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 BC: Microfluidics III: Electrokinetics |
Hide Abstracts |
Chair: Leslie Yeo, Monash University Room: Tampa Marriott Waterside Hotel and Marina Grand Salon AB |
Sunday, November 19, 2006 11:00AM - 11:13AM |
BC.00001: Screening of Electroosmotic Flow by Long-Chain Polymer Coatings Ping He, Rui (Jim) Qiao Electroosmotic flow (EOF) is a popular transport mechanism used in micro/nanofluidic systems. Polymer coating has been widely used to control EOF with varying degrees of success. However, the fundamental mechanism of flow control is not well-understood. One key difficulty is that when a surface is coated with polymers, there can exist a two-way coupling between the flow and polymer conformation. Therefore, fluid flow and polymer conformation must be solved self-consistently to predict the screening of flow. Here we report on the particle simulation of screening of electroosmotic flow by neutral polymer grafted on a charged surface. Our simulation results indicate that, as the unscreened electroosmotic velocity increases, the effective polymer thickness decreases and the polymers tend to orient in the flow direction. Such variations of the polymer conformation become less sensitive to the unscreened flow velocity as the unscreened velocity increases. The simulation results indicate that the degree of flow screening decreases as the unscreened flow velocity increases, in consistent with earlier theoretical predictions (Harden, Long and Ajdari, \textit{Langmuir}, \textbf{17}, 705, 2001) [Preview Abstract] |
Sunday, November 19, 2006 11:13AM - 11:26AM |
BC.00002: Molecular views of surface driven transport : electro-osmosis, diffusio-osmosis, and beyond Lyderic Bocquet, Christophe Ybert, Armand Ajdari We investigate the molecular mechanisms associated with surface driven transport, such as electro-, diffusio- or thermo-osmosis, which generate flows by application of a macroscopic gradient (of electric potential, solute concentration or temperature). The driving force (e.g., electric or osmotic) is located typically within a nanometric distance to the surface and the resulting effects are thus expected to be strongly affected by modifications of surface properties. We show in this context that moderate departures from the no-slip hydrodynamic boundary condition can result in very large enhancement of most interfacially driven transport, - up to two orders of magnitude for hydrodynamic slip lengths in the nanometer range ! -. The amplification of the effect due to slippage scales as the ratio between the slip length and the interfacial characterisic length (debye length, or solute attraction or depletion range). These predictions are confirmed qualitatively and quantitatively by molecular dynamics simulation of electro- and diffusio -osmosis. We will discuss the pertinence of these effects for flow enhancement in nano- or micro- fluidic geometries, but also fortransport of macromolecules in externally applied or self- generated gradient, in line with recent experiments. [Preview Abstract] |
Sunday, November 19, 2006 11:26AM - 11:39AM |
BC.00003: Flow of DNA solutions in planar 90 degree micro-bends Shelly Gulati, Susan Muller, Dorian Liepmann The characterization of flows containing macromolecules is critical for the optimal design of microfluidic systems for biochemical analyses. The effects on lambda-DNA transport in microscale flows are significant because the flow behavior may be influenced by molecular interactions, both viscous and elastic forces dominate inertial forces at this length scale, and the macromolecular length scale L approaches the device length scale D. Our previous studies of flow of semi-dilute DNA solutions in micro-contractions (L/D $\sim $ 0.17) indicate strongly elastic behavior through the observation of vortex enhancement with increasing Re and Weissenberg (Wi) numbers; here Wi is the ratio of the polymer relaxation time to the characteristic flow time scale. In the present work, the viscoelastic flow of semi-dilute DNA solutions in planar 90 degree micro-bends (L/D $\sim $ 0.09), another canonical microfluidic structure, is investigated; macromolecular flows in this geometry on a macro or microscale have been previously unexplored. The onset of a new flow instability occurs at Wi $\sim $ 1. A recirculation region is present in the interior corner of the bend and is enhanced with increasing Re (6 x 10$^{-7} <$ Re $<$ 3 x 10$^{-4})$ and Wi (1 $<$ Wi $<$ 190); this vortex is absent in Newtonian flows. [Preview Abstract] |
Sunday, November 19, 2006 11:39AM - 11:52AM |
BC.00004: Micro pumping methods based on AC electrokinetics and Electrorheologically actuated PDMS valves Gaurav Soni, Todd Squires, Carl Meinhart We have developed 2 different micropumping methods for transporting ionic fluids through microchannels. The first method is based on Induced Charge Electroosmosis (ICEO) and AC flow field-effect. We used an AC electric field to produce a symmetric ICEO flow on a planar electrode, called `gate'. In order to break the symmetry of ICEO, we applied an additional AC voltage to the gate electrode. Such modulation of the gate potential is called field effect and produces a unidirectional pumping over the gate surface. We used micro PIV to measure pumping velocities for a range of ionic concentration, AC frequency and gate voltage. We have also conducted numerical simulations to understand the deteriorating effect of lateral conduction of surface charge on the pumping velocities. The second method is based on vibration of a flexible PDMS diaphragm actuated by an electrorheological (ER) fluid. ER fluid is a colloidal suspension exhibiting a reversible liquid-to-solid transition under an electric field. This liquid-to-solid transition can yield very high shear stress and can be used to open and close a PDMS valve. Three such valves were fabricated and actuated in a peristaltic fashion in order to achieve positive displacement pumping of fluids. [Preview Abstract] |
Sunday, November 19, 2006 11:52AM - 12:05PM |
BC.00005: Generating a Dipole Momentum Source with a Non-Uniform AC field: A New Long-Range Electro-kinetic Micro-Pump Design Jason Gordon, Zachary Gagnon, Hsueh-Chia Chang An AC electrokinetic funnel flow is generated by a planar micro-coil resister design capable of sustaining 3500V without Faradaic reactions. The final line of the serpentine coil exhibits a transverse electro-osmotic slip velocity exceeding 1 cm/sec. Due to side boundaries, a large back pressure is established in the slip direction to produce an intense vortex cylinder. As the coil design stipulates a vortex circulation that is longitudinally non-uniform, the longitudinal gradient in the back pressure drives a secondary flow to transform the vortex flow into a funnel flow. This funnel flow resembles a dipole momentum line source with large field-line penetration and can be used to effectively transport fluid at the microscale. Unlike all previous electrode-based AC electrokinetic micropump designs, fluid motion is not confined to the polarized electrode surface, which allows one to generate long-range pumping without having to employ large-scale electrode arrays that span the entire length of the microchannel. [Preview Abstract] |
Sunday, November 19, 2006 12:05PM - 12:18PM |
BC.00006: Einstein's Tea Leaf Analogy to Electrohydrodynamically-Driven Microfluidic Blood Plasma Separation. James Friend, Leslie Yeo, Dian Arifin The application of a voltage upon a sharp electrode tip, mounted at an inclination angle above the liquid surface of a microfluidic chamber, generates an electrohydrodynamic air thrust that shears the liquid surface and hence induces liquid recirculation consisting of a primary azimuthal rotation and a secondary bulk meridional flow. Colloidal particles suspended within the liquid are then observed to be swept by the recirculation in a helical swirl-like motion and deposited at a stagnation point located centrally at the bottom of the microfluidic chamber. This is due to a delayed centrifugal force and an enhanced inward radial force at the base. We propose that the flow, which is similar to Batchelor flows arising between rotating and stationary disks, is analogous to Einstein's paradoxical observation of tea leaves centrally accumulating at the base of a stirred teacup and show that the phenomenon can be exploited for bioparticle trapping and concentration. In particular, we demonstrate the rapid separation of red blood cells from blood plasma for miniaturized blood diagnostic kits. [Preview Abstract] |
Sunday, November 19, 2006 12:18PM - 12:31PM |
BC.00007: Electric field driven motion of flexible polyelectrolytes in solution Tak Shing Lo, Andreas Acrivos, Joel Koplik, Boris Khusid Our work aims to study dielectrophoresis of biomolecules in micro/nanofluidics, by combining electrohydrodynamics with molecular theories for the macromolecule polarization caused by the distortion of the counterion cloud. Molecular dynamics (MD) is used to simulate the transport of a flexible polyelectrolyte suspended in a solvent, with or without added salt, under the action of electric fields. We used a molecular model with explicit solvent atoms which includes automatically hydrodynamic interactions and finite atom size effects. The polyelectrolyte is modeled as a negatively charged freely-jointed bead-spring chain, and its responses in dc and ac fields are studied in detail. We focused on investigating the effects of the electric field on various physical quantities in comparison with equilibrium. We also developed a procedure to compute the dipole moments of the molecule and of the surrounding double layer, which are required for understanding the dielectrophoretic behavior of these molecules in nanoscopic channels. [Preview Abstract] |
Sunday, November 19, 2006 12:31PM - 12:44PM |
BC.00008: Ultra-Concentration and Microjet Ejection of Charges by AC Tangential Charging at the Poles of Conducting Granules Hsueh-Chia Chang, Shau-Chun Wang, Hsien-Hung Wei Counter-ions charged into the extended double layer of a conducting granule of size a by an AC field E is shown by fluorescent imaging to migrate tangentially towards the pole in each half cycle if $\Gamma $=(EaF/RT) is large. The charges accumulate at the pole and extend the local double layer thickness to a macroscopic value $a/\sqrt \Gamma $. The accumulated charges reach a concentration more than a million times, estimated to be the Boltzmann factor $e^\Gamma $, higher than the bulk value before being discharged in a dramatic microjet ejection event. The pole charging time can be estimated by the electro-migration time around the granule, which is (1/ $\Gamma )$ times the diffusion time a$^{2}$/D, and ejection can be prevented if the frequency is below the inverse charging time. The microjet is shown to be confined by the field lines bounding the extended double layer at the pole and reaches a distance five times its thickness. Implication of this intense particle dipole formation on dielectrophoresis and alpha impedance relaxation will be discussed [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