Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R15: Microscale Flows: Mixing and Reactions |
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Chair: Hirofumi Shintaku, Kyoto University Room: 3022/3024 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R15.00001: Fluid Mechanics of the ``Vortex Fluidic Device'' Stuart Dalziel, Joshua Britton, Colin Raston The Vortex Fluidic Device (VFD) provides a new ``green'' alternative for many industrially important organic chemistry processes including the generation of biodiesel. Improved chemical kinetics have also been demonstrated for a number of reactions. This relatively simple device, comprising essentially of a rapidly rotating tube, provides advantages ranging from reduced energy requirements and waste streams to high flow rates and the avoidance of clogging. The VFD is effective due to the interplay between fluid mechanics and chemistry providing near optimal conditions for the required reactions. This contribution provides an insight into the rich fluid mechanics of the device. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R15.00002: Nuclemeter: A Reaction-Diffusion Column for Quantifying Nucleic Acids Undergoing Enzymatic Amplification Haim Bau, Changchun Liu, Chitvan Killawala, Mohamed Sadik, Michael Mauk Real-time amplification and quantification of specific nucleic acid sequences plays a major role in many medical and biotechnological applications. In the case of infectious diseases, quantification of the pathogen-load in patient specimens is critical to assessing disease progression, effectiveness of drug therapy, and emergence of drug-resistance. Typically, nucleic acid quantification requires sophisticated and expensive instruments, such as real-time PCR machines, which are not appropriate for on-site use and for low resource settings. We describe a simple, low-cost, reactiondiffusion based method for end-point quantification of target nucleic acids undergoing enzymatic amplification. The number of target molecules is inferred from the position of the reaction-diffusion front, analogous to reading temperature in a mercury thermometer. We model the process with the Fisher Kolmogoroff Petrovskii Piscounoff (FKPP) Equation and compare theoretical predictions with experimental observations. The proposed method is suitable for nucleic acid quantification at the point of care, compatible with multiplexing and high-throughput processing, and can function instrument-free. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R15.00003: ABSTRACT WITHDRAWN |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R15.00004: Viscous thread behavior in branching microchannels Thomas Cubaud, Xiaoyi Hu, Martin Sauzade We experimentally study the properties of viscous core-annular flows using miscible fluids in bifurcating microchannels. A viscous filament is first generated using a square hydrodynamic focusing junction by injecting a thick fluid into the central channel and a thin fluid from the side-channels. This method allows us to produce miscible fluid threads of various sizes and lateral positions in the channel, and enables the systematic study of thread transport and stability from low to moderate Reynolds numbers in branching microfluidic networks. We examine, in particular, the role of viscous buckling instabilities on thread behavior and the formation of complex viscous mixtures and stratifications at the small-scale. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R15.00005: Stirring and Mixing in Microstreaming Transport Flows Sascha Hilgenfeldt, Bhargav Rallabandi, Lin Guo, Cheng Wang Microfluidic mixing in closed geometries has inspired a wealth of theoretical models focusing on the simplest flow ingredients necessary for efficient micromixing. For transport devices with given throughput, often required in applications, such fundamental descriptions have largely been absent. We suggest microbubble streaming flows, for which we have developed an analytical flow field description, as a model system for these transport flows. Conducting experiments with nanoparticle tracers, we focus on the advection field (the stirring dynamics) in flows driven by modulated ultrasound. A simplified theory of stirring yields predictions of rapid decay of mixing variance measures for a limited time, indicating optimum time scales for modulation of the driving. Experiments confirm that the model captures the physical features of the stirring (and thus mixing) mechanism. Set-ups with multiple bubbles can be used to further optimize the outcome. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R15.00006: Efficiency enhancement of membraneless fuel cells by using Dean vortices Massimiliano Rossi, Christian J. K\"{a}hler To prove the concept of efficiency enhancement of membraneless fuel cells (Ferrigno et al., J. Am. Chem. Soc., 2002) by means of Dean vortices, we performed detailed experiments using Astigmatic Particle Tracking velocimetry (Cierpka et al., Meas. Sci. Technol., 2011). The basic idea is to use transversal secondary flows to stir the fluid inside the two co-laminar streams of the fuel cell (Yoon et al., Lab chip, 2006). To systematically characterize the performance of this approach, we proposed to measure simultaneously the voltage/current intensity output of the device and the corresponding 3D velocity field for different geometries and flow regimes. In this work, we show the first results obtained on a fuel cell with rectangular cross-section of 600 $\mathrm{\mu}$m x 400 $\mathrm{\mu}$m and radius of curvature $r$ = 1 mm. A device with the same cross-section and a straight microchannel was used as a reference. Different flow rates were investigated leading to Reynolds numbers from 3.6 to 18. Additionally, to study the implications of possible variations of the co-laminar stream configuration, the topology of the interface between the two streams was measured using a particle-based interface reconstruction approach (Rossi et al., Meas. Sci. Technol., 2011). [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R15.00007: Boundary layer enrichment for reversible surface reactions in stirred microfluidic flows Joseph Kirtland, Lennon McCartney The local rate of scalar transport from a fluid stream to a reactive surface depends on the concentration of scalar in the fluid elements incident on the surface and the character of the flow. Unstirred, uniaxial flows tend to develop thick depleted regions adjacent to the reactive surface (concentration boundary layers) that grow to the full scale of the flow, resulting in low rates of scalar transport. In the case of an irreversible surface reaction, this effect can be mitigated through the introduction of a three-dimensional flow to achieve mixing in the bulk: efficient mixing will result in the consistent presentation of fluid with the average scalar concentration to the reactive surface and the three-dimensional flow will maintain a thinner concentration boundary layer. Coupled reversible surface reactions, such as the oxidation and reduction of a single species occurring at opposite surfaces of an electrochemical flow cell, will complicate the analysis of such a system: depletion of the reactant of the forward reaction is accompanied by enrichment of the product of the forward reaction, which is necessarily the reactant of the reverse reaction. In certain flows, this fluid enriched with reactant for the reverse reaction can bypass the stirred bulk, leading to increased scalar transport, even in cases of inefficient bulk mixing. We present numerical and experimental results in several such flows and discuss situations where this enrichment can be beneficial or detrimental. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R15.00008: Symmetry-breaking bifurcations and enhanced mixing in microfluidic cross-slots Rob Poole, Simon Haward, Paulo Oliveira, Manuel Alves We investigate, both experimentally and numerically, a new subcritical bifurcation phenomenon for a Newtonian fluid flowing through three-dimensional cross-slot geometries. At low Reynolds numbers the flow remains steady and symmetric. For the case of square inlets and outlets, at a critical Reynolds number of approximately 40 (based on average velocity) a pitchfork bifurcation is observed beyond which the unstable symmetrical solution is replaced by a pair of steady asymmetric solutions. Sensitivity of this critical Reynolds number to the initial conditions of the simulation, resulting in a small degree of hysteresis, suggests a subcritical instability. At higher flowrates the flow becomes unsteady. The effects of channel aspect ratio are investigated on the critical conditions and excellent agreement is found between three-dimensional finite volume simulations and flow visualisation experiments in microfluidic channels. Finally we suggest this new flow bifurcation could be an effective method of enhancing mixing in microfluidic channels as significant increases in mixing quality are observed beyond the bifurcation. This enhancement occurs at flowrates more than a factor of two smaller than those observed in the well-known T-channel micromixer. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R15.00009: Mixing and Moffatt Eddies in the Rectangular Channel Miron Kaufman, Petru S. Fodor A detailed analysis of the fluid flow in the infinitely long rectangular channel is performed by combining numerical and analytical methods. While finite element analysis numerical models are used to extract the transversal velocity field, an analytical model in the limit of zero Reynolds numbers is used to determine the longitudinal component of the fluid velocities. Using this high resolution 3D model developed for the fluid flow in channels with aspect ratios ranging from 1 to 10, we identify the position and extent of Moffatt eddies, which impede mixing. We study the fluid mixing by using entropic measures. This model is relevant to polymer processing extruders used as mixers. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R15.00010: Studies on two-phase ionic liquid-aqueous flows in small channels of various sizes Dimitrios Tsaoulidis, Maxime Chinaud, Qi Li, Panagiota Angeli Two-phase flows in intensified small-scale systems find increasing applications in (bio)chemical analysis and synthesis, fuel cells, polymerisation, and separation processes (solvent extraction). Ionic liquids are emerging as a useful chemical in different areas of interest because of their unique properties such as negligible volatility and flammability, and good thermal and radiation stability. In this work, the hydrodynamic characteristics during plug flow have been investigated in detail. Experiments were carried out in Teflon channels of different sizes, i.e. 0.5, 1, and 2 mm internal diameter using two-phase systems relevant to spent nuclear fuel reprocessing, i.e. TBP/ionic liquid (30{\%}, v/v)-nitric acid solutions. Important mixing characteristics and circulation patterns within the aqueous plugs have been studied by means of Particle Image Velocimetry (PIV). Finally, the mechanism of plug flow formation and the resulting plug size were investigated using Computational Fluid Dynamic (CFD). [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R15.00011: Buckling instability of thin films as a means to control or enhance fluid flow within microchannel Behrouz Tavakol, Aschvin Chawan, Douglas Holmes Here we show that the buckling of thin, flexible plates can be used for pumping fluids, controlling the flow rate, and mixing different media within a microfluidic channel. A confined, dielectric elastomeric film buckles out of the plane when exposed to an electric field. We use an electrolytic fluid solution as the electrode to enable buckling at relatively low voltages, and to enhance the rate of deformation. When embedded in a microfluidic channel, this mechanism can be used as a microvalve that controls the flow rate, or as a micropump that alters the flow rate. A similar mechanism can be used to aid diffusion between two adjacent laminar streams and improve mixing. This novel means for dielectric actuation may improve voltage application, and the buckling microstructures may be used in variety of applications to accurately control and manipulate fluid flow in a microchannel. [Preview Abstract] |
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