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 G18: Microfluids: General IV |
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Chair: Ali Beskok, Old Dominion University Room: 321 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G18.00001: Thermally-Induced Mixing of Microscale Droplets for Detection of Gas-Phase Analytes Meysam Barmi, Chrysafis Andreou, Mehran Hoonejani, Brian Piorek, Martin Moskovits, Carl Meinhart In this talk, we investigate the absorption and mixing kinetics inside of sessile droplets, subject to a specified temperature gradient and evaporation rate. Such microscale droplets have free-surface interface that enables the adsorption of airborne molecules and their subsequent identification by Surface Enhanced Raman Spectroscopy (SERS). A droplet of SERS-active colloidal suspension can be mixed efficiently with the adsorbed analyte due to the Marangoni effect. This stimulates an aggregation process that results in the creation of the so called ``SERS hotspots.'' The droplets are interrogated using a Raman spectrometer to obtain highly sensitive and specific analyte detection. Efficient mixing is critical for practical measurements in many chemical and biological applications. We investigate specified thermal gradients and evaporation rates for precision mixing, and optimize these parameters for high-performance vapor detection. Numerical simulations using COMSOL Multiphysics are used to investigate droplet dynamics, by predicting fluid motion and analyte-induce aggregation kinetics. For the velocity field inside of the sessile droplet, Micro-PIV experiments are used to validate the numerical simulations. The system is tested using gas phase 4-Aminobenzenethiol as a model analyte. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G18.00002: Monodisperse droplet generation for microscale mass transfer studies Christine Roberts, Rekha Rao, Anne Grillet, Carlos Jove-Colon, Carlton Brooks, Martin Nemer Understanding interfacial mass transport on a droplet scale is essential for modeling liquid-liquid extraction processes. A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets for microscale mass transport studies. Surface treatment of the microfluidic device allows creation of both oil in water and water in oil emulsions, facilitating a large parameter study of viscosity and flow rate ratios. The unusually thin channel height promotes a flow regime where no droplets form. Through confocal microscopy, this regime is shown to be highly influenced by the contact angle of the liquids with the channel. Drop sizes are found to scale with a modified capillary number. Liquid streamlines within the droplets are inferred by high speed imagery of microparticles dispersed in the droplet phase. Finally, species mass transfer to the droplet fluid is quantitatively measured using high speed imaging. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G18.00003: Stationary microdroplets in a Hele-Shaw Cell: anchoring mechanism and flow field Sungyon Lee, R\'emi Dangla, Fran\c{c}ois Gallaire, Charles Baroud When a droplet is confined inside a Hele-Shaw cell, it adopts a ``pancake''-like shape from its relaxed spherical state, resulting in increase in its surface energy. Variations in the level of confinement lead to the gradient in surface energy, or a force, to ``anchor'' droplets against the mean external flow. In this talk, we discuss the experimental and theoretical findings of this novel anchoring mechanism. In addition, a peculiar three-dimensional flow pattern has been observed on the boundary of the anchored drop. We show the robustness of this flow pattern over various flow conditions and provide the theoretical justification based on the Marangoni stresses on the droplet interface. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G18.00004: Flow patterns of microbubble streaming in microfluidic settings Cheng Wang, Shreyas V. Jalikop, Sascha Hilgenfeldt Steady streaming flows from oscillating microbubbles have demonstrated a number of promising applications in microfluidics, including manipulating microparticles, mixing enhancement, transporting liquid and lysing vesicles. Earlier experimental studies have shown that oscillating bubbles located on or close to solid boundaries produce a variety of streaming flow patterns, such as ``fountain'' and ``anti-fountain'' flows. These flow patterns depend on the viscosity of the liquid, and on the driven frequency and amplitude. Here we take advantage of the micro-fabrication technology to create microbubbles of controlled size, position and also with different exposure to solid boundaries to study bubble streaming. Our new experimental results show that the flow topology depends not only on the frequency and amplitude, but also significantly on the solid boundaries near the bubbles. The findings are of importance towards the fundamental understanding of bubble streaming flows and helpful on guiding practical microfluidic device designs as well. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G18.00005: Bubble shrinkage and growth: A solution to carbon dioxide dissolution and solubility Milad Abolhasani, Eugenia Kumacheva, Axel Guenther Dynamic and equilibrium aspects of carbon dioxide transport across gas-liquid interfaces impact a wide range of technical, physiological and geological applications. We investigate carbon dioxide transport by continuously guiding a train of uniformly sized carbon dioxide bubbles and non-saturated liquid segments through a microchannel. The bubbles initially shrink and later expand. We quantitatively link the evolution of the bubble size to kinetic and equilibrium characteristics of carbon dioxide dissolution. While the initial velocity of carbon dioxide bubbles and the length of liquid segments significantly affect the dissolution of carbon dioxide, these parameters cannot be externally imposed, due to the dynamic nature of microscale gas-liquid flows. We use an automated microfluidic platform (gas impermeable) in combination with an image-based feedback control strategy to keep the dependent parameters constant and systematically determine the rate of carbon dioxide dissolution and the equilibrium solubility of carbon dioxide-liquid mixtures for a wide range of pressures, temperatures and liquids in a flowable format. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G18.00006: Dissolution without shrinking: the Epstein-Plesset problem in a channel flow Suin Shim, Jiandi Wan, Howard Stone The dynamics of dissolution of CO2 bubbles in microfluidic channels is studied experimentally and theoretically. The results show first a rapid dissolution regime followed by a second apparent equilibrium regime where the bubble radius is constant. We observed that, regardless of the surfactant concentration, bubbles stopped shrinking after $\sim$30ms following generation. In the equilibrium regime, the bubble sizes are larger at low concentration of surfactants than at high concentrations. We interpret the results by considering the pressure variation along the microfluidic channel and modify the Epstein-Plesset model for bubble dissolution. Our modified model with a time dependent pressure term as the bubbles move along the channel explains the transient and steady behaviors of CO2 bubbles in a channel flow. In particular, the model rationalizes why the bubbles continue to dissolve even though the bubble radius is constant, since the pressure change accompanying bubble translation causes a shape change compensating the shrinkage due to dissolution. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G18.00007: Cusps and spouts in microfluidic systems Aur\'elien Duboin, Florent Malloggi, Fabrice Monti, Patrick Tabeling By injecting mineral oil (inner phase) and polymer solutions (outer phase), in a microfluidic flow focusing geometry, we observed the formation of cusps. These cusps undergo a transition from a steady state, to a thin cylindrical spout (oil in polymer). These oil spouts, do not touch the walls, and are surprisingly stable (they do not break into droplets). We study the nature of the cusp-spout transition, and find it is of first order. By taking advantage of the stability of the jet, we expect to synthesize micro-wires with this approach. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G18.00008: Formation of Satellite and subsatellite droplets in a flow-focusing junction for viscoelastic fluids Denis Funfschilling, Odile Carrier, Huai-Zhi Li The formation of a cascade of satellite and subsatellite oil droplets is observed in a flow-focusing microfluidic junction (250 micrometer of characteristic length) in the presence of surfactant (Sodium Dodecyl Sulfate), and polymer (100 to 1000 ppm of PAAm of high molecular mass). The size and distribution of the satellite and subsatellite droplets is quite reproducible. One and only one satellite droplet is formed in the dripping regime in case of Newtonian fluids [1]. When PAAm is added to the solution, the viscosity becomes viscoelastic and satellite droplets are many. The mechanism of breakup leading to multiple satellite droplets is self-repeating, as observed in previous work [2, 3]. At low frequency, the number of satellite droplet can go up to 7 or more. The distribution is generally very structured: a unique mother satellite droplet is surrounded by two daughter droplets, each of these daughter droplet is surrounded again by two grand-daughter droplets so that there are 4 daughter droplets in total. The ratio in volume between each generation is about 30. [1] Funfschilling D., Debas H., Li H.-Z.and Mason T. G., 2009, Phys. Rev. E, 80, 015301 [2] Muzzio F. J., Tjahjadi M. and Ottino J. M. 1991, Phys. Rev. Lett. 67, 54-57 [3] Tjahjadi M., Stone H. A. and Ottino J. M. 1992, J. Fluid Mech. 243, 297-317 [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G18.00009: Capillary focusing in the microfluidic Hele-Shaw channel Alexander Leshansky, Yuli Halupovich, Len Pismen For many droplet-based mircofluidic applications it is desirable to create monodisperse droplets with adjustable volume and production rate directly in the microfluidic device. In this work we address the recently developed ``step emulsification'' technique that relies on an abrupt change in the aspect ratio of a single shallow and wide microchannel that merges into a deep reservoir. Such geometry yields a rapid and well controlled destabilization of confined co-flowing streams of immiscible liquids at the step separating the channel and the reservoir forming highly monodisperse droplets of the size comparable to the channel depth. The distinctive feature of the process is the capillary focusing effect, whereas the interface between the two fluids takes the shape of a tongue narrowing in the flow direction just ahead of the step; at the step the tongue tip becomes unstable generating droplets. We present theoretical modeling of the capillary focusing effect. The developed small-capillary-number asymptotic theory and the results of VOF numerical simulations show a good agreement with the experiments and provide some important insights into the underlying physics of the phenomenon. [Preview Abstract] |
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