Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NA: Micro Fluids: General VI |
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Chair: K. Sallam, Oklahoma State University Room: Salt Palace Convention Center 150 A-C |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NA.00001: Transient Temperature Measurements of Electrokinetically-driven Microflows Zheyan Jin, Satoshi Someya, Koji Okamoto, Hui Hu While electrokinetically driven techniques (electroosmosis and electrophoresis) can greatly simplify fluid transport in microfluidics compared to conventional pressure-driven methods, a significant drawback is the internal heat generation (commonly referred to as Joule heating) caused by current flow through working fluid. This internal heat source can significantly increase fluid temperature and generate temperature non-uniformity in microfluidics. The performance of the microfluidics is therefore strongly affected by the temperature-dependent electrical conductivity, permittivity, and viscosity of the working fluid. In this study, we present the research progress made in our recent effort to develop and implement a novel molecular tagging thermometry (MTT) technique for the transient temperature measurements of working fluid inside microchannels to further our understanding about Joule heating and micro-scale heat transfer processes in microfluidics in order to minimize the detrimental effects of Joule heating for improved performances and extended capacities of microfluidic systems. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NA.00002: Electrically-driven size separation of giant vesicles Sigolene Lecuyer, Olivier Vincent, William D. Ristenpart, Howard A. Stone Giant unilamellar lipid vesicles are widely used due to their potential as model systems in biophysics. The typical preparation method, electroformation, yields very polydisperse suspensions ($\simeq$1-100 $\mu$m). Giant vesicles are fragile and size separation is problematic. Here we present an electric field technique inside a microfluidic device to separate large vesicles from a polydisperse suspension. We establish that electrohydrodynamic (EHD) flows, of the sort known to induce aggregation of colloidal particles near electrodes [Trau et al., Science 272, 706, 1996], also act on vesicles; the competition between dipolar, EHD and gravitational effects leads for the latter to a complex aggregation process, of which main features are described. This results in large vesicles stacked on top of smaller ones. After adhering small vesicles, a gentle flow can be used to remove large ones. We measure the vesicle size distributions and demonstrate that the majority of small vesicles (of diameter $\leq$20 $\mu$m) is removed from a polydisperse suspension. We discuss how this versatile technique could be used to sort more complex systems and eventually be integrated in more advanced microfluidics devices. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NA.00003: Instabilities of charged liquid jet of polymer solution and electrospinning of Nanofibers. Khaled Sallam, Anu Osta, Abdel-Rahman El-Leathy An experimental study of the mechanism of beads formation during electro-spinning of nanofibers is presented. Liquid jets of a polymer solution (Poly Ethylene Oxide (PEO)) were electrospun and the resulting PEO nanofibers were deposited on a grounded substrate. The collected nanofibers exhibited bead-like structures depending on: the polymer concentration in the electro spun solution, the applied voltage, the injection pressure and the needle-electrode distance. The micro jet near the nozzle exit was observed in-flight using digital holographic microscopy and the collected nanofibers were examined using Scanning Electron Microscopy (SEM). The measurements include initial liquid jet diameter and the wave lengths of the jet instabilities. The present results show a strong relationship between the initial jet instability (near the injector exit) and the bead formation on the collected nanofibers. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NA.00004: Columnar Transitions in Microscale Evaporating Liquid Jets Hanif Hunter, Ari Glezer Microscale evaporating liquid jets that are injected into a quiescent gaseous medium having adjustable ambient pressure are investigated over a range of jet speeds using a shadowgraph technique. The jets are formed by a laser-drilled 10 $\mu $m nozzle from a small-scale pressurized reservoir, and sub-atmospheric ambient pressure is maintained using a controllable, metered Venturi pump. The near-field jet features are captured by shadowgraph imaging using a pulsed ND-Yag laser and a 12 bit CCD camera where the field of view measured 200 $\mu $m on the side. As the ambient pressure is reduced, the jet column undergoes a series of spectacular transitions that are first marked by the appearance of vapor bubbles within the jet column. The transitions progress from columnar instabilities to series of column bifurcations to high-order branching and film formation and culminate in conical atomization of the jet column. In addition to the effects of the ambient pressure, the present investigation also considers effects of the liquid surface tension and vapor pressure on the onset, evolution, and hysteresis of the columnar transitions. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NA.00005: Double emulsions in a microfluidic device Nicolas Pannacci, Thibaut Lockhart, Herv\'e Willaime, Patrick Tabeling Double emulsions (emulsion of two liquids dispersed in a third liquid phase) are widely used in cosmetics, medicine or food industry. We are interested in producing very well controlled double emulsions in a microfluidic device and predicting the morphology (complete engulfing, non-engulfing or partial engulfing called ``janus'') from classical energetic considerations. We use a double flow focusing geometry with a 100 micrometers cross section for the PDMS microsystem. We compare theoretical and experimental morphologies flowing thirty triplets of immiscible fluids. We observe quite a good agreement and show that microfluidic technology may permit to get non standard objects. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NA.00006: Vibration Induced Microfluidic Colloidal Island Self-Assembly and Erasure Leslie Yeo, Haiyan Li, James Friend We demonstrate rich and complex pattern formation arising from the nonlinear dynamics associated with nanometer-order amplitude standing wave vibrations induced along the surface of a piezoelectric substrate upon which a small liquid drop containing a colloidal suspension is placed. At low input powers insufficient to generate streaming in the fluid, colloidal islands are assembled along the drop interface due to the action of the vibration forces directly on the particles. The position and number of these islands directly correlate with the intersection between the nodal positions of the surface waves and that of the induced interfacial capillary wave. As the input power is increased to a critical value corresponding to the onset of fluid streaming, a transient metastable state is observed in which the system oscillates between colloidal island self-assembly in a quiescent fluid and the subsequent temporal erasure of these islands due to the generation of local streaming vortices in the antinodal positions of the surface and capillary waves. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NA.00007: Charged species transport and the formation of ionic depletion/enrichment zone in the junction between micro- and nano-channel Kuan-Da Huang, Ruey-Jen Yang Charged species transport exhibits ionic exclusion and enrichment effect when the channel scale down to the magnitude of the electric double layer thickness. The effect as a semipermeable interface results in a wide range ionic depletion zone and enrichment zone in the adjacent microchannel when an electrical field passes through the nanochannel. We utilize an alternating depletion and enrichment effect to achieve the goal of the species concentration in microchannels. Besides, the bulk charge layer originates from the theory of the electroosmosis second kind is used to explain the formation of high speed recirculation within the ionic depletion zone in this study. We also used latex particle to visualize these recirculation structures to evidence the specific flow field in the junction between micro- and nano-channel. [Preview Abstract] |
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