Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session HN: Micro Fluids IV |
Hide Abstracts |
Chair: Kenneth Christensen, University of Illinois at Urbana-Champaign Room: 201 |
Monday, November 24, 2008 10:30AM - 10:43AM |
HN.00001: ABSTRACT WITHDRAWN |
Monday, November 24, 2008 10:43AM - 10:56AM |
HN.00002: Diffusio-osmotic control in microfluidic devices Benjamin Abecassis, David Huang, Cecile Cottin-Bizonne, Christophe Ybert, Lyderic Bocquet We present recent work aiming at exploring the possibilities offered by diffusio-phoretic phenomena in the context of microfluidic devices. Such phenomena are associated to the generation of surface-induced liquid flows in response to an imposed external gradient of solute concentration. Using a $\psi$-shaped microchannel to establish a well-controlled gradient of electrolyte, we first demonstrate experimentally that diffusiophoresis phenomena can be used to perform various microfluidic functionalities: particle manipulation, sorting, concentration, etc. Moreover, we provide a theoretical description that quantitatively accounts for all observed behaviors [1]. We then extend the theoretical analysis to incorporate the possible modification of surface hydrodynamical properties offered when moving to hydrophobic or superhydrophobic surfaces. Diffusioosmosis responses are found amplified by the presence of surface slippage to an extend that can reach up to 3 orders of magnitude with superhydrophobic surfaces. On the basis of such predictions, a source-free osmotic pumping device is proposed for microsystems [2]. \\[0pt] [1] B. Abecassis {\it et al.}, {\it Nature Mat.} (2008), \textbf{in press}. \\[0pt] [2] D. Huang {\it et al.}, {\it Phys. Rev. Lett.} (2008), \textbf{in press}. [Preview Abstract] |
Monday, November 24, 2008 10:56AM - 11:09AM |
HN.00003: Structural Characteristics of Transitional Flow in Microscale Capillaries K.T. Christensen, V.K. Natrajan Microscopic particle image velocimetry measurements of flow through a 536\,$\mu$m capillary for Reynolds numbers in the range 1900\,$<$\,Re\,$<$\,4500 are utilized to study the maturation of the flow from a laminar to a fully-turbulent state. Examination of the instantaneous velocity fields reveals that transitional capillary flow is composed of patches of disordered motion that are bounded by regions of flow that are nominally laminar. Consistent with that noted for transitional wall-bounded flows at the macroscale, the non-laminar fraction of the flow increases progressively with increasing Re. Further, the intensity of these disordered motions grows with Re and quadrant analysis supports a gradual maturation of the instantaneous Reynolds-shear-stress-producing events as the flow transitions toward a fully-turbulent state. Proper orthogonal decomposition of the transitional datasets indicates that the large-scale structures embedded within the disordered motions play a dominant role in the transport of both kinetic energy and Reynolds-shear-stress. Visualization of these large-scale structures reveals spatial signatures consistent with hairpin- like vortices that organize to form larger-scale hairpin vortex packets that are commonly noted in studies of transitional and turbulent wall-bounded flows at the macroscale. [Preview Abstract] |
Monday, November 24, 2008 11:09AM - 11:22AM |
HN.00004: Dispersion in isotachophoresis Moran Bercovici, Juan G. Santiago Isotachophoresis (ITP) is a widely used separation and preconcentration technique, which has been utilized in numerous applications including drug discovery, toxin detection, and food analysis. In ITP, analytes are segregated and focused between relatively high mobility leading ions and relatively low mobility trailing ions. These electromigration dynamics couple with advective processes associated with non-uniform electroosmotic flow (EOF). The latter generates internal pressure gradients leading to strong dispersive fluxes. This dispersion is nearly ubiquitous and currently limits the sensitivity and resolution of typical ITP assays. Despite this, there has been little work studying these coupled mechanisms. We performed an analytical and experimental study of dispersion dynamics in ITP. To achieve controlled pressure gradients, we suppressed EOF and applied an external pressure head to balance electromigration. Under these conditions, we show that radial electromigration (as opposed to radial diffusion as in Taylor dispersion) balances axial electromigration. To validate the analysis, we monitored the shape of a focusing fluorescent zone as a function of applied electric field. These experiments show that ITP dispersion may result in analyte widths an order of magnitude larger than predicted by the typical non-dispersive theory. Our goal is to develop a simplified dispersion model to capture this phenomenon, and to implement it in a numerical solver for general ITP problems. [Preview Abstract] |
Monday, November 24, 2008 11:22AM - 11:35AM |
HN.00005: Producing colloids with microfluidics Nicolas Pannacci, Herve Willaime, Patrick Tabeling Submicronic emulsions are commonly used in pharmaceutical, food, cosmetic and material industries. Standard microfluidic tool is particularly convenient to produce in a very controlled way either droplets of typical diameter ranging from 10 to 300 microns with a perfect monodispersity ($<$3{\%}), or double emulsions as well as double droplets (janus). We report the use of microfluidic devices to produce submicronic objects. We use a hydrodynamic flow-focusing that has the advantage to generate nanodrops in a way that is slightly dependent on the fluids used. The control on such a flow authorizes the adjustment of the diameter of the colloids formed. We will show brownian particles from 860 nm to 1.3 $\mu $m in diameter obtained in such way and their clustering into crystals thanks to their high monodispersity. These first experimental results are very promising and make evident the great potential of micro and nano-fluidics to produce nano-emulsions or colloids with very controlled size that metamaterials can require. [Preview Abstract] |
Monday, November 24, 2008 11:35AM - 11:48AM |
HN.00006: Topology optimization of induced-charge electro-osmotic flows Misha Marie Gregersen, Fridolin Okkels, Martin Z. Bazant, Henrik Bruus Nonlinear induced-charge electro-osmotic (ICEO) flows with both AC and DC forcing have recently been observed around isolated and inert (but polarizable) objects, offering the possibility to manipulate liquids and suspensions of nanoparticles. ICEO is the nonlinear electro-osmotic slip that occurs in an electrolyte when an applied electric field acts on the ionic charge it induces around a polarizable surface of any dielectric in contact with an electrolyte. We present a numerical study of ICEO of electrolytes in microfluidic channels containing a fixed dielectric structure with a complex geometry. The full nonlinear coupled equation system is solved, enabling studies of finite Debye layers and confinement effects. A method has been established for optimizing the shape of the dielectric structure for any given objective function. Optimized structures have been achieved for maximizing the pumping velocity generated by asymmetric structures. [Preview Abstract] |
Monday, November 24, 2008 11:48AM - 12:01PM |
HN.00007: Simulation of highly non-linear electrokinetics using a weak formulation Gaurav Soni, Todd Squires, Carl Meinhart In most electrokinetic simulations, the electrical double layer is modeled as a boundary condition using a linear capacitance model. However, the linear model is valid only for very low zeta potentials \textbf{\textit{$\zeta $}} and thus does not predict correct results for experimental conditions involving high \textbf{\textit{$\zeta $}}. We have formulated a highly nonlinear but very stable double layer boundary model which allows us to investigate electrokinetic phenomena even at very high \textbf{\textit{$\zeta $}}. The model is a time dependent partial differential equation for the double layer surface charge density \textbf{\textit{q}}, establishing a balance between accumulation and normal plus lateral fluxes of the charge. We first make this model stable by changing the dependent variable from \textbf{\textit{q}} to \textbf{\textit{$\zeta $}}. In other words, we solve the PDE for \textbf{\textit{$\zeta $}} instead of \textbf{\textit{q}}. A nonlinear capacitance formula is used for changing the variables. Since the magnitudes of zeta potential are limited by geometry and applied voltage, the PDE becomes well behaved. Then we also transform the surface conduction term in such a way that it behaves like a diffusion term and thus makes our solution much more stable without changing the essential physics. We have simulated the effects of nonlinear capacitance and surface conduction at high applied voltages and compared our slip velocity results with experimental data. Our model reduces the existing discrepancy between the numerical and experimental results significantly. [Preview Abstract] |
Monday, November 24, 2008 12:01PM - 12:14PM |
HN.00008: Frequency dependence of Brownian driving force exciting microcantilevers in fluids Matt Clark, Mark Paul, John Sader, Jason Cleveland The spectral density of the Brownian force acting on an oscillating infinite cylinder in a viscous fluid is expected to be frequency dependent from theoretical considerations. Using analytics, numerics, and experimental measurements we explore the frequency dependent nature of the Brownian force, for a micron scale rectangular and V-shaped cantilever in fluid. Surprisingly, measurements using a V-shaped atomic force microscope cantilever indicate an almost constant frequency dependence of the Brownian driving force. The origin of this unexpected behavior is explored using three-dimensional finite element simulations of the fluid-structure interaction that includes the precise cantilever geometry, where we find that it is necessary to include contributions from multiple bending modes. [Preview Abstract] |
Monday, November 24, 2008 12:14PM - 12:27PM |
HN.00009: Towards High Speed, Whole Volume Velocimetry of Unsteady Microscale Flows Steven A. Klein, Jonathan D. Posner Development of a diagnostic platform for measuring three dimensional velocity fields in whole microscopic volumes is presented. The imaging system is based on Nipkow spinning disk based confocal microscopy. The confocal system provides for optical sectioning thinner than 500 nm which allows for rejection of light originating from out of focus particles. Out of focus light in standard microPIV typically results in depth averaging, poor SNR, and the inability to measure unsteady flows. Spinning disk confocal imaging provides depth resolved imaging, high SNR images with increased particle volume fractions, and imaging rates of 5000 frames per second. Volumetric scanning is obtained by rapid translation of the high numerical aperture objective using a piezo stage. High speed optical sectioning and volumetric scanning of microscopic volumes can be used for real time visualization and velocimetry of three dimensional flows and cellular processes. [Preview Abstract] |
Monday, November 24, 2008 12:27PM - 12:40PM |
HN.00010: Whipping charged jet instabilities within dielectric liquid baths Guillaume Riboux, Alvaro G. Marin, Ignacio G. Loscertales, Antonio Barrero Capillary liquid flows have shown their ability to generate micro and nano structures of interest in several technological fields. Most of these techniques resort to deforming and stretching a liquid thread by applying forces of different nature (hydrodynamic, electrical, etc). Electrospray, employing electrical forces, is the most popular and powerful technique to produce particles in the nanometric range. A charged liquid jet issued from a Taylor cone may develop a special type of nonaxisymmetric instability which manifests itself as a series of fast and violent lashes of the charged jet. Recently, we have found that this instability is also present in electrosprays within a liquid bath which is essentially the same phenomenon as in air. However, within a liquid bath, the jet forms more easily and its oscillations are much slower. Taking advantage of this situation, we have used a high speed camera to experimentally characterize the whipping instability taking place inside liquid baths in terms of the governing parameters: flow rate and applied electric field. [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