Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E18: Microfluids: General III |
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Chair: Francisco Diez, Rutgers University Room: 321 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E18.00001: Tracer Particles Zeta Potential Evaluation from Time Resolved $\mu $PIV Measurements Miquel Sureda, Andrew Miller, Francisco J. Diez A method is proposed to directly measure the zeta potential of the tracer particles in aqueous solutions using time resolved microPIV (TR$\mu )$. The zeta potential of tracer particles in microPIV measurements used in electrokinetic flow analysis in microchannels must be known to accurately calculate the tracer particles electrophoretic velocity. By subtracting this velocity from the observed microPIV velocity, the electroosmotic velocity of the flow is obtained. Using a high-speed laser and camera, the electrophoretic velocity of the tracer particles can be directly calculated from microPIV measurements. The measurements are obtained during the first microsecond of the transient flow that is generated when a potential drop is imposed across a micro-channel. This allows fully visualizing the temporal development of the electroosmotic flow (EOF) in microchannels. For the measurements, 830 nm diameter polystyrene particles are diluted in 100 mM, 10 mM and 1 mM Borate Buffer solutions. A comparison between the proposed TR$\mu $ technique and an improved Closed Cell technique validated the results. An advantange of this proposed technique is that measurements can be performed in situ in any microchannel configuration where EOF is needed, without the need for additional calibrations providing reliable data in an easy and quick way. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E18.00002: The effect of multivalent counterions on electrokinetically driven flows Necmettin Cevheri, Minami Yoda Various numerical and experimental studies have shown that even trace amounts of multivalent counterions can greatly affect electroosmosis by changing the wall zeta-potential $\zeta_{\mbox{w}}$ via both electrostatic and chemical interactions with the diffuse and Stern layer parts of the electric double layer (EDL). We have previously shown that replacing 1{\%} of the monovalent cation by $\mbox{Ca}^{++}$ can almost halve the electroosmotic mobility, and hence the inferred $\zeta_{\mbox{w}}$ of a symmetric monovalent electrolyte solution [Datta \textit{et al}. (2009)]. These counterions can, however, also affect electrophoresis of the tracers by changing the particle zeta-potential $\zeta_{\mbox{p}}$. Evanescent-wave particle velocimetry was used to study how trace amounts of divalent cations such as $\mbox{Ca}^{++}$and $\mbox{Mg}^{++}$ affect velocities in the electrokinetically driven flow of various aqueous monovalent electrolyte solutions through channels with a minimum dimension $H\approx 30 \mu \mbox{m}$. In all cases, the Debye length $\ll H$. The $\zeta_{\mbox{p}}$ of the tracers were characterized by light scattering, and the steady-state distribution of tracers over the first 300 nm next to the wall was determined from these data to investigate the interactions between the negatively charged particle and wall surfaces and the mobile divalent counterions. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E18.00003: Interaction of Ion-Concentration Shock Waves in Microfluidics Supreet S. Bahga, Robert D. Chambers, Juan G. Santiago Electrophoresis based separation techniques, such as capillary electrophoresis and isotachophoresis (ITP), are routinely used in microfluidics to separate ionic species from complex mixtures. Nonlinearities in these electrophoretic processes can result in formation of shock and rarefaction waves. We here focus on shock waves which form in ITP between regions of high and low mobility ions. Depending on the charge of ions, these shocks can propagate either towards anode or cathode, and may interact with each other. We here demonstrate simultaneous anionic and cationic ITP process, in which shock waves approach each other and then interact. Using simulations and experimental visualizations, we show that the interaction of these shock waves can modify the electrophoretic conditions and result in formation of new shock and rarefaction waves. We show two applications where we use shock interaction to couple different electrophoretic processes: (i) where we first preconcentrate DNA fragments in anionic ITP and then use shock interaction to initiate DNA separation, and (ii) where we use shock interaction to elongate ITP zones for higher sensitivity. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E18.00004: Concentration polarization and desalination in nanochannels: Effect of surface charge dynamics Mathias B. Andersen, Henrik Bruus, Ali Mani, Martin Z. Bazant Mani, Zangle, and Santiago (\textit{Langmuir}, 25, 3898--3916) have shown that at microchannel-nanochannel junctions the coupled effect of concentration polarization and surface conduction can lead to long range propagation of bulk ion-depletion shocks. Essential for this phenomena is the surface charge which for many materials depends on both the concentration and the pH of the local bulk electrolyte. Standard models predict that the surface charge decreases with decreasing concentration leading to the contradictory expectation that there is little or no surface charge in the depleted region and hence no mechanism to sustain long range propagation of desalination shocks. We show that this simple prediction fails to take into account axial transport terms. As such, we couple a surface charge model with the Poisson--Nernst--Planck equations for electric potential and ionic species combined with the Navier--Stokes and continuity equations for fluid velocity. Motivated by experimental work we consider steady-state solutions at the dead end of a nanochannel against a membrane, a scenario where especially space charge and electroosmotic flow are important. Our results suggest that the surface charge density remains finite and does not vanish, and even grows, as the depletion front propagates through the channel. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E18.00005: Electromigration dispersion: theory vs. experiment Sandip Ghosal, Zhen Chen When the concentration of sample ions in zone electrophoresis is significant compared to that of the carrier electrolyte, axial variation of electrical conductivity along the capillary could arise. The consequent variations of the electric field results in electromigration dispersion of the sample peak. It has been shown (S. Ghosal and Z. Chen, Bull. Math. Biol. 2010 vol.72, pg. 2047), that, in an idealized three-ion system consisting ofthe sample ion, a co-ion and a counter-ion, all of equal diffusivity, the time evolution of the sample concentration is governed by a single nonlinear one dimensional transport equation. If the solute concentration is not too large, this equation reduces to Burger's equation that permits exact analytical solution. In the aforementioned analysis, ionic dissociation and recombination was neglected, so that the model only describes systems of strong electrolytes. Here we show that the one dimensional model derived earlier also describes electrophoresis of a solute in the presence of a buffer consisting of a single weak acid (or base). A simple approximate formula is derived for the number of theoretical plates in the presence of electromigration dispersion and compared with published experimental data. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E18.00006: Electromigration dispersion in the presence of a zeta-potential Zhen Chen, Sandip Ghosal Electromigration dispersion is observed in zone electrophoresis when the concentration of sample ions is comparable to that of the background ions, so that the local electrical conductivity is significantly altered in the sample zone. It was shown (S. Ghosal and Z. Chen Bull. Math. Biol. 2010, vol. 72, pg. 2047) that under certain simplifying assumptions, the concentration profile is described by Burgers' equation. Here we consider a more general situation where the walls of the separation channel may have a non-zero zeta potential and is therefore able to sustain an electro-osmotic bulk flow. The main result is a one dimensional nonlinear advection diffusion equation for the area averaged concentration that accounts for the Taylor-Aris dispersion resulting from the variation in the electro-osmotic slip velocity along the wall. It is shown that in a certain range of parameters, the electro-osmotic flow can actually reduce the total dispersion by delaying the formation of a concentration shock. However, if the electro-osmotic flow is sufficiently high, the total dispersion is increased because of the Taylor-Aris contribution. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E18.00007: Fluid properties in the electrical double layer-effect on streaming potential at charged interfaces Alexander Barbati, Brian Kirby Fluid properties in the electrical double layer are central to electrokinetic phenomena and affect interfacial measurement. We use a flat plate streaming potential apparatus to measure the interfacial potential of novel (and previously unmeasured) films with unique surface properties. The nature of the parallel plate geometry permits pre- or post-experiment measurement of surface properties (AFM, XPS, etc...) to further interrogate the surface structure. Analytical approximations and a numerical framework are used to directly compute experimental outcomes by combining potential and flow simulations to predict electrical currents generated by the convection of the ion distribution near the liquid-solid boundary; specifically we introduce electric field sensitive variation in the viscosity and permittivity to determine, quantitatively, property effects on linear electrokinetic measurements, with application to novel interfaces and liquids. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E18.00008: Velocity field measurements of electrokinetic flow past a conductive cylinder Cetin Canpolat, Ali Beskok Using the micro particle-image-velocimetry technique, electrokinetic (EK) flow past a conductive circular cylinder (D=0.67 mm) is measured in a rectangular cross-section PDMS/glass microchannel (H=0.1 mm, W=1.0 mm and L=5.3 mm). EK transport in such a system experiences electrophoresis (EP) of the PIV particles, electroosmotic flow (EOF) due to the channel walls, and induced charge electroosmotic (ICEO) flow due to the conductive cylinder. Experiments are conducted using 1xPBS buffer diluted in DI water, and the buffer pH is fixed at 2.05 using HCl solution. This pH value is shown to nearly eliminate the electrophoresis of 0.5 micron carboxylate modified spherical micro-particles used in the PIV studies. Suppression of EP enabled direct measurements of local ICEO flow and its interaction with the global EOF in the channel. By systematically varying the applied electric field from 5 V to 40 V, changes in the velocity field are recorded and correlated with the theoretical trends of EOF and ICEO flow. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E18.00009: Electrothermal micromixing in 96 well plate Paul Kauffmann, Sophie Loire, Igor Mezic Diagnostic and pharmacology processes could be greatly accelerated by appropriate mixing. Here electrothermal flows are explored to provide mixing of conductive physiological solutions (=1.6 S/m) in a 96 well plate. Three interdigitated electrodes provide an electric field ($<$ 15Vpp, 1MHz) beneath each well. Polarization and conduction phenomenon of the fluid in a well will be first modeled numerically and compared to an electrical circuit model. Due to high conductivity and permittivity of the fluid, the impedance of the array of filled wells collapse dramatically (96 wells: R = 1Ohm, C=250nF). The power supply challenges accordingly raised by arrays of electrothermal micromixers will be then analyzed. The efficiency of different methods of mixing in those wells will be also compared: the addition of low frequency signal leading to AC electro-osmotic perturbations, a blinking vortices method. The experimental results will be compared to simulations. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E18.00010: Electrokinetic flows through a parallel-plate channel with slipping stripes on walls Henry C.W. Chu, Chiu-On Ng Electrohydrodynamic flows through a periodically-micropatterned plane channel are considered. One unit of wall pattern consists of a slipping and non-slipping stripe, each with a distinct zeta potential. The problems are solved semi-analytically by eigenfunction expansion and point collocation. In the regime of linear response, the Onsager relation for the fluid and current fluxes are deduced as linear functions of the hydrodynamic and electric forcings. The phenomenological coefficients are explicitly expressed as functions of the channel height, the Debye parameter, the slipping area fraction of the wall, the intrinsic slip length, and the zeta potentials. We generalize the theoretical limits made in previous studies on electrokinetic flow over an inhomogeneously slipping surface. One should be cautious when applying these limits. First, when a surface is not 100{\%} uniformly slipping but has a small fraction of area being covered by no-slip slots, the electroosmotic enhancement can be appreciably reduced. Second, when the electric double layer is only moderately thin, slipping--uncharged regions on a surface will have finite inhibition effect on the electroosmotic flow. [Preview Abstract] |
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