitude smaller than in liquid metals. Electrochemistry can be used to accomplish an ion flux, j (to induce FB) and/or create or deplete paramagnetic species to change CP and 3CP and effect F3B and F3C, respectively. Therefore, a brief primer on electrochemical processes is presented here before elaborating further

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 the inner 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 of micrometers from the electrode as shown in (a); D is the diffusion coefficient and t is the time. (c) RMHD with FB around a single microband electrode held at an oxidizing potential;21 flow is tracked with 10-µm beads. Ion gradients parallel to the electrode from radial diffusion form a nonzero cross product with B to produce circular flow. (d) RMHD-generated flow tubes resulting from circular flow at microdisk electrodes for the same reasons as those in (c).19 (e) Gradient forces confine purple cation radical generated at a NdFeB disk electrode held at oxidizing potentials (lower image), offsetting natural density gradients that cause convection in the absence of a magnetic field (upper image, Au disk electrode).10 Component of j perpendicular to B is not sufficient to produce MHD flow at these millimeter-sized electrodes.

on magnetoconvective applications. More thorough discussions on electrochemistry can be found in textbooks such as Bard and Faulkner.9 In an electrolyte system, controlling the potential or current between two (or more) electrodes can affect the chemical species in the solution, producing a current or potential, respectively, that can be monitored. The current involves two processes: charging (Figure 2a) and faradaic (Figure 2b). Charging at the electrodesolution interface occurs when ions migrate to accommodate an electrode potential that differs from that of the solution (the potential of zero charge). This leads to a double layer at the electrode-solution interface, often represented electrically as a capacitor. The distance away from the electrode across which the electric field can mobilize ions (diffuse layer, Figure 2a) can be very short (approximately three to hundreds of angstroms) in solutions containing electrolyte because of screening of the field by other ions in the double layer. A faradaic process at the electrode-solution interface (Figure 2b) occurs when a chemical species undergoes an electron transfer, gaining or losing electrons. This process leads to a net

ion flux, j, from diffusion of electroactive ions and counter ion movement. If a one-electron process is involved, then the paramagnetic species concentration also changes, contributing to 3CP and CP.10 In contrast to charging processes, faradaic processes can affect solution quite far from the electrode. In the diffusion layer, where the concentration gradients reside, thickness (hundreds of micrometers) evolves with time and can be further influenced by the convection. Early work with faradaic processes in electrochemistry in the presence of magnetic fields has been reviewed.11-14 More recent fundamental studies on this topic include those by Leventis and coworkers at millimeter-sized electrodes3,5,10,15 and by White and coworkers at microelectrodes16-19 and investigations in electrodeposition.8,11,12 Figure 2 illustrates examples of redox-MHD (RMHD) (2c and 2d) and gradient forces (2e) that lead to control and containment of flow through changes in solution composition, placement and dimensions of electrodes, and orientation relative to the magnetic field. Note that density, viscosity, and temperature gradients also have an effect on fluid motion (e.g., upper image, Figure 2e). Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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

To determine flow velocities and profiles, colored chemical species and beads (Figure 2c-e) can be tracked. Measuring changes in faradaic current is more convenient and has been used extensively 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 not be able to perturb the diffusion layer enough at the electrode to cause quantifiable changes in current. APPLICATIONS TO ANALYTICAL CHEMISTRY Non-redox MHD Pumping in Channels. Several reports on the development of direct current (dc) MHD pumps for aqueous solutions have appeared in the literature. The first report of dc MHD pumping of saline solution appeared in 1832.22 One of the first micropumps, however, was developed by Jang and Lee in 2000.23 They demonstrated pumping of a saline solution in silicon microchannels using Al electrodes with a NdFeB permanent magnet. Bau and coworkers have also reported on dc MHD in microchannels using low temperature co-fired ceramic (LTCC) as a substrate with electrodes patterned along the channel walls.24,25 These studies demonstrated pumping of mercury slugs, water, and saline solution. With devices using side-wall vertical electrodes embedded in SU-8 microchannels, Lee and coworkers26 used the bidirectional pumping capability of dc MHD to pump and sort biological cells in phosphate buffer solution. However, in all of these reports of dc MHD micropumps, bubble generation due to electrolysis of water caused interferences in flow and, in some cases, electrode dissolution occurred. In order to alleviate some of the problems caused by dc micropumps, devices which isolate the pumping channel from bubbles generated by the electrolysis have been developed, thereby reducing interferences to the flow.27,28 3414

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Using alternating current (ac) (achieved by applying current or potential) can maximize the contribution from charging of the double layer and limit faradaic contributions such as electrolysis. The ac approach is also better suited to the smaller range of channel dimensions for microfluidics (tens of micrometers compared to hundreds of micrometers for dc systems). However, the current must be synchronous with a corresponding alternating magnetic field, making the devices more complex than dc systems. There have been several reports of ac MHD micropumps. In 2000, Lemoff and Lee demonstrated a micropump that produced a continuous flow of several electrolyte solutions using an electromagnet.29 Later, in 2003, they reported an ac MHD microfluidic switch, which combined two ac MHD pumps to switch flow from one channel to another.30 In these systems, the peak-to-peak voltage is still large enough to induce faradaic current, and bubble formation remains a problem. Higher frequencies should resolve this issue because a larger fraction of the potential drops across the double layer, but inductive heating is a drawback.31 Nuclear Magnetic Resonance. There is interest in using MHD as a pump for integrated microfluidic/NMR systems. In one study,32 a dc MHD pump generated fluid velocities of 2.8 mm/s in a 7 T superconducting NMR magnet. Bubble generation was a problem only when the system was used for >30 minutes, and Joule heating became an issue only when the applied voltage was >∼20 V. Separations. Eijkel and coworkers proposed a circular ac MHD micropump for chromatographic applications.31 The device (Figure 3a) consisted of a 30-µm-high, 200-µm-wide circular channel with gold walls. Five ferrite electromagnets generated a

Figure 4. Demonstration of mixing via MHD-induced convection. (Reprinted from ref. 36 with permission from Elsevier.) (a) Schematic representation 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 NdFeB magnet provided the magnetic field (0.4 T).

B-field of 0.1 T (Figure 3b). The maximum flow velocity was 40 µm/s, as measured by tracking fluorescent beads in 1 M KNO3. This theoretically corresponds to a chromatographic efficiency of 0.2 plates/s, which is much lower than conventional separation methods. Again, bubble formation and inductive heating were problems. Polymerase Chain Reaction. AC MHD actuation has also been used for DNA amplification via PCR.33 MHD devices used in this work consisted of a ring-shaped channel with copper/ platinum electrodes on the walls (Figure 3c). Video microscopy of 6-µm particles monitored fluid flow (e.g., 342 µm/s, Figure 3d). MHD-induced convection was observed for solutions containing KCl and PCR reagents. Electrolysis, however, became a severe problem at the elevated temperatures required for PCR because bubbles blocked channels and byproducts were formed that could interfere with the PCR chemistry. MHD Mixing. Traditionally, magnetic stir plates and mechanical shakers have been used for large-scale solution mixing. Stirring in microdevices, however, is not trivial because low Reynolds numbers (Re , 1) prevent turbulent flow, which is beneficial for mixing.34 Mixing is critical for carrying out chemical reactions (e.g., synthesis, DNA sequencing, and immunoassays). To date, only a few reports in the literature describe microscale mixing using MHD.35,36 Qian and Bau developed a MHD device for stirring and pumping (Figure 4).36 A Y-shaped microchannel fabricated from PDMS with copper electrodes installed along opposite walls of the channel was placed on a NdFeB permanent magnet. A potential was applied in different time and space variations between electrodes to achieve mixing. Initially (t ) 0), wellseparated red and green dyes were introduced into the conduit. A potential difference of ∆V ) 2.5 V was alternately applied at 4 second 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. Adding redox species alleviates the problems of electrode degradation and water electrolysis because the resulting faradaic processes generate high currents while maintaining low applied voltages. RMHD has been demonstrated for pumping in LTCC channels with gold electrodes on opposing sidewalls.20,37 The redox species nitrobenzene (NB, 0.5 M) in an electrolyte in acetonitrile was used to generate a faradaic current that produced a velocity of 5.0 mm/s in the presence of a NdFeB permanent magnet.20 This example demonstrates the use of RMHD for microfluidics with nonaqueous solvents. Figure 5a shows how simply switching the polarity of the electrodes reverses flow direction. Also, in RMHD, flow velocity is easily controlled with applied potential and a change in redox species concentration (Figure 5b). An equimolar mixture of oxidized and reduced forms of a redox couple (e.g., Fe(CN)63and Fe(CN)64-) offers greater protection than a single form because both the anode and the cathode have electrochemical alternatives to electrode dissolution and electrolysis.20,37 These RMHD micropumps apply potential (or current) to generate ion gradients, which cause ion flux (j) in a thermodynamically uphill (electrolytic) process. An alternative is to choose chemical species that spontaneously react (in a galvanic process) to avoid the need for an outside energy source and its associated instrumentation. Leventis and Gao3 reported such a self-propelled RMHD micropump to move fluids in channels. RMHD has also been investigated to increase sensitivity and achieve lower detection limits in anodic stripping voltammetry (ASV) for trace metal analysis.38,39 High concentrations of “pumping” redox species (Fe3+) are added to the solution and a low reducing voltage is applied. This generates FB (where reduction of Fe3+ to Fe2+ dominates j) to accelerate co-deposition of a Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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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 from both 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) Dependence of 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. 39sreproduced by permission of The Royal Society of Chemistry.)

Hg film with analytes Cd, Pb, and Cu (at ppb levels) onto an electrode. The enhanced deposition step was followed by ASV to quantify the deposited analytes. Disposable electrodes (screen-printed carbon [SPC] on a LTCC substrate) were used with small volumes (150 µL) and permanent magnets with the ultimate objective of making portable devices (Figure 5c). ASV peak areas for analytes increased by 75% in the presence of a magnetic field when compared to those in the absence of a magnetic field (Figure 5d). More recently, a method was developed for investigating trajectories and velocities of RMHD fluid flow at microelectrode arrays in confined volumes using microbeads (Figure 2c).21 A fairly flat flow profile between pumping electrodes without the need of nearby channel walls indicates possible use for separations and reconceptualization of wall placement in microfluidic devices. A drawback of RMHD is the need for high concentrations of redox species to provide a large enough ion flux that a sufficient magnitude of FB can be generated in the low magnetic fields of permanent magnets. High concentrations could cause 3416

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interferences in analysis and detection methods. By tracking microbeads, velocities have been measured recently for concentrations in the low tens of millimolar range.21 To further decrease the necessary concentration of redox species, new electrode patterns that take advantage of reinforcing flowsand therefore provide higher fluid velocitiessare being evaluated. Creating new channel designs that separate pumping solution containing redox species from sample/analyte solution is another approach that addresses this challenge. LESSONS LEARNED OUTSIDE OF ANALYTICAL CHEMISTRY Electrodeposition and Electrodissolution. There are probably more theoretical and experimental studies of magnetoconvection in solution in the field of electrodeposition than reported in any other field. This body of work largely concerns varying the properties of deposited pure metals and alloyssboth ferromagnetic and non-ferromagneticsby controlling convection at electrode surfaces during reduction in magnetic fields.8,11,12 These

MHD systems naturally involve redox species and faradaic chemistry. Contribution to j by reduction of H+ and the change in pH that accompanies electrodeposition has been considered.40 The influence on convection of concentration gradients of paramagnetic species in uniform fields and magnetic field gradients caused by deposits of ferromagnetic materials in a uniform external magnetic field have been discussed.8,12 The lessons learned from these diverse ion-gradient and generation/ depletion studies could be transferred to magnetoconvection-based µTAS. Coey and coworkers8 give an interesting comparison of the relative magnitude of mass transport forces for a set of copper electrodeposition conditions at a 1 T field and gradient of 1 T/m. For this case, diffusion and migration forces are the strongest and paramagnetic gradient and natural convection forces are seven orders of magnitude lower. The roles that these different forces play on a microfluidic scale will change depending on the conditions. In addition, the electrodeposition literature contains significant contributions toward modeling and simulations of magnetoconvection.41,42 However, system dimensions are millimeters or larger rather than the micrometer scale suitable for µTAS, in which laminar flow dominates.34 Particle image velocimetry (PIV)42 has shown complex flows in a closed cylindrical electrodeposition cell. Microconvection has been demonstrated inside the diffusion layer and attributed to gradient forces.43,44 The role that complex microconvective flows would play in smaller confined µTAS devices has yet to be determined. The process known as “electrodissolution”, “anodic dissolution”, or “corrosion” has also been addressed.12,42,45 The “disadvantage” of electrode corrosion claimed by some investigators of MHD microfluidics is predicted by and explained through this literature. A better understanding of the process could lead toward intentional sacrificing or recycling of electrodes in future applications. Metallurgy. The history of MHD in metallurgy is longstanding, with origins over a century ago, but the past three decades have brought the greatest attention to metallurgical MHD and a maturing of its understanding and application.46 However, discoveries made in metallurgical MHD are not always directly applicable to analytical chemistry systems because the typical conditions are quite different (higher electrical conductivities, boundary layers between solid and liquid metal, and high temperatures). Thus, care should be taken when translating lessons learned from this maturing discipline. Ferrofluids. Applications of ferrofluids in microfluidics are limited. A “magnetocaloric pump” that has no moving mechanical parts was recently reported.47 A ferrofluid within a channel is attracted into a region containing a magnetic field where subsequent heating weakens this attraction and cooler ferrofluid displaces the warm one. This process can then push other nonferrofluids through the microsystem. A challenge has been to find suitable channel materials that are easy to clean and compatible with both oil- and water-based ferrofluids.

possible with existing microfluidic pumping methods. Stirring, pumping in a loop, and changing flow direction are possible with MHD. Reversing the polarity of the electrodes can switch the level of containment, release, or motion of paramagnetic species when gradient forces are involved, which has no equivalent in other microfluidic approaches. Because the focus so far has been on using FB for microfluidics, we will likely see growing investigations on the horizon in using the gradient forces F3C and F3B for µTAS applications as well. Channel dimensions such as those associated with electrokinetic pumping are achievable with ac MHD, but more attention needs to be focused on the relative contributions of charging and faradaic current. Future work must focus on simplifying the fabrication and implementation of ac electromagnets. Stirring appears to be the most practical application for ac MHD at this time because it requires only permanent magnets. Large dimensions of hundreds of micrometers are needed to achieve practical microfluidic pumping in a channel with dc MHD and permanent magnets with low B fields (for simplicity and portability), which fill a gap between narrow channels of electrokinetic pumping and wider ones of mechanical methods. Linear velocities (tens of micrometers to millimeters per second) for dc MHD are on the low end of those for electrokinetic pumping (millimeters to centimeters per second). But because channels can be larger, the volume flow rate can be much higher with MHD. In fact, magnetoconvection in general opens a new dimension of µTAS devices because channel walls are not required to direct flow patterns. With a better understanding of the electrochemistry at electrodes and in solution, problems that were previously cited in the literature are likely to be overcome. For example, there is already evidence that the addition of redox species to the solution lowers the voltages applied and avoids bubble formation and electrode corrosion, thus suggesting that RMHD will invigorate microfluidic applications in analytical chemistry in both analysis and separations. Efforts to commercialize RMHD as a unique microfluidic technology in the form of pumping in a loop are underway.48 More fundamental studies of magnetoconvection in solution are needed to achieve a better understanding of the influence of the forces on fluid flow and optimization of parameters for µTAS. Not only is a better account of the charging and faradaic processes desirable, but better integration of the fluid dynamics and the physics of magnetic fields is needed. Investigators must bridge traditional boundaries of scientific and engineering disciplines to accomplish this. Computer simulations that begin to apply these different phenomena to µTAS dimensions are in progress.49,50 Development of future µTAS might combine the most successful features of magnetoconvection, such as RMHD for pumping and enhancement of analysis signal with ac MHD for stirring, or unite the benefits of magnetoconvection with other more established microfluidic technologies. Finally, we anticipate seeing activities in upcoming years that tap into existing knowledge on magnetoconvection in fields adjacent to and far from analytical chemistry for small-scale, magnetoconvective analytical devices.

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

ACKNOWLEDGMENT Our work in this area is supported by grants from the National Science Foundation (research grants CHE-0096780 and CHE0719097 and a REU grant 0243978 that partially supported M. C. Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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W) and the Arkansas Biosciences Institute. We acknowledge Drs. Emily C. Anderson and Prabhu U. Arumugam for their early RMHD studies and Dr. Christine Evans of SFC Fluidics, LLC for useful discussions about commercial applications. Melissa C. Weston is currently a graduate student in the Chemistry and Biochemistry program at the University of Arkansas Fayetteville (UAF). Her research interests involve analytical chemistry applications of RMHD fluidics. Matthew D. Gerner recently received a M.S. degree from the Microelectronics/Photonics graduate program at UAF. His research focuses on fundamental relationships among fluid flow, electrochemistry, and magnetic fields. Ingrid Fritsch is a professor at UAF in the Department of Chemistry and Biochemistry. Her research interests are in the development of multifunctional, miniaturized analytical devices and sensors with integrated components on a single substrate, including protein and DNA-hybridization microarrays interfaced to electrochemical detection, novel microelectrochemical strategies for detection of small molecules, and microfluidics. Address correspondence to Ingrid Fritsch, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701 ([email protected]).

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