Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D10: Microscale Flows: General |
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Chair: Linda Cummings, New Jersey Institute of Technology Room: 110 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D10.00001: Topological transitions in unidirectional flow of nematic liquid crystal Linda Cummings, Thomas Anderson, Ensela Mema, Lou Kondic Recent experiments by Sengupta et al. (Phys. Rev. Lett. 2013) revealed interesting transitions that can occur in flow of nematic liquid crystal under carefully controlled conditions within a long microfluidic channel of rectangular cross-section, with homeotropic anchoring at the walls. At low flow rates the director field of the nematic adopts a configuration that is dominated by the surface anchoring, being nearly parallel to the channel height direction over most of the cross-section; but at high flow rates there is a transition to a flow-dominated state, where the director configuration at the channel centerline is aligned with the flow (perpendicular to the channel height direction). We analyze simple channel-flow solutions to the Leslie-Ericksen model for nematics. We demonstrate that two solutions exist, at all flow rates, but that there is a transition between the elastic free energies of these solutions: the anchoring-dominated solution has the lowest energy at low flow rates, and the flow-dominated solution has lowest energy at high flow rates. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D10.00002: Quasiparallel flow of a binary gas mixture: the Stefan tube revisited S.J.S. Morris Placed in the bottom of a vertical tube open at the top, volatile liquid (species 1) evaporates at a rate set by diffusion of vapour through the carrier gas (species 2). In the textbook solution, due to J. Stefan, species 2 is assumed to be stationary, but numerical solutions of the governing equations show that species 2, in fact, recirculates (Mills and Chang 2013; and references therein). But although Stefan's solution is based on an incorrect assumption, the same numerical solutions show that it predicts the evaporation rate to within a few percent (Mills and Chang, below eq.12). Assuming the ratio $L/a$ of tube length to radius to be large, we use lubrication theory to give an elementary solution determining the velocity profiles for each species, including the effect of slip. It is shown that, in the limit as $L/a\to \infty$, the Stefan solution correctly determines the total evaporation rate; this conclusion is independent of the precise form of the boundary condition placed on the species velocities at the tube wall. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D10.00003: Boundary integral simulations of dissolving drops in segmented two-phase flows Arun Ramchandran, Thomas Leary Recent years have seen an upsurge in the literature reporting the microfluidic measurement of the kinetics of `fast' gas-liquid reactions by recording the shrinkage of bubbles in segmented flows of these gas-liquid combinations in microfluidic channels. A critical aspect of the data analysis in these experiments is the knowledge of how dissolution influences the velocity field in the liquid slug, and hence, the mass transport characteristics. Unfortunately, there is no literature on this connection for dissolving bubbles. Our research addresses this gap using boundary integral simulations. The effects of the dissolution rate on the film thickness and the inter-drop separation are examined as a function of the capillary number and the viscosity ratio. The results demonstrate that dissolution can enhance the degree of mixing appreciably from one slug to the next. A curious result is that the film thickness and the droplet separation distance can change significantly beyond a critical capillary number, producing flow patterns completely different from those known for the undissolving bubble case. These results will guide the selection of operating regimes that enable convenient interpretation of data from experiments to deduce kinetic constants. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D10.00004: Elastic deformations in a Hele-Shaw cell driven by local non-homogeneities of fluid properties Shimon Rubin, Amir Gat, Moran Bercovici We consider a Hele-Shaw chamber with an elastic top plate, and study the effect of spatial variations in fluid properties on deformations of the plate. Specifically, we present analytical solutions for the pressure and depth-averaged flow field for axially-symmetric variations in slip velocity, viscosity, slip length, and channel height. We then focus on electroosmotic flow, which may be a practical method for obtaining gradients in slip velocity via non-uniform zeta-potential patterning of the surface. We derive an equation which relates elastic deformations of a Kirchhoff-Love plate to gradients in zeta potential, and obtain an analytical solution for the zeta potential distribution which gives rise to a local Gaussian deformation. Owing to the fact that any surface can be represented by superposition of Gaussians, we are thus able to determine the zeta potential necessary for creation of arbitrary deformations. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D10.00005: Blood Perfusion in Microfluidic Models of Pulmonary Capillary Networks: Role of Geometry and Hematocrit Hagit Stauber, Dan Waisman, Josue Sznitman Microfluidic platforms are increasingly used to study blood microflows at true physiological scale due to their ability to overcome manufacturing obstacle of complex anatomical morphologies, such as the organ-specific architectures of the microcirculation. In the present work, we utilize microfluidic platforms to devise in vitro models of the underlying pulmonary capillary networks (PCN), where capillary lengths and diameters are similar to the size of RBCs ($\sim$ 5-10 $\mu$m). To better understand flow characteristics and dispersion of red blood cells (RBCs) in PCNs, we have designed microfluidic models of alveolar capillary beds inspired by the seminal ``sheet flow'' model of Fung and Sobin (1969). Our microfluidic PCNs feature confined arrays of staggered pillars with diameters of $\sim$ 5,7 and 10 $\mu$m, mimicking the dense structure of pulmonary capillary meshes. The devices are perfused with suspensions of RBCs at varying hematocrit levels under different flow rates. Whole-field velocity patterns using micro-PIV and single-cell tracking using PTV are obtained with fluorescently-labelled RBCs and discussed. Our experiments deliver a real-scale quantitative description of RBC perfusion characteristics across the pulmonary capillary microcirculation. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D10.00006: Imbibition of ''Open Capillary'': Fundamentals and Applications Marie Tani, Ryuji Kawano, Koki Kamiya, Ko Okumura Control or transportation of small amount of liquid is one of the most important issues in various contexts including medical sciences or pharmaceutical industries to fuel delivery. We studied imbibition of ``open capillary'' both experimentally and theoretically, and found simple scaling laws for both statics and dynamics of the imbibition, similarly as that of imbibition of capillary tubes. Furthermore, we revealed the existence of ``precursor film,'' which developed ahead of the imbibing front, and the dynamics of it is described well by another scaling law for capillary rise in a corner [1]. Then, to show capabilities of open capillaries, we demonstrated two experiments by fabricating micro mixing devices to achieve (1) simultaneous multi-color change of the Bromothymol blue (BTB) solution and (2) expression of the green florescent protein (GFP) [2]. [1] A. Ponomarenko, D. Quere and C. Clanet, J. Fluid Mech., 666, 146 (2011). [2] M. Tani, R. Kawano, K. Kamiya and K. Okumura, Sci. Rep. 5, 10263 (2015). [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D10.00007: Experimental investigation of non-Newtonian/Newtonian liquid-liquid flow in microchannel Eynagelia-Panagiota Roumpea, Weheliye Weheliye, Maxime Chinaud, Panagiota Angeli Plug flow of an organic phase and an aqueous non-Newtonian solution was investigated experimentally in a quartz microchannel with I.D. 200 $\mu $m. The aqueous phase was a glycerol solution where 1000 and 2000 ppm of xanthan gum was added while the organic phase was silicon oil with 155 and 5 cSt viscosity. The two phases were brought together in a T-junction and their flowrates varied from 0.3 to 6 ml/hr. High speed imaging was used to study the characteristics of the plugs and the effect of the liquid properties on the flow patterns while a two-colour micro-PIV technique was used to investigate velocity profiles and circulation patterns within the plugs. The experimental results revealed that plug length was affected by both flowrate and viscosity. In all cases investigated, a film of the continuous phase always surrounded the plugs and its thickness was compared with existing literature models. Circulation patterns inside plugs were obtained by subtracting the plug velocity and found to be depended on the plug length and the amount of xanthan gum in the aqueous phase. Finally, the dimensionless circulation time was calculated and plotted as a function of the plug length. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D10.00008: Destabilization of highly viscous fluid threads in complex microgeometries Thomas Cubaud High-viscosity multiphase flows in microchannels encompass a broad range of fluid phenomena, including self-lubrication and viscous buckling instabilities. Here, a series of experiments is conducted to study the dynamic response of miscible fluid threads to a change in carrier flow velocity due to varying microgeometries. The structural stability of core-annular flows is systematically investigated in simple and complex microchannels, such as square, bifurcating, and corrugated channels, from low to high flow rates of injection and for a variety of fluid viscosities. Focus is on flow regimes of practical interest for the improvement of mixing and separation processes between fluids having large viscosity contrasts at the small scale. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D10.00009: The motion of long drops in rectangular capillaries at low capillary numbers Harris Wong, Sai Rao The immiscible liquid-liquid drop flow in rectangular capillaries has found extensive industrial applications. However, the flow patterns and pressure-flow rate relations are not well understood. We study the steady motion of a long drop of length LW (L\textgreater \textgreater 1) in a rectangular microchannel of width W and height BW (B$\ge $1). The drop is moving at a velocity U such that the capillary number Ca$=$mU/s \textless \textless 1, where m is the viscosity of the carrier liquid and s is the interfacial tension. The drop is non-wetting so that a carrier liquid film separates the drop from the channel wall. We find that the carrier liquid either pushes the drop (plug flow) or bypasses the drop through corner channels (corner flow). When LCa$^{1/3}$ \textgreater \textgreater 1, the plug flow dominates, whereas the corner flow dominate when LCa$^{1/3}$ \textless \textless 1. The plug flow and the corner flow are coupled through the corner interface. Hence, when the corner flow dominates, the carrier liquid bypasses the drop and drags the drop fluid forward faster than the drop velocity. To conserve mass, the drop fluid circulates from the front to the back of the drop along the center region. The pressure-flow rate relation is linear when LCa$^{1/3}$ \textless \textless 1 or \textgreater \textgreater 1, and is nonlinear when LCa$^{1/3}$ $\sim$ 1. The coupled flow is studied for B $=$ 1, 1.2, 1.5, and 2, and for viscosity ratio R $=$ 0.001 to 100, where R is the ratio of drop viscosity to carrier liquid viscosity. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D10.00010: Broadband light based optoelectric tweezers Avanish Mishra, Katherine Clayton, Steve Wereley Trapping, sorting and transport of particles are fundamental operations in microfluidic platforms. However, very few methods exist that can dynamically trap and manipulate particles with high spatial resolution and accuracy. Recently, a new set of methods have emerged that can trap and sort particles by optically controlling electrokinetic effects. Rapid Electrokinetic Patterning (REP) is such an emerging optoelectric technique. It utilizes a laser activated electrothermal (ET) vortex and particle-electrode interactions for trapping particles. Trapped particles can be translated by optically steering the laser or by moving the trapping chamber. Previously demonstrated applications of REP have utilized a 1064 nm infrared laser, integrated in an inverted microscope, to create the necessary temperature rise for producing the ET flow. Use of an external laser for REP trapping is expensive and time intensive to integrate, making it difficult to design a portable REP system. Using experiments and simulations, we show that a non-coherent incandescent broadband light source can be used for REP trapping and manipulation. This allows for a microscope with a broadband lamp to be used for REP trapping without integrating an external laser. [Preview Abstract] |
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