Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N37: Focus Session: Microfluidic Physics II: Electrokinetics |
Hide Abstracts |
Sponsoring Units: DFD Chair: Armand Ajdari, PCT-ESPCI-CNRS Room: LACC 512 |
Wednesday, March 23, 2005 8:00AM - 8:36AM |
N37.00001: Electrokinetic Microfluidic Systems Invited Speaker: Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microchip systems with no moving parts. Electrokinetics involves the interaction of solid surfaces, ionic solutions, and electric fields. Electric fields can be used to generate bulk fluid motion (electroosmosis) and to separate charged species (electrophoresis). Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microsystems with no moving parts. This seminar reviews progress at Stanford including methods for sample stacking in capillary electrophoresis assays and fundamental studies of electrokinetic flow instabilities. Field amplified sample stacking (FASS) leverages conductivity gradients as a robust method of increasing sample concentration prior to electrophoretic separation. A major challenge to achieving robust, high-efficiency FASS is the role of electrokinetic instabilities (EKI) generated by a coupling of electric fields and ionic conductivity gradients. This coupling results in electric body forces in the bulk liquid that can generate instabilities. Suppression and/or control of electrokinetic flow instabilities is critical as they dramatically increase dispersion rates and thereby limit stacking efficiency. We have identified the key physical mechanisms in EKI; developed generalized models for electrokinetic systems; and validated the models with experiments. We have applied this understanding to the development of chip systems that achieve signal increases of more than 20,000 fold using FASS. This stacking ratio is over 200 times larger than previous on-chip FASS devices. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N37.00002: AC electric field driven microfluidic control and mixing Nicolas Green, Hywel Morgan, Antonio Ramos, Antonio Gonzalez, Antonio Castellanos The AC electrokinetic movement of fluids has its origin in two different physical mechanisms: AC electroosmosis, the interaction of the Electrical Double Layer induced on microelectrodes by an applied potential and the generated electric field; and electrothermal Electro-hydrodynamics, the interaction of an electric field with gradients in polarisability of the fluid produced by non-uniform heating. Both mechanisms are dependent on a range of factors: applied voltage, signal frequency, fluid properties and the use of AC electric fields requires significantly less voltage ($\sim $10V) than DC electrokinetics, therefore presenting a range of different applications in microfluidic systems. This paper presents results of the use of AC electrokinetics for a range of applications in pumping, mixing and microfluidic control. Patterned microelectrode structures were used for rapid, switchable mixing of multiple fluid streams in microchannels, enhancing diffusive mixing. The mixing occurred over a short distance in the microchannel and could be switched on and off rapidly. Also presented is the use of AC electrokinetics for the local modification of streamlines and deflection of fluid streams in microchannels. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N37.00003: Microfluidic pumps based on AC electro-osmosis: non-lienear effects Armand Ajdari, Vincent Studer, Chen Yong, Anne Pepin We report on experiments demonstrating the possibility to pump electrolyte solutions in closed microchannel loops using asymmetric arrays of micro-electrodes addressed with AC voltages. Velocities of a few mm/s can be obtained with voltages of a few Volts in this integrable pumping scheme. The variation of pumping velocity with frequency, amplitude and ionic strength demonstrate the occurrence of complex mechanisms, far beyond the quasi-linear models present in the literature. In particular a reversal of the pumping direction at high frequencies is reported, which could be of practical use. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N37.00004: Travelling Wave Electro-Osmosis: Nonlinear Double Layer Analysis and Application To Pumping of Liquid Antonio Gonz\'alez, Antonio Ramos, Antonio Castellanos Steady motion of aqueous solutions can be produced through ac electro-osmosis, due to the coupling between ac fields and the induced charge at the double layer close to microelectrodes. Continuous unidirectional fluid motion can be obtained when the solution is placed on top of an array of microelectrodes subjected to a travelling wave potential. In this paper we consider a simple model, consisting of a single mode travelling wave, and its extension to an square wave signal. To describe the double layer we use the nonlinear Gouy-Chapman theory. A numerical solution is obtained for this model, and the results are compared with experiments. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N37.00005: Microfluidic experiments demonstrating induced-charge electro-osmosis Jeremy Levitan$^{1,3}$, Yuxing Ben$^{2,3}$, Todd Thorsen$^{1,3}$, Martin Bazant$^{2,3}$ Motivated by recent work on AC electro-osmosis, a general theory of ``induced-charge electro-osmosis'' (ICEO) has been developed, and a variety of ICEO-based pumping and mixing strategies for micro-fluidics have been proposed, using both DC and AC applied voltages. As in the electrophoresis of metal colloids (studied in the Russian literature), ICEO slip of a liquid electrolyte occurs at polarizable (metal or dielectric) surfaces in response to applied electric fields. Due to the nonlinear coupling of the applied field and its nonuniform and time-dependent induced surface charge, the ICEO slip velocity depends on the field amplitude squared, and thus it provides electrohydrodynamic rectification of AC forcing, especially in asymmetric geometries.Although many theoretical predictions have been made, here we provide clear experimental demonstrations of steady ICEO flows near metal structures in polymer microchannels. We investigate the effect of AC frequency, applied voltage, and geometry, and find reasonable agreement with theoretical predictions, allowing for Stern-layer capacitance. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N37.00006: Impact of double-layer charging dynamics on induced-charge electro-osmotic flows Kevin T. Chu, Yuxing Ben, Martin Z. Bazant Induced charge electro-osmotic (ICEO) flows depend crucially on a combination of diffuse layer charge build up and a non-trivial tangential electric field at the electrode surface. Moreover, since time-dependent fields are commonly used to drive ICEO flows, charging dynamics play a critical role in determining the magnitude and direction of the resulting fluid flow. Unfortunately, at strong applied fields, the traditional model of the electrochemical cell as a linear RC circuit breaks down and the impact of bulk diffusion on the charging cannot be ignored. To gain a deeper understanding of the dynamics of diffuse layer charging, we consider the following simple problem: What is the response of a metallic sphere in an electrolyte solution to a suddenly applied uniform electric field. Even in the weak-field limit, we find that there is a non-trivial temporal and spatial dependence to the charge build up at the surface of the sphere, which may impact transient fluid flows. At strong fields, we find that surface conduction begins to be important as the diffuse layer builds up sufficient charge to induce surface diffusion and electromigration. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N37.00007: Simulations of induced-charge electro-osmosis in microfluidic devices Yuxing Ben$^{1,3}$, Kevin T. Chu$^{1,3}$, Jeremy Levitan$^{2,3}$, Martin Z. Bazant$^{1,3}$ Theories of nonlinear electrokinetic phenomena generally assume a uniform, neutral bulk electroylte in contact with a polarizable thin double layer near a metal or dielectric surface, which acts as a "capacitor skin". Induced-charge electro-osmosis (ICEO) is the general effect of nonlinear electro-osmotic slip, when an applied electric field acts on its own induced (diffuse) double-layer charge. In most theoretical and experimental work, ICEO has been studied in very simple geometries, such as colloidal spheres and planar, periodic micro-electrode arrays. Here we use finite-element simulations to predict how more complicated geometries of polarizable surfaces and/or electrodes yield flow profiles with subtle dependence on the amplitude and frequency of the applied voltage. We also consider how the simple model equations break down, due to surface conduction, bulk diffusion, and concentration polarization, for large applied voltages (as in most experiments). [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N37.00008: The Response of a Colloidal Microparticle near an Electrode to an AC Electric Field Paul Sides, Jeffrey Fagan, Dennis Prieve We monitored the elevation of single colloidal polystyrene microparticles near an electrode in response to an oscillating electric field. The media were HNO$_{3}$, NaHCO$_{3}$, and KOH, and the frequency band was10 kHz. At low frequencies, large oscillations at the driving frequency with small superimposed Brownian excursions were observed. At high frequencies deterministic oscillations in elevation were negligible compared to Brownian fluctuations, which allowed direct transformation of data into potential energy profiles. The ac field drew the particle closer on average to the electrode in KOH solutions (compared to the no-field average elevation) and the field pushed the particle farther from the electrode in NaHCO$_{3}$. In HNO$_{3}$ a reversal of average height was observed at a frequency of 300 Hz at 1.7 kV/m with the particle being drawn closer to the electrode at low frequencies, and being pushed away at higher frequencies. Analysis of the data at a high frequency (10 kHz) revealed a net force that was attractive in KOH, and repulsive in HNO$_{3}$. This net force scaled with E$^{2}\omega ^{-1}$, where E is the amplitude and $\omega $ is the frequency. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N37.00009: `Designer' porous channels for electrokinetic injection and microfluidic manipulation Todd Squires, Max Narovlyansky, George Whitesides Microfabrication techniques allow effective `porous' media in microchannels to be designed with specified properties. In this talk, we present a general and intuitive framework for such systems. For electrokinetic phenomena, specifying the `pore' geometry is akin to effectively determining the dielectric constant. Pressure-driven systems, on the other hand, are even richer, since an effective permeability and volume fraction can be independently controlled. Furthermore, anisotropy can be deliberately designed into the channel properties, opening a range of possibilities for microfluidic applications. We present simple, intuitive examples to highlight the basic effect, and demonstrate how such ideas can be used for applications of practical interest, such as using electrokinetic injection to form sharp sample plugs for high-resolution separations. Both theoretical and experimental results will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N37.00010: Patterning electrohydrodynamic flows with conductive obstacles in microfluidic channels C.K. Harnett, T.F. Hill, A.J. Skulan, L.M. Barrett, G.J. Fiechtner, E.B. Cummings, B.A. Simmons Flow patterns with both recirculating and unidirectional characteristics are useful for controlled mixing and pumping within microfluidic devices. We have developed a fabrication process that converts injection-molded polymer chips into devices that demonstrate induced-charge electroosmosis (ICEO) effects (1,2) in AC fields. Polymeric insulating posts are coated with metal to produce a nonuniform zeta potential under an applied electric field. Induced flows are analyzed by particle image velocimetry. Stable, recirculating flow patterns are discussed, along with their potential to produce well-characterized and reversible streamlines for on-chip mixing in chemical separation and synthesis devices. Asymmetric conductive features can bias the flow direction, generating unidirectional pumping in an AC field. This pumping approach will be discussed in comparison with DC electrokinetic pumps we have studied. 1) M. Z. Bazant and T. M. Squires, Phys. Rev. Lett. 92, 066101/1-4 (2004). 2) T. M. Squires and M. Z. Bazant, J. Fluid Mech. 509, 217 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N37.00011: Control of electroosmotic flow in a nanofluidic channel using grafted polymer chain Gary W. Slater, Frederic Tessier Electroosmotic flow (EOF) refers to fluid flow past a surface induced by an external electric field. It initially arises near a solid-fluid boundary due to the net charge density in the Debye layer, but the bulk of the fluid is dragged into a uniform flow by viscosity. The phenomenon is ubiquitous in DNA capillary electrophoresis, and is bound to play a critical role in emerging nanopore technologies. However, ways in which it may be controlled or quenched rest mostly on empirical evidence. The most common approach consists in coating the inner capillary surface with adsorbed or grafted polymer chains, but the definite mechanism by which this modulates the EOF remains elusive. We report on large-scale Molecular Dynamics computer simulations of EOF in a nanoscale cylindrical capillary, and discuss the impact of grafted polymers chains on the properties of EOF. We present data for the velocity of the generated flow field as a function of the polymer brush density and the size of individual grafted polymers, and compare our results with theoretical scaling laws derived in the thin Debye layer limit. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N37.00012: Electroosmosis through a Bottleneck: Formation of Eddies and Theory for Arbitrary Debye Lengths Stella Park, Christopher Russo, Howard Stone, Daniel Branton Although using an applied electrical field to drive flows becomes desirable as channels become smaller, most discussions of electroosmosis treat the case of thin Debye layers. Here electroosmotic flow (EOF) through a constricted cylinder is presented for arbitrary Debye lengths $\kappa^{-1}$ using a perturbation approach. The varying diameter of the cylinder produces radially and axially varying effective electric fields, as well as an induced pressure gradient. We predict the existence of eddies for certain constricted geometries and propose the possibility of electrokinetic trapping in these regions. Eddies can be found both in the center of the channel and along the perimeter, and the presence of the eddies is a consequence of the induced pressure gradient that accompanies electrically driven flow into a narrow constriction. An experimental system is also presented in which we observe regions of recirculation in EOF eddies in the small Debye length limit. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N37.00013: Electrokinetics for control of on-chip chemical reactions. David Erickson, Roberto Venditti, Xuezhu Liu, Ulrich Krull, Dongqing Li It is well known that electrokinetics affords precise control over flow and species transport in microfluidic systems through simple manipulation of externally applied electric potentials. In this work it is demonstrated how electrokinetic effects can be extended to provide simultaneous control over on-chip chemical reactions through manipulation of the local thermal (ohmic/joule heating), shear (electroosmosis) and electrical (electrophoresis) energies at the reaction site. The coupling of the electrical, flow and ``whole-chip'' thermal effects in both the fluidic and substrate domains are investigated through extensive finite element simulations and experimentally validated using microscale fluorescence thermometry. The simulations reveal changes in viscosity and local conductivity on the order of 50{\%} induced by changes in the fluidic geometry. General chip design guidelines for maximizing or minimizing these effects will also be discussed. The degree of precision available and clinical utility of the technique is demonstrated through the detection of a single base pair mutation (single nucleotide polymorphism) in a DNA microarray integrated into a PDMS/glass microfluidic chip. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700