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 LN: Micro Fluids V |
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Chair: Alexander Alexeev, Georgia Institute of Technology Room: 201 |
Monday, November 24, 2008 3:35PM - 3:48PM |
LN.00001: Directional Liquid Spreading on Asymmetric Nanostructured Surfaces Kuang-Han Chu, Rong Xiao, Evelyn N. Wang We investigated the ability to manipulate the directionality of liquid spreading by using asymmetric nanostructured surfaces. The nanostructures were composed of silicon pillars with diameters of 250 nm with one side coated with a gold film of thicknesses ranging from 250 nm to 400 nm. Due to the thermal expansion mismatch of the materials, the pillars deflected to angles ranging from 5 to 15 degrees, where the deflection angle was dependent on the thickness of the gold layer. We demonstrated that such asymmetrical structures allow the advancing side of the droplet to spread, while pinning the receding side of the droplet. Detailed experiments were performed to characterize the effect of material properties and nanostructure deflection angle on spreading dynamics. The surface tension of the liquid was also varied to examine the effect on spreading velocity. To interpret the data, we developed a model using an energy minimization approach, which accounted for both the effects of material properties and geometry. This work provides insight into designing asymmetric structures for controlling microfluidic systems. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LN.00002: Pulsatile flow transport in microscale cavities Derek Rinderknecht, Morteza Gharib Critical to the impact of microfluidics is the ability to transport fluids and biomolecules effectively, particularly at the size scales involved. In this context a bio-inspired pumping mechanism, the valveless impedance pump, was explored for applications in microfluidics ranging from micro total analysis systems to microchannel cooling with the aim of using the pulsatile flow output of the pump to augment transport at low Reynolds numbers. Micro PIV was used to study the affect of both steady and pulsatile flows on transport in microscale rectangular cavities. Ventilation of the cavity contents was examined in terms of the residence time or average time a particle remains in the cavity region. Empirical velocity fields were analyzed using Lagrangian Coherent Structures to determine the impact of unsteadiness on time dependent boundaries to fluid transport present in the flow. Experimental results show that there are both frequencies which are beneficial and detrimental to cavity ventilation as well as certain frequencies which more evenly distribute particles originating in the cavity throughout the freestream. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LN.00003: Three-dimensional transport of an optically induced electrothermal microvortex Stuart Williams, Sean Peterson, Aloke Kumar, Steve Wereley A novel, 3D microfluidic instability is induced from intense laser irradiation applied to a fluid sample contained within a parallel-plate electrode microfluidic chamber. Particles follow the streamlines of this typically toroidal vortex, traveling into and out of focus, which can be visualized with the periodically-varying diffraction ring patterns. These microfluidic vortices assist particle manipulation schemes (e.g. concentration, separation, patterning) as well as show promise as microfluidic mixers. The three-dimensional structure of these vortices is explored using a diffraction-based extension of micron resolution particle image velocimetry ($\mu $PIV) in which the radius of the outermost circle in the diffraction pattern is employed as a metric for the out-of-plane position of the tracer. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LN.00004: Generating double emulsions W/O/W in a PDMS system by controlling locally the wetting properties of the channel Herve Willaime, Nicolas Pannacci, Pol Grasland-Mongrain, Emilie-Marie Soares, Michael Benzaquen, Patrick Tabeling In microfluidic systems, it has been shown that wetting properties of the wall of the microchannel are of crucial importance for the generation of emulsions [1]: to generate an emulsion in a fluid, the continuous phase must wet the walls of the channel better than the dispersed phase. In the particular case of alternate double emulsions (water in oil in water), it is necessary to pattern the wetting properties of the channel. In this paper, we present preliminary works on the control of wetting properties on PDMS microchannels for the generation of double emulsion. The method we have chosen is inspired by the works of Allbritton [2] and Kumacheva [3] by UV-grafting a hydrophilic polymer onto the surface inside the microchannel. Once the channels are grafted, it is possible to obtain alternate double emulsions. We will present the objects obtained with such microchannels and will focus on their structures and on their stabilities. [1]$^{ }$Dreyfus R., Tabeling P., Willaime H., \textit{Physical Review Letters}, \textbf{2003}, 90(14). [2] Hu S., Ren X., Bachman M., Sims C., Li G.P., Allbritton N. \textit{Anal. Chem}. \textbf{2004, }76, 1865-1870 [3] Seo M., Paquet C., Nie Z., Xua S., Kumacheva E. \textit{Soft Matter}, \textbf{2007}, 3, 986--992 [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LN.00005: Enzymatic Reactions in Microfluidic Devices W.D. Ristenpart, J. Wan, H.A. Stone We establish simple scaling laws for enzymatic reactions in microfluidic devices, and we demonstrate that kinetic parameters obtained conventionally using multiple stop-flow experiments may instead be extracted from a single microfluidic experiment. Introduction of an enzyme and substrate species in different arms of a Y-shaped channel allows the two species to diffuse across the parallel streamlines and to begin reacting. Measurements of the product concentration versus distance down the channel provide information about the kinetics of the reaction. In the limit where the enzyme is much larger (and thus less diffusive) than the substrate, we show that near the entrance the total amount of product ($P$) formed varies as a power law in the distance $x$ down the channel. For reactions that follow standard Michaelis-Menten kinetics, the power law takes the form $P\sim(V_{max}/K_m) x^{5/2}$, where $V_{max}$ and $K_m$ are the maximum reaction rate and Michaelis constant respectively. If a large excess of substrate is used, then $K_m$ is identified by measuring $V_{max}$ far downstream where the different species are completely mixed by diffusion. Numerical simulations and experiments using the bioluminescent reaction between luciferase and ATP as a model system are both shown to accord with the model. We discuss the implications for significant savings in the amount of time and enzyme required for determination of kinetic parameters. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LN.00006: AC Electrowetting and nanodrop ejection on Conducting Parallel Electrodes Lu Zhang, Nishant Chetwani, Peter Mushenheim, Yingxi Elaine Zhu, Hsueh-Chia Chang The variation of contact angle for a drop of size $a$ on conducting parallel electrodes is shown via goniometry to be a strong function of the frequency of an applied electric field. While the contact line demonstrates the usual DC electrowetting behavior at low frequencies, no electrowetting was observed at frequencies higher than $\omega _c \sim \frac{D}{\lambda a}$, corresponding to the inverse \textit{RC} time scale for electrode screening. Below this screening frequency, the electric field is focused towards the contact line and leads to nanodrop ejection at a threshold voltage that is frequency dependent. Because of electrode screening, this threshold voltage for nanodrop ejection corresponds to a unique threshold electric field, which is captured with an asymptotic analysis. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LN.00007: Simulation of gas and water management strategies in PEM fuel cells for UAV power Nasir Wade, Sonya Smith Proton exchange membrane fuel cells (PEMFC) a involve a number of complex fluid phenomena that are not well understood. The focus of this research is to design a fuel cell that addresses the issues of gas and water management for the power requirements for an Unmanned Arial Vehicle (UAV). Often in conventional stack design, PEM fuel cells are connected electrically in series to create the desired voltage and feed from a common fuel or oxidant stream. This method of fueling, often leads to an uneven distribution of fluid within the stack, causing issues such as cell flooding, dehydration of membrane and inevitably poor fuel cell performance. Generally, fuel cell designers and developers incorporate higher stoichiometric gas flow rates and use flow field designs with high pressure drops in order to counter this phenomenon, ensuring even gas distribution. This method, although effective for water removal, leads to added cost and higher levels of wasted fuel. Using a simulation based approach we demonstrate the feasibility and effectiveness of an individual fuel and oxidant flow distribution, integrated with an individual sequential exhaust technique for a 6-8 cell stack which outputs 300-500 Watts of power. Using varied exhaust configurations the most optimal active gas management strategy will be outlined and recommended to give the best stack performance. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LN.00008: Inducing Rapid Fluid Flows In Microchannels with Surface Wave Vibrations Ming Tan, James Friend, Leslie Yeo The application of MHz-order traveling wave vibrations along a microchannel cut into a piezoelectric substrate results in rapid fluid flows along the channel in the direction of the vibration propagation, up to 2~cm/s in a 50$\times$100~$\mu$m rectangular channel in our device, much faster than other methods known to the authors. The vibration energy carried along the sides and bottom of the channel is diffracted into the channel, imparting momentum to the fluid through streaming. Given the intended application of most microfluidic devices, the fluid would reasonably be expected to carry particles, and introducing micro and nanoparticles into the flow exposes transitions to chaotic behavior, particle collection, and rapid vortex formation and shedding ideal for mixing. We show experimentally and numerically what conditions are necessary for these behaviours, and explain other peculiarities of the chosen system, such as particles traveling upstream from induced forces applied via standing waves formed across the channel during mixing, and transitions between steady and chaotic flow depending on the intensity of the traveling wave vibrations. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LN.00009: Transmitting High Power RF Vibration via Fluid Couplants into Superstrates for Microfluidic Actuation James Friend, Leslie Yeo, Ming Tan Surface acoustic wave (SAW) devices provide acoustic radiation to effectively transport fluids and particles within them for applications in microfluidics, yet require the use of piezoelectric substrates with fabrication chemistry incompatible with industry standard silicon and polymer MEMS materials. Here we couple leaky SAW acoustic radiation transmitted along a lithium niobate-based device through a fluid coupling into a thin glass plate. Though simple application of Snell's law would suggest propagation of the acoustic radiation from the couplant into the glass plate is impossible, we demonstrate the radiation's propagation as a Lamb wave to the top surface of the glass plate with sufficient power to transport small fluid droplets at up to 10~mm/s. Further, we illustrate why this occurs with numerical analysis and experimental measurement of the acoustic radiation. This enables the use of standard processing techniques to fabricate an inexpensive and disposable microfluidics device together with the power transmission capabilities of SAW devices with an easily renewable coupling. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LN.00010: Vibration Induced Microfluidic Atomization Leslie Yeo, Aisha Qi, James Friend We demonstrate rapid generation of micron aerosol droplets in a microfluidic device in which a fluid drop is exposed to surface vibration as it sits atop a piezoelectric substrate. Little, however, is understood about the processes by which these droplets form due to the complex hydrodynamic processes that occur across widely varying length and time scales. Through experiments, scaling theory and numerical modelling, we elucidate the interfacial destabilization mechanisms that lead to droplet formation. Droplets form due to the axisymmetric break-up of cylindrical liquid jets ejected as a consequence of interfacial destabilization. Their 10 $\mu$m size correlates with the jet radius and the instability wavelength, both determined from a viscous-capillary dominant force balance and confirmed through a numerical solution. With the exception of drops that spread into thin films with thicknesses on the order of the boundary layer dimension, the free surface is always observed to vibrate at the capillary-viscous resonance frequency despite the surface vibration frequency being several orders larger. This is contrary to common assumptions used in deriving subharmonic models resulting in a Mathieu equation, which has commonly led to spurious predictions in the droplet size. [Preview Abstract] |
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