Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A10: Microscale Flows: Electrokinetics and Electrohydrodynamics |
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Chair: Cullen Buie, Massachusetts Institute of Technology Room: 3005 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A10.00001: Influence of Film Thickness and Substrate Geometry on the Growth of Taylor Cones in Perfectly Conducting Films Theodore Albertson, Sandra Troian There are a growing number of space based applications ranging from miniature mass spectrometry devices to small focused ion beam units which rely on nanoscale fluid flow controlled by Maxwell stresses. These new technologies require ever improved understanding of the process by which a liquid stream or droplet is transformed from an initial smooth shape into a steepening cone known as the Taylor cone. While ongoing studies in the literature have elucidated how the cone-jet transition controls the delivery of mass and charge flux, less attention has been paid to the case of microscale films and how frictional effects influence the shape and timescale of evolving conical elongations. Here we describe recent efforts in our group using moving mesh and phase field methods to capture the influence of substrate geometry and film thickness on the formation of transient Taylor cones in perfectly conducting films. The computational model fully couples electrohydrodynamic fluid flow with active pressure and electrical potential fields resulting from the rapidly evolving film shape. We examine the asymptotic behavior of the film deformation process as a function of the electric field strength and substrate curvature, which in experimental systems can be easily tuned. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A10.00002: Experimental electrokinetic flow characteristics in cross-shaped microchannels with groove and wavy geometrical inhomegeneities Diego Oyarzun, Alvaro Socias, Amador Guzman Electrokinetic flow instabilities (EKI) in cross-shaped microchannels occur when a critical local Rayleigh number is reached. To know the range of Rayleigh numbers when either stable or unstable electrokinetic flows happen is very important for suppressing or enhancing the EKI effect on processes such as injection or mixing. One way of enhancing flow mixing is by incorporating geometrical inhomogeneities on the microchannel walls such as groove and/or waves that can disturb or suppress convective and absolute instabilities for highly critical electrokinetic flow regimes. We, first experimentally investigate the flow pattern in a cross-shaped microchannel without groove and waves at the channel walls under an external electric field to determine the Rayleigh number range for stable and unstable flow regimes and the dominants frequencies associated to EKI. Then, we investigate the effect of grooves and waves at the microchannel walls on the electrokinetic flow characteristics for determining the effect of the existence of the geometrical inhomogeneities on the electrokinetic flow patterns for the range of sub-and super-critical Rayleigh numbers. Our primary results for local Rayleigh numbers based on a cross-shaped microchannel with flat walls indicate that at least for subcritical flow regimes there are no unstable, but stable flow regimes. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A10.00003: Dynamics of micro-vortices induced by ion concentration polarization in electrodialysis Joeri de Valenca, R.M. Wagterveld, Rob Lammertink, Peichun Amy Tsai We experimentally investigate the coupled dynamics of global ion transport and local electroconvective flow of an electrolyte solution close to a charge selective membrane under an electric forcing. At small dc electric currents, due to the membrane permselectivity counterions (cations) transport diffusively through the cation exchange membrane (CEM) whereas the passage of co-ions (anions) is inhibited, thereby forming ion concentration polarization or gradients. At large currents, our simultaneous measurements of voltage drop and flow filed reveal several distinct dynamical regimes. Initially, the electrodialysis system exhibits a linear Ohmic electric resistance and then a rate-limiting regime with a voltage jump. Subsequently, electro-osmotic micro-vortices set in and grow linearly both in size and speed with time. After this linearly growing electroconvective regime, the measured voltage drop levels off around a fixed value. The average vortex size and speed saturate as well, however the individual vortices are unsteady and dynamical. Furthermore, the influence of micro-patterned CEM on the couple dynamics will be presented and discussed. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A10.00004: Investigation of Material Dependence in Electrothermal Vortex Jian Wei Khor, Avanish Mishra, Xudong Pan, Steven Wereley Rapid Electrokinetic Patterning (REP) is an optoelectric method which can be used for manipulation of diverse set of particles with laser. However, requirement of high laser intensity remains a stumbling block to proliferation of this method. In this presentation, we demonstrate that careful selection of the electrode material is critical in producing a cost effective REP chip that requires low laser power. The electrodes in REP provide two aspects of the phenomenon; electrical conduction from the electrical power source that produces the electric field and heat absorption from the laser heating that produces the temperature gradient. Consequently, the physical and thermal properties of the electrodes used in REP are crucial for the formation of the electrothermal vortex, which plays a major role in REP as a manipulation technique. Currently, Indium Tin Oxide (ITO) layer is used as the electrodes. ITO produces the electrothermal vortex needed for REP and also provides a viewing window in to the chip. In this study, possibilities of utilizing other materials to produce equivalent or better REP effects than ITO will be investigated. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A10.00005: Highly selective creation of hydrophilic micro-craters on super hydrophobic surface using electrohydrodynamic jet printing Jaehyun Lee, Sangyeon Hwang, Fariza Dian Prasetyo, Vu Dat Nguyen, Jungwoo Hong, Jennifer H. Shin, Doyoung Byun Selective surface modification is considered as an alternative to conventional printing techniques in high resolution patterning. Here, we present fabrication of hydrophilic patterns on the super hydrophobic surface, which makes structure on the hydrophilic region. The super hydrophobic surface is able to be chemically changed to hydrophilic with alcohols. As a consecutive process, electrohydrodynamic (EHD) jet printing was utilized to fabricate local hydrophilic craters with 30-200 $\mu m$ sizes. 3 kinds of target liquids were deposited well on hydrophilic region; PEDOT (poly 3,4 ethylenediocythiophene), polystyrene nano-particles, and salmonella bacteria medium. Additionally, qualitative analysis were presented for modification mechanism and surface properties on super hydrophobic/hydrophilic by analysis of surface energy with contact angle, SEM (scanning electron microscopy) image, and SIMS (secondary ion mass spectroscopy) analysis. This new simple modification method provides possibility to be utilizing in bio-patterning engineering such as cell culturing microchip and lab on a chip. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A10.00006: Characterization of self-assembled colloidal particle bands in combined electroosmotic and Poiseuille flow Necmettin Cevheri, Minami Yoda Periodic and steady electric fields have long been used to manipulate and assemble colloidal particles suspended in conducting fluids, usually aqueous solutions. Most of these studies have, however, focused on suspensions at rest. Recent studies have shown that a combination of steady electric fields and shear flow can be used to manipulate radii $a = 245$ nm particles in a dilute suspension flowing through $\sim 30~\mu$m deep microchannels. When the electric field is in the opposite direction from the Poiseuille flow (which is essentially simple shear flow near the wall), the particles are first attracted to the wall, then self-assemble into nearly periodic concentrated bands aligned with the flow direction, as the electric field magnitude $|E|$ increases. This talk will discuss the characteristics of these bands, {\it e.g.} how their average spacing depends on $|E|$ and the near-wall shear rate $\dot{\gamma}$, as well as the dynamics of the particles within the bands, which are moving in the same direction as the flow and appear to be in a disordered liquid ({\it vs.} crystalline) state. Bands only form above a threshold value of $|E|$, and this value depends on parameters such as $\dot{\gamma}$, the particle radius $a$, and the particle zeta-potential $\zeta_p$. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A10.00007: Repulsive and Attractive Colloidal Particle-Wall Interactions in Poiseuille and Electroosmotic Flow through Microchannels Minami Yoda, Necmettin Cevheri Manipulating near-wall polystyrene particles in a dilute suspension flowing through a microchannel is important in microfluidics. Such particles experience wall-normal lift forces in electroosmotic (EO) flows driven by electric fields of magnitude $E$, and in Poiseuille flows driven by pressure gradients $\Delta p/L$ beyond the forces predicted by DLVO theory. Recent evanescent-wave particle tracking studies of combined EO and Poiseuille flow have shown that 245 nm radii particles are repelled from, or attracted to, a fused-silica wall when the EO flow is in the same, or opposite, direction as the Poiseiulle flow, respectively. Estimates of the lift force magnitude $\mathcal{F}$ suggest that it scales with the shear rate $\dot{\gamma}$ for Poiseuille flow, and not the $\dot{\gamma}^2$ typical of electroviscous lift. Surprisingly, when the force is repulsive, $\mathcal{F}$ exceeds the sum of the forces observed for EO flow at the same $E$ and Poiseuille flow at the same $\Delta p/L$ and appears to scale as $\dot{\gamma}^{1/2}$. Furthermore, in both cases $\mathcal{F}$ appears to be proportional to $E$, suggesting that this lift force is distinct from those observed in ``pure" EO and ``pure" Poiseuille flows. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A10.00008: Electrohydrodynamic manipulation of particles on drop surfaces Edison Amah, Kinnari Shah, Ian Fischer, Pushpendra Singh We have recently shown that particles adsorbed on the surface of a drop can be self-assembled at the poles or the equator of the drop by applying a uniform ac electric field, and that this method can be used to separate on the surface of a drop those particles experiencing positive dielectrophoresis from those experiencing negative dielectrophoresis. In this talk we show that the frequency of the electric field is an important parameter which can be used to modify the intensities of the dielectrophoretic and the hydrodynamic-flow induced forces, and thus control the distribution of self-assembled monolayers. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A10.00009: Effect of electrode geometry on field strength in plastic microfluidic devices and application to cell membrane permeabilization Marc Chooljian, Jacobo Paredes, Dorian Liepmann We have developed a method that allows embedding of electrodes in up to 3 walls of a plastic microfluidic channel. Electric field strength and homogeneity of various electrode geometries is analyzed theoretically and experimentally by evaluating the efficiency of on-chip lysis of cells. Electric field-mediated disruption of membranes is an important tool in diagnostics, basic biology, and synthetic biology due to the ability to permeabilize the cell membrane without changing the chemical composition of the buffer. Typically, fields of the required magnitude are applied to the cell by discharging a capacitor through a mixture of cells in a cuvette, resulting in a transient high-voltage pulse. We demonstrate that is possible to substitute a spatially varied DC electric field along a microchannel and to control the timing of the pulses by changing the electrode spacing and the flow rate. Homogeneity of the field with respect to the cross section of the channel is key to achieving critical field strength regardless of the cell's lateral position in the channel. A comparison of 2D versus 3D electrode geometries on the efficiency of electroporation and on side-effects arising due to the electric field (recirculating flows and hydrolysis) is presented. [Preview Abstract] |
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