Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session HC: Microfluidics VII: Electroosmotic Flows and Dielectrophoresis |
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Chair: Hsueh-Chia Chang, University of Notre Dame Room: Tampa Marriott Waterside Hotel and Marina Grand Salon AB |
Monday, November 20, 2006 2:00PM - 2:13PM |
HC.00001: Electroosmotic Flow and Particle Transport in Micro/nano Nozzles and Diffusers Lei Chen, Pradeep Gnanaprakasam, A.T. Conlisk Electroosmotic flow in a converging or diverging micro-nozzle is calculated using the lubrication approximation. The pressure driven component and the electroosmotic component of the velocity is superimposed and the pressure distribution is obtained by satisfying mass continuity. Electroosmotic flow in micro-nozzles is important in many applications, for example in electrical measurements on living cells, for the injection and the manipulation of DNA fragments and in drug delivery systems. In this work the velocity, potential and concentration distribution are predicted as a function of nozzle (diffuser) geometry, ionic strength and other parameters. In the Debye-Huckel limit valid for small potentials analytical solutions may be obtained for the velocity and potential. Asymptotic analysis is used to calculate a uniformly valid solution for thin electric double layers and is valid for potentials far outside the Debye-Huckel limit. The transport of long chain polymers through these nozzles and diffusers is also considered. If time permits we will compare the results with experimental data. Supported by NSF through the NSEC Center for the Affordable Nanoengineering of Polymeric Biomedical Devices. [Preview Abstract] |
Monday, November 20, 2006 2:13PM - 2:26PM |
HC.00002: Electroosmotic micro-pump array for local control of droplets. Amit Gupta, Amir Hirsa, Diana-Andra Borca-Tasciuc Droplet-based microfluidic devices have a wide range of applications in various fields such as diagnostics and clinical testing, drug delivery and opto-electronics. This paper presents a novel microfluidic device for actuation and control of individual droplets employing electroosmotic pumping across a nanoporous membrane. To fabricate the device, arrays of gold electrodes pairs are first patterned on both sides of an anodic alumina membrane (Whatman, $\sim $50 $\mu $m in thickness, with parallel cylindrical pores of 150 nm in diameter). One side of the membrane is then attached to a liquid reservoir while the other side is covered partially with Teflon to prevent droplet spreading. When voltage is applied between the two aligned top and bottom gold electrodes electroosmotic flow occurs from the liquid reservoir through the membrane and a droplet forms onto the Teflon-coated surface of the membrane. Actuation time and droplet shape are investigated by video microscopy in order to assess the effect of electrode configuration and electrolyte ionic strength Possible applications for the device include addressable liquid microlens arrays, fast-response droplet switches and fast, sample collection devices for brain microdialysis. [Preview Abstract] |
Monday, November 20, 2006 2:26PM - 2:39PM |
HC.00003: The Effect of Induced Electro-Osmosis on a Cylindrical Particle Next to a Surface Hui Zhao, Haim Bau The effect of induced electro-osmosis on a cylindrical particle positioned next to an insulated wall is studied theoretically. We calculate analytically the induced hydrodynamic (electro-osmotic) and electrostatic forces using a thin double layer approximation and numerically with a multi-ion model. The forces are calculated as functions of the particle and medium dielectric constants, the electrical double layer thickness, and the distance between the particle and wall. Not surprisingly, these forces decrease as the particle's dielectric constant decreases and the distance from the wall increases. The induced resultant and hydrodynamic forces are always directed normal to the direction of the imposed electric field and away from the wall. The electrostatic force that acts on the particle (excluding the adjacent electric double layer) is directed, respectively, away and towards the wall at low and high particle dielectric constants. At low and high electric field intensities, respectively, all the forces increase linearly and sublinearly with the square of the electric field intensity. Among other things, the work has important implications for PIV-based, near wall measurements in electro-osmotic flows. [Preview Abstract] |
Monday, November 20, 2006 2:39PM - 2:52PM |
HC.00004: A Thick Double Layer Theory for Dielectrophoresis of Nano-Colloids Sagnik Basuray, Zachary Gagnon, Hsueh-Chia Chang, Franck Plouraboue An analytical theory is reported for the AC dielectrophoretic cross-over frequency of nano-colloids whose dimension is comparable to the double layer thickness. Unlike larger particles, the counter-ions are dispersed in a large ion cloud around the particle and can be shifted asymmetrically relative to the particle. A linear theory based at the Debye-Huckel limit produces a Poisson equation with complex coefficients, whose solution can be expressed in terms of complex spherical Bessel functions of fractional order with a nearly uniform far field. The key polarization mechanism is due to tangential migration and concentration of the charges to the far pole of the particle to form a concentrated cap whose dimension is frequency dependent. The characteristic time for this cross-particle migration is the geometric mean of the double layer relaxation time and the diffusion time across the particle, whose inverse is the cross-over frequency. This is favorably compared to literature data for latex particles, for which the classical dielectric polarization theory predicts no cross-over, and is consistent with our data for high-conducting drops which possess two. [Preview Abstract] |
Monday, November 20, 2006 2:52PM - 3:05PM |
HC.00005: Double-Layer AC Tangential Conduction Effects on Polarization, Assembly and Dielectrophoretic Mobility of Particles in Ionic Liquids Zachary Gagnon, Hsueh-Chia Chang Polarization of gas bubbles and liqud drops by an AC field in ionic liquids is shown to exhibit anomalous induced-dipole behaviors that cannot be explained by classical Maxwell-Wagner dielectric polarization theories. A dielectrophoretic cross-over frequency exists even though the ionic liquid has much higher permittivity and conductivity than the particle, even when conducting Stern layer effects are included. Moreover, the cross-over frequency is observed to be field and particle size dependent. Drops of different size can hence have anti-parallel induced dipoles and are observed to coalesce at an intermediate frequency between the cross-over frequencies of the two particles. ~A scaling theory based on a thin-layer tangential conduction model is found to collapse our measured cross-over frequency and dielectrophoretic velocity data. [Preview Abstract] |
Monday, November 20, 2006 3:05PM - 3:18PM |
HC.00006: A New Theory for Dielectrophoresis due to Polar Double-Layer Charging and Tangential Electro-Migration at Large Fields. Hsien-Hung Wei, Shau-Chun Wang, Hsueh-Chia Chang A charged particle with a thin double layer generally renders the Debye screening effect that prevents normal penetration of the external field. However, this screening can break down at the poles, leading to normal charging or extension of the double layer locally, when the applied field is sufficiently large. With the aid of a scaling analysis, we identify that the DEP crossover frequency $\omega $ is not only size-dependent, but also exhibits three different scales that depend on the field. At fields insufficient to drive polar charging but still large enough to induce tangential migration, $\omega $ is the inverse of diffusion time across the particle D/a$^{2}$. At intermediate fields that permit polar charging but does not extend the local double layer, $\omega $ is still relaxed by tangential diffusion, but further modulated by the field and the double layer thickness. At strong fields, the extended double layer and charging at the pole will furnish a strong electro-migration flux for tangential conduction and $\omega $ becomes the inverse Maxwell-Wagner relaxation time $\varepsilon $/$\sigma $ attenuated by the square-root of the field. For the first time, diverse experimental results for different bulk conductivity, particle size and field strength can be explained and unified under the same theoretical framework. [Preview Abstract] |
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