Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session NB: General Microfluidics |
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Chair: Brian Storey, Olin College of Engineering Room: Hilton Chicago Waldorf |
Tuesday, November 22, 2005 11:01AM - 11:14AM |
NB.00001: Viscous folding in microfluidics Thomas Cubaud, Thomas G. Mason We explore folding instabilities of viscous threads co-flowing with miscible Newtonian liquids in diverging microchannels. Surface tension does not play a role in the instabilities, yet we find a remarkably wide range of flow morphologies, such as folds, heterogeneous viscous flow, and the formation of “viscous droplets”, dendrites, and plumes. By systematically varying the relative flow rate and the relative viscosity, we have created a detailed map of these diverse instabilities over several orders of magnitude in a dimensionless dynamical “phase” space. Our results highlight the advantages of the microfluidic approach and have important implications in geophysics and materials processing. [Preview Abstract] |
Tuesday, November 22, 2005 11:14AM - 11:27AM |
NB.00002: Designing elastic sheets to self-assemble in a viscous environment. Silas Alben, Michael Brenner A recent work by Boncheva et al. (Proc. Nat. Acad. Sci. 2005 102: 3924-3929) has raised some basic issues about designable self-assembly within the context of planar elastic sheets which fold into 3D structures under magnetic forces. While being agitated in water, millimeter-scale structures were shown to fold with varying success depending on the locations of magnets on the sheets. Our work considers how to design such structures, an understanding of which will be necessary when moving this process to the micron scale. Among the important parameters are the geometry of the flat sheet, the configurations of the magnets, and the ratios of magnetic to elastic forces. We consider this problem using a numerical model of an elastic sheet, and restrict to the simpler case of electrostatic forces in a quasi-static limit. We identify a simple algorithm for choosing configurations of electrostatic charges, and select ratios of charge strength to elastic energy using physical arguments. We then demonstrate our algorithm on dynamical foldings of a sphere and more general geometries, in the overdamped viscous regime. [Preview Abstract] |
Tuesday, November 22, 2005 11:27AM - 11:40AM |
NB.00003: Electrokinetics due to gas phase Plasma Polarization. Siddharth Maheshwari, Hsueh-Chia Chang Exposed AC drop electrodes are shown to generate a local negative plasma cloud due to asymmetric gas-phase ionization reactions. This negative plasma can be transferred onto a disjoint flat interface during the cathodic half cycle of the generating electrode. The resulting interfacial electrokinetics are quite different depending upon the location of the charge, with normal and tangential interfacial Maxwell stresses dominating for each case respectively. Microjets eject from the plasma-generating drop with normal Maxwell stress. The microjet velocity and radius are determined by a balance among the Maxwell, extensional and capillary stresses, which we decipher via matched asymptotics. Internal vortices are produced without significant interfacial distortion with tangential Maxwell stress. Both electrokinetic phenomena are strong functions of the AC frequency and are most pronounced at a frequency corresponding to the inverse charge relaxation time of the gas plasma. [Preview Abstract] |
Tuesday, November 22, 2005 11:40AM - 11:53AM |
NB.00004: Simple and double emulsions via electrospray Antonio Barrero, Alvaro G. Marin, Ignacio G. Loscertales Generation of nanoemulsions is of great interest in medical and pharmaceutical applications; drug delivery or antiviral emulsions are typical examples. The use of electrosprays for dispersing liquids inside liquid insulator baths have been recently reported, (Barrero et al. \textit{J. Colloid Interf. Sci.}\textbf{\textit{ }}\textbf{272}, 104, 2004). Capsules, nanotubes and coaxial nanofibers have been obtained from electrified coaxial jets (Loscertales et al. \textit{Science }\textbf{295}, n. 5560, 1695, 2002; \textit{J. American Chem. Soc. }\textbf{126}, 5376, 2004). Here we present a method for making double emulsions (both water-oil-water and o/w/o) based on the generation of compound electrosprays inside insulator liquid baths. Basically, a conducting liquid injected throughout a capillary needle is electroatomized in cone-jet mode inside a dielectric liquid bath. A third insulating liquid is injected inside the Taylor cone to form a second meniscus. Then, a steady coaxial jet, in which the insulating liquid is coated by the conducting one, develops. A double emulsion forms as a result of the jet breaking up into compound droplets electrically charged. Experimental results carried out with glycerine and different oils in a bath of heptane are reported. [Preview Abstract] |
Tuesday, November 22, 2005 11:53AM - 12:06PM |
NB.00005: Cusp formation in drops inside Taylor cones Alvaro G. Marin, Ignacio G. Loscertales, Antonio Barrero Here, we report the formation of cusp in insulating drops inside compound Taylor cones. The action of the electrical shear stress acting on the outer interface, which is transmitted by viscous forces inside the Taylor cone, tends to deform the drop of insulating liquid placed inside. For appropriate values of the capillary number, the insulating drop develops a steady cusp angle which depends on both the capillary number and the conducting to insulating viscosity ratio. A self-similar analysis has been developed to qualitatively describe the flow inside these compounds Taylor cones. Any perturbation of the cusp gives rise to an intermittent emission of tiny droplets; this effect may recall the tip-streaming observed by G.I. Taylor in his four-roll mill device. This emission can be stabilized by an appropriate control of the injected flow rate of the insulating liquid. When the capillary number increases, the cusped interface turns into a spout which flows coated by the conducting liquid forming the electrified coaxial jet which has been successfully employed for the production of nanocapsules, coaxial nanofibers and nanotubes (\textit{Science} 295, n. 5560, 1695, 2002; \textit{JACS} 126, 5376, 2004). [Preview Abstract] |
Tuesday, November 22, 2005 12:06PM - 12:19PM |
NB.00006: Electrohydrodynamic instability in microchannels: time dependent forcing Brian Storey, David Boy The interaction of fluid electrical conductivity gradients and applied electric fields are known to be susceptible to electrohydrodynamic instabilities. In microfluidic applications, it has been shown that such instabilities can generate chaotic flows at low Reynolds number. This work considers stability in a flow channel with an electric field applied perpendicular to a diffuse interface of two fluids with different electrical conductivities. The applied electric field, which drives the instability, is taken to have both AC and DC components. The time dependent nature of the electric body force can have a stabilizing or destabilizing effect relative to the DC case. The linearized analysis is validated with direct numerical simulations. [Preview Abstract] |
Tuesday, November 22, 2005 12:19PM - 12:32PM |
NB.00007: Effect of Aspect Ratio on the Performance of Spiral-Channel Viscous Micropump M.I. Kilani, A. Al-Salaymeh, A.T. Al-Halhouli, M. Gad-el-Hak The effect of aspect ratio on the flow field in a newly developed spiral-channel viscous micropump has been investigated. An approximate 3-D analytical solution to the flow field in the lubrication limit that ignores channel curvature but accounts for finite wall-height is first developed. A number of models for spiral pump with different aspect ratios are then built and analyzed using the finite-volume method. The numerical and analytical results are in good agreement and tend to support one another. The results are also compared with an approximate 2-D analytical solution developed for infinite aspect ratio, which neglects the effect of sidewalls and assumes uniform velocity distribution across the channel width. This approximation was found valid for aspect ratios of 10 or greater. For aspect ratios less than 10, the flow rate deviates from the 2-D approximation. Shape factors were developed in the present work to express the effect of the pressure difference and boundary velocity on the flow rate at various aspect ratios for both moving and stationary walls. It has been found that the flow rate varies linearly with both the pressure difference and boundary velocity, which validates the linear lubrication model employed at the microscale. [Preview Abstract] |
Tuesday, November 22, 2005 12:32PM - 12:45PM |
NB.00008: Squeeze film flow analysis of pulsed microjet actuators Max Roman, Arnaud Goullet, Nadine Aubry Microfabrication (MEMS) offers a platform to build miniaturized inexpensive, reliable, light-weight, and low power actuators and sensors. Such small actuators can have a very unique function in microfluidics, where they can serve as micromixers, pumps, and non-invasive cell manipulators. In this work, theoretical modeling and computer simulation is used to analyze pulsed microjet actuators. We have derived a low dimensional theoretical model, which takes into account the coupling between the electrostatic actuation, the solid deformation of the membrane, and the squeeze flow in the cavity. The pressure generated in the cavity by the deforming membrane is described in terms of actuation frequency and membrane deflection amplitude. The cavity pressure characterizes the performance of the microjet, which is measured in terms of nozzle exit velocity, and the microjet's operation is optimized for a minimum voltage input. To validate the model, we use computer simulation to evaluate the pressure and the nozzle exit velocity over the range of parameters of the problem. [Preview Abstract] |
Tuesday, November 22, 2005 12:45PM - 12:58PM |
NB.00009: Deploying Liquid Filaments and Suspensions with an Electrohydrodynamic Liquid Bridge D.A. Saville, S. Korkut, H.F. Poon, C.-H. Chen, I.A. Aksay We show that a dynamic liquid bridge can be formed by deploying the filament issuing from a Taylor Cone onto a surface with the nozzle and surface held at different electric potentials. This configuration differs sharply form the familiar `electrospinning' configuration where the filament whips violently. Nevertheless, although the aspect ratio (length/diameter) exceeds the Plateau limit by more than two orders of magnitude the bridge is stable. Here we report on the stability characteristics and show that such a bridge can be used to `print' sub-micron scale features on a moving surface with both clear fluids and suspensions. [Preview Abstract] |
Tuesday, November 22, 2005 12:58PM - 1:11PM |
NB.00010: Evaluation of a compact model for prediction of liquid film thickness in stratified two-fluid microchannel flows Julie E. Steinbrenner, S\'{e}bastien Vigneron, Fu-Min Wang, Carlos H. Hidrovo, Jae-Mo Koo, Eon-Soo Lee, Ching-Hsiang Cheng, John K. Eaton, Kenneth E. Goodson Interaction between gas and liquid phases in separated flow through a channel governs flow regimes and influences the behavior of each phase. However, this interaction is not well modeled by traditional single-phase parameters. A compact model is presented which accounts for the interaction of the two phases by employing a modification to the single-phase friction factor formulation for rectangular channels. The modification represents the interaction between phases using a multiplicative factor derived from an analytical solution to stratified flow between parallel plates. Film thickness and pressure drop predictions from the model are compared with analytical solutions to two-fluid flow in a rectangular duct. Computational results are compared with experimental measurements of the liquid film thickness in stratified two-phase flow in rectangular microchannels (D = 50-500 $\mu$m) for various aspect ratios. A physical interpretation of experimental and computational results is presented. [Preview Abstract] |
Tuesday, November 22, 2005 1:11PM - 1:24PM |
NB.00011: Two-Stage Particle Trapping by Aggregation and DEP Reversal in Micro-Vortices Zachary Gagnon We report a new negative DEP trapping mechanism to induce migration of small particles into electrode gaps. A high frequency AC voltage is applied to a serpentine wire above an aqueous dilute suspension of micron size particles to produce strong electro-osmotic vortex flows. Such flows convect particles from the bulk suspension to the high field regions of the serpentine wire but the particles are suspended away from the wire due to a balance between positive DEP and buoyancy forces. The suspended particles aggregate within the vortex due to induced-dipole interaction. The larger aggregates have a lower DEP crossover frequency and suffer a negative DEP force in an electrolyte whose permittivity is properly tuned with zwitter ions. The aggregates then migrate from the vortex and are trapped in the low-field gap region. [Preview Abstract] |
Tuesday, November 22, 2005 1:24PM - 1:37PM |
NB.00012: Microscopic Rayleigh Droplet Beams R.B. Doak, D. Starodub, U.J. Weierstall, J.C.H. Spence A periodically triggered Rayleigh Droplet Beam (RDB) delivers a perfectly linear and periodic stream of identical, monoenergetic droplets that are phase-locked to the trigger signal.\footnote{ Image at: http://physics.asu.edu/doak. \hspace{0.1 cm} Funded by NSF 0429814.} The droplet diameter and spacing are easily adjusted of choice of nozzle diameter and trigger frequency. Any liquid of low viscosity may be emloyed as the beam fluid. Although the field of nanofluidics is expanding rapidly, little effort has yet been devoted to ``external flows'' such as RDB's. At ASU we have generated RDB's of water and methanol down to 2 microns in droplet diameter. Nozzle clogging is the sole impediment to smaller droplets. Microscopic Rayleigh droplet beams offer tremendous potential for fundamental physical measurements, fluid dynamics research, and nanofabrication. This talk will describe the apparatus and techniques used at ASU to generate RDB's (surprisingly simple and inexpensive), discuss the triboelectric phenomena that play a role (surprisingly significant), present some initial experimental fluid dynamics measurements, and briefly survey RDB applications. Our particular interest in RDB's is as microscopic transport systems to deliver hydrated, undenatured proteins into vacuum for structure determination via serial diffraction of x-rays or electrons. This may offer the first general method for structure determination of non-crystallizable proteins. [Preview Abstract] |
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