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 D18: Microfluids: General II |
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Chair: H, Auradou, FAST, Paris Room: 321 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D18.00001: Programming fluid flow with microstructures Hamed Amini, Mahdokht Masaeli, Dino Di Carlo Flow control and fluid interface manipulation in microfluidic platforms are of great importance in a variety of applications. Current approaches to manipulate fluids generally rely on complex designs, difficult-to-fabricate 3D platforms or use of active methods. Here we show that in the presence of simple cylindrical obstacles (i.e. pillars) in a microchannel, at moderate to high flow rates, streamlines tend to turn and stretch in a manner that, unlike intuition for Stokes flow, does not precisely reverse after passing the pillar. The asymmetric flow behavior up- and down-stream of the pillar due to fluid inertia manifests itself as a total deformation of the topology of streamlines that effectively creates a \textit{net }secondary flow which resembles the recirculating Dean flow in curving channels. Confocal images were taken to investigate the secondary flow for a variety of microstructure settings. We also developed a numerical technique to map the fluid motion in the channel which is utilized to characterize the secondary flow as well as to engineer the fluid patterns within the channel. This passive method creates the possibility of exceptional control of the 3D structure of the fluid within a microfluidic platform which can significantly advance applications requiring fluid interface control (e.g. optofluidics), ultrafast mixing and solution control around cells. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D18.00002: Mapping low Reynolds number cavity flow phenomena inside microfluidic devices Ramy Fishler, Josue Sznitman Small-scale cavity flows are known for their usefulness in microfluidic applications. These include devices such as passive micro-mixers and cell diagnostic applications. Concurrently, small-scale cavity flows are physiologically relevant in capturing respiratory flow structures pertinent to the alveolar region of the lung. However, studies in this latter field are typically restricted to computational fluid dynamic (CFD) simulations and scaled-up experimental models based on hydrodynamic similarity matching. In the present study, we investigate low Reynolds number cavity flow phenomena using a microfluidic screening platform featuring rectangular channels with cylindrical cavities. Based on experiments and CFD simulations, we map the detailed cavity flow patterns characterizing a wide range of dimensionless geometric parameters and Reynolds numbers. We find that attached flow is observed at low aspect ratios and large opening angles, while separated flow, characteristic of alveolar flows, is associated with high aspect ratios and small opening angles. These findings provide design guidelines for microfluidic cavity applications and serve as a steppingstone towards designing in vitro microfluidic models of alveolated airways. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D18.00003: Distribution of two miscible fluids at a T-junction Casey Karst, Brian Storey, John Geddes When a fluid comprised of multiple phases or constituents flows through a network, how the phases distribute throughout the network is a fundamental and important question. If one focuses at a single bifurcation where an incoming branch splits into two daughter branches, many systems have a phase fraction in the daughter branches which is different than the incoming branch. In this work, laminar stratified flow of two miscible fluids with different viscosity at a T junction is explored with experiments and simulation. The phase distribution as a function of flow ratio in the daughter branches is explored. The distribution depends on the incoming Reynolds number, viscosity ratio, and incoming volume fraction. Good agreement between experiment and simulation is found. The results may be relevant for microfluidic networks with two or more fluids, or networks involving blood flow (either microfluidic or microvascular) where the red blood cells and plasma distribute unevenly throughout the network. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D18.00004: Flow distribution in 4 channels junctions at low Reynolds numbers as a function of the junction angle J.P. Hulin, L. Talon, D. Etien, H. Auradou, M. Cachile, J.M. Gomba Flow distribution in junctions is a key issue in microfluidics processes. We study a junction between $4$ straight channels facing each other by pairs and in which fluid is injected in two facing channels at a low Reynolds number (fluorescent dye is added in one channel). LIF measurements are performed in transparent millimetric models together with 2D FEM simulations using the Stokes equation. For equal injection flow rates on both sides, the fraction of the injected fluorescent fluid leaving in the outflow channel at the lowest angle $\alpha$ from the injection is always larger than $0.5$ (or equal for $\alpha = 90^{\circ}$) and increases as $\alpha$ decreases. Surprisingly, this fraction becomes equal to $1$ below a threshold non-zero value $\alpha_c \simeq 30^{\circ}$ (the fluid ``bounces back'' at the junction). For $\alpha \le \alpha_c$, recirculation cells appear at the center of the junction and increase in size and number as $\alpha$ goes to zero. Moving the LIF measurement plane perpendicular to that of the junction shows a $2D$ structure of the flow at low Reynolds numbers while $3D$ features appear at larger ones. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D18.00005: $\mu$PIV characterization of a toroidal microfluidic vortex driven by opto-electrokinetic methods Jae-Sung Kwon, Steve Wereley The simultaneous application of a uniform AC electric field and a focused laser to a fluid induces a toroidal microvortex with its center at the laser beam waist. In this paper we analyze the vortex quantitatively using $\mu$PIV technique. Also the vortex is characterized with respect to electric field strength, AC frequency, laser power and nonlinear coupling effect of the thermal gradient in the fluid and the electric field. As a result the flow vorticity at a certain constant AC frequency and laser power is found to increase as the square of the electric field strength. And it does not change appreciably in 10--70 kHz range under a fixed electrical voltage and laser power, but starts decreasing from the frequency ($\sim$100 kHz) at which a charge relaxation of the fluid occurs. Also at a constant AC frequency and voltage, the vorticity is significantly enhanced as the laser power is increased from 20 to 170mW due to the nonlinear interaction of the two driving sources---the laser and electric field. These results provide important insights to optimize the design and operation of a novel microvortex based mixer for rapid, dynamic on-chip mixing in progresses. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D18.00006: Light field particle image velocimetry Bryce McEwen, Jesse Belden, Tadd Truscott Three-dimensional flow field measurement of microscopic environments requires innovative solutions. We propose a system capable of measuring instantaneous, three- dimensional velocities in microscopic flow fields using light field microscopy. The light field microscope used in these experiments consists of a camera that images the back focal plane of a micro lens array (similar to the system proposed by Levoy 2006). The lens array enables capture of the light field in a single image, which can then be reparameterized to render synthetically-refocused images at different focal depths post-capture. A three-dimensional volume can be reconstructed from this synthetic focal stack, and particles extracted for velocity measurements. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D18.00007: Measurements of turbulent velocity statistics in a microscale rectangular confined impinging jets reactor Michael Olsen, Vishwanath Somashekar, Rodney Fox Microscale chemical reactors capable of operating in the turbulent flow regime, such as the confined impinging jets reactor (CIJR), offer many advantages for rapid chemical processing at the microscale, especially in application such as flash nanoprecipitation used for the production of functional nanoparticles. In the presented work, microscopic particle image velocimetry (microPIV) was employed on a microscale rectangular CIJR to obtain instantaneous velocity fields at jet Reynolds numbers of 200, 1000 and 1500, which corresponds to completely laminar, weakly turbulent, and fully turbulent regimes respectively in the reaction zone. For each Reynolds number, approximately 2000 instantaneous velocity fields were collected to analyze the flow fields and calculate pointwise and spatial turbulence statistics. Large eddy simulation (LES) was then performed to obtain time resolved simulated velocity fields which were then compared with the experimental results. Good agreement was observed between the experimental results and the LES results, demonstrating the viability of LES could be used as a tool for designing microscale reactors. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D18.00008: Some turbulent like flow property observed in microfluidics G. Wang, F. Yang, W. Zhao We report some flow properties that are similar to what normally observed in many conventional turbulent flows, can also be achieved in an electrokinetic flow in microfluidics where the Reynolds number (Re) is in the order of 0.1. One important issue in microfluidic devices is the relatively slow mixing due to laminar flow at low Reynolds number. In conventional fluid dynamics, mixing can be enhanced by manipulate flow into turbulent flow. However, in mirofluidics, where Re is usually $<$ 1, it is always believe that the flow is laminar. When mixing is enhanced (e.g. using AC electrical excitation) in microfluidics, it is believed that the flow is chaotic, not turbulence. If there is turbulence in microfluidics, another challenge is how to measure it. Here AC electrokinetics is used to excite a pressure driven flow in a microchannel with conductive sidewall to generate turbulent like flow at Re in the order of 0.1. First the flow is found to be random. With a recently developed laser induced fluorescence photobleaching anemometer (LIFPA) having high temporal and spatial resolution, we observe quantitatively random fluctuation of flow velocity and concentration in the flow. The scalar mixing is so rapid that the flow exhibits diffusion property in conventional turbulent flows. Visualization indicate that the flow may not be cahotic. Furthermore, we also observe that the power spectra of velocity and concentration exhibited a continuous decay with a span of more than one decade, again, indicating a multiscale property of turbulent flow. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D18.00009: Application of interferometry in analyzing gas flow in microchannels Yongli Li, David Newport, Shiju Joseph, Juergen J. Brandner Interferometry is a noninvasive measurement and based on this technique, the measurement or visualization of changes in physical properties of transparent objects can be achieved by detecting the refractive index changes. Gas pressure and temperature can be related to their refractive index, so interferometry can be used for local measurement of changes of these properties along the channel. A Mach--Zehnder interferometer was built with a laser with a wavelength of 633 nm, a high speed camera and two acousto-optic modulators (AOMs). Due to small channel characteristic length and sometimes low gas pressure, the measurement could be much influenced by noise. The AOMs can introduce frequency shifts into the system by acousto-optic effect, which can avoid mechanical noise generally by translating piezoelectric mirrors. The channel sides with optical access are made from crystalline silica (Quartz) that does not show speckle effects and any laser absorbance. For initial tests, the local gas pressure drop distribution along microchannel is studied at room temperature. [Preview Abstract] |
Sunday, November 20, 2011 4:07PM - 4:20PM |
D18.00010: Ultrasonic cleaning of root canals Bram Verhaagen, Christos Boutsioukis, Lei-Meng Jiang, Ricardo Macedo, Luc Van der Sluis, Michel Versluis A crucial step during a dental root canal treatment is irrigation, where an antimicrobial fluid is injected into the root canal system to eradicate all bacteria. Agitation of the fluid using an ultrasonically vibrating miniature file has shown significant improvement in cleaning efficacy over conventional syringe irrigation. However, the physical mechanisms underlying the cleaning process, being acoustic streaming, cavitation or chemical activity, and combinations thereof, are not fully understood. High-speed imaging allows us to visualize the flow pattern and cavitation in a root canal model at microscopic scales, at timescales relevant to the cleaning processes (microseconds). MicroPIV measurements of the induced acoustic streaming are coupled to the oscillation characteristics of the file as simulated numerically and measured with a laser vibrometer. The results give new insight into the role of acoustic streaming and the importance of the confinement for the cleaning of root canals. [Preview Abstract] |
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