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 AN: Micro Fluids I |
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Chair: Debjyoti Banerjee, Texas A&M University Room: 201 |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AN.00001: Particle ordering in inertially focused microfluidic flows Katherine Humphry, Pandurang Kulkarni, Dino Di Carlo, Jon Edd, Mehmet Toner, Jeffrey Morris, David Weitz, Howard Stone We study inertially driven focusing of particles [1], which has recently been exploited in a controlled fashion in microfluidic devices [2]. In particular, we characterize the focusing as a function of particle and channel Reynolds number by reporting particle position in directions perpendicular to the flow, and a large distance from the fluid inlet. Focusing of dilute suspensions leads to a linear arrangement of particles whose spacing is primarily a function of concentration and channel aspect ratio. All results are compared with simulations, which provide mechanistic insights into particle behavior.\\ \\ $[1]$ G. Segr\'{e} and A. Silberberg, Nature \textbf{189}, 209 (1961). \\ $[2]$ D. Di Carlo, D. Irimia, R. G. Tompkins, and M. Toner, Proc. Nat. Acad. Sci. U.S.A. \textbf{104}, 18892 (2007). [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AN.00002: The effective slip length and vortex formation in laminar flow over a rough surface Anoosheh Niavarani, Nikolai Priezjev The flow of viscous incompressible fluid over a periodically corrugated surface is considered by the numerical solution of the Navier-Stokes equation. We define the effective slip length with respect to the level of the mean height of the surface roughness. With increasing corrugation amplitude the effective no-slip boundary plane is shifted towards the bulk of the fluid what implies a negative effective slip length. Analysis of the flow streamlines shows that a flow circulation is developed in the grooves of the rough surface provided that the local boundary condition is no-slip. By applying a local slip boundary condition, the location of vortex is displaced towards the bottom the grooves and the effective slip length increases. For values of the local slip length larger than the period of the surface corrugation, the vortical structure disappears, the flow streamlines are deformed to follow the surface curvature, and the effective slip length saturates to a constant value. Inertial effects promote vortex flow formation in the grooves and reduce the effective slip length. [Preview Abstract] |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AN.00003: Dual-tracer laser-induced fluorescence thermometry in microchannels Myeongsub Kim, Minami Yoda Laser-induced fluorescence (LIF) thermometry measures liquid temperatures based on changes in fluorescence intensity. In dual-tracer or ratiometric LIF thermometry, liquid temperatures are based on the ratio of the fluorescent signals from two different fluorophores, since this ratio is independent of changes in fluorescence intensity due to variations in the excitation. Recently, a dual-tracer LIF technique using two species with opposite temperature sensitivities, fluorescein 27 and Kiton Red (sulforhodamine B), excited at 532~nm has been reported with temperature sensitivities as great as 7\% per $^\circ$C [Sutton {\it et al.} (2008) {\it Exp. Fluids} DOI 10.1007/s00348-008-0506-4]. We describe here a similar technique using fluorescein and sulforhodamine B, which have intensities that increase by 2.2\% and decrease by 1.3\%, respectively, per $^\circ$C when volumetrically illuminated at 514 nm. The ratio of these two signals gives temperature sensitivities as great as 9\% per $^\circ$C. This LIF technique is used to measure temperature distributions in water flowing through a 500 $\mu$m $\times$ 1000 $\mu$m polydimethylsiloxane (PDMS) microchannel covered with a glass lid with localized heating to create temperature gradients up to about 15 $^\circ$C /mm. The results are compared with FLUENT predictions. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AN.00004: Nonuniform particle distributions in near-wall particle-image velocimetry Haifeng Li, Minami Yoda Multilayer nano-particle image velocimetry ({\mbox MnPIV}) uses fluorescent colloidal tracers illuminated by evanescent waves to visualize the flow within the first 500~nm next to the wall. Because the evanescent-wave intensity decays exponentially with wall-normal distance {\it z}, the {\it z-} position of each tracer particle can be correlated to the intensity of its image, assuming that the particle image and illumination intensities behave in a similar fashion. Recent experimental calibrations suggests that the z-position of 100~nm fluorescent polystyrene spheres can be determined with an accuracy of about 20~nm [Li \& Yoda (2008) {\it Meas. Sci. Technol.} {\bf 19}, 075402]. Near-wall particle distributions were obtained as a function of {\it z} for the Poiseuille flow of monovalent electrolyte solutions at various pH and ionic strengths through bare hydrophilic and coated hydrophobic fused- quartz microchannels with similar nominally rectangular cross- sections. The tracers were then divided into three sub-layers, each containing about $1/3$ of the particles, based on the measured particle distribution, and the average velocities in each layer were placed at the average {\it z-}position sampled by the particles in that layer. The effect of pH and wall properties on the near-wall particle distributions and the resultant {\mbox MnPIV} data is discussed. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AN.00005: Drop break-up and pressure measurements in a microfluidic device Suzie Protiere, Howard A. Stone, David A. Weitz We study experimentally the flow of an emulsion passing through one or a few constrictions placed in a microfluidic channel. Using a high-speed differential manometer placed in the same device (M. Abkarian et al. PNAS 200:16407104 (2006)) we have measured the dynamic pressure as a drop breaks up when it meets one or several constrictions. We can then study how a global measurement of the pressure drop indicates the sequence of phenomena occurring in the channel (breakup, trapped and squeezed drops etc.). In a separate set of experiments with a microfluidic model of a two-dimensional porous medium through which drops flow we can observe the various phenomena and thus correlate the pressure fluctuations to single events at the pore scale. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AN.00006: Effect of divalent ions on electroosmotic flow Subhra Datta, A.T. Conlisk, Haifeng Li, Minami Yoda The electroosmotic flow (EOF) rate in fused silica microchannels is found to decrease when trace quantities of salts containing the divalent cations $\rm {Ca}^{2+}$ and $\rm {Mg}^{2+}$ are added to a background electrolytic solution (BGE) containing a salt of monovalent ions. Moreover, the observed effect is quantitatively different for the two ions $\rm {Ca}^{2+}$ and $\rm{Mg}^{2+}$. Since electrostatic interactions should be identical for ions of the same valence modeled as point charges, a description of the electric double layer (EDL) based on the Poisson-Boltzmann equation alone cannot account for these experimental observations. Experiments to measure EOF in the presence of $\rm {Ca}^{2+}$ and $\rm{Mg}^{2+}$ in the BGE were carried out using nano-particle image velocimetry (nPIV). A model for the charge development at the silica-BGE interface (site binding model) that accounts for the chemical interactions of the BGE ions with the silica surface is developed. The model predictions are in good agreement with the experimental observations on the effect of divalent cations as well as data from the literature on how properties such as pH and ionic strength affect electroosmotic flow rates in a BGE containing only monovalent ions. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AN.00007: Dynamics of polymer solutions and polymer/vesicle mixtures during microchannel flow Michael Graham, Samartha Anekal, Juan Hernandez-Ortiz Addition of small amounts of long-chain polymers to blood has been found to have dramatic effects on its flow in the microcirculation. To address the mechanisms underlying these phenomena, we use a real-space P$^3$M method for Stokes flow including Brownian fluctuations to study the dynamics of polymer solutions and polymer/vesicle mixtures in microscale flows. Both a simple slit geometry and a grooved cavity flow are studied and polymer concentrations from ultradilute up to near the overlap concentration are considered. As concentration increases, the hydrodynamic migration effects observed in dilute solution unidirectional flows become less prominent, virtually vanishing as the overlap concentration is approached. In a grooved channel geometry, the groove is almost completely depleted of polymer chains at high Weissenberg number in the dilute limit, but at finite concentration this depletion effect is dramatically reduced. In suspensions of vesicles, the presence of polymer molecules has a substantial effect on the dynamics of pair collisions and on migration of the vesicles from microchannel walls. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AN.00008: Nanoscale Wicking Structures Conan Zhang, Carlos Hidrovo Heat pipes have been used extensively as heat transfer systems due to their low maintenance and lack of moving parts. These characteristics allow the heat transfer system to be compact for increasingly miniature electronic devices. The fluid flow is possible through the pressure gradient induced by the capillary force and is unaided by external power sources. However, since there is no external force driving the flow, the fluid is entirely dependent on the dimensions and capillary characteristics of the wicking structure. Since the convective and latent heat transfer is strongly dictated by mass flow of the fluid, the wicking structure dimensions should be optimized in order to achieve a maximum flow rate. In order to optimize the dimensions of the wicking structure, a fluidic model was developed to simulate fluid flow based on existing capillarity, Bernoulli and Stokes flow equations for a nanoscale posts array. This fluidic analysis was the initial platform on which thermal performance was based. These results were then compared with experimental data to validate and further examine the effects of wicking structure geometry and wetting characteristics. [Preview Abstract] |
Sunday, November 23, 2008 9:44AM - 9:57AM |
AN.00009: Flow of a rarefied gas in a micro channel caused by oscillatory heating of a wall Toshiyuki Doi A rarefied gas in a micro channel where one of the channel wall is heated periodically in time is studied numerically on the basis of the linearized Boltzmann equation for a hard sphere molecular gas. The time-dependent motion of the gas caused by the heating is investigated with the aid of a deterministic numerical method for a wide range of the Knudsen number (=mean free path/channel with $D$) and the Strouhal number (=frequency $\times D$ /sound speed). The gas motion is highly induced by the heating when Sh $=3\sim 4$, for $0.1\le \textrm{Kn} \le 10$, so the normal stresses of the gas acting on the walls are. At about Sh$=1.5\sim 2$, the normal stress acting on the heated wall exhibits a rather sharp minimum with respect to Sh, while no such sharp minimum is found in that on the other wall. [Preview Abstract] |
Sunday, November 23, 2008 9:57AM - 10:10AM |
AN.00010: A new electrohydrodynamic flows due to field-induced conductivity gradient in dielectric liquids Hyun Jin Park, Jae Chun Ryu, Jun Kwon Park, Kwan Hyoung Kang A dielectric liquid is often preferred as a host fluid of a colloidal system under an electric field, because one can utilize the full benefits of a strong electric field with little concern for occurrence of electrolysis. Hence, dielectric liquids have been employed in many practical applications such as electrorheological fluids, electrophoretic deposition, and electrophoretic display. Nonetheless, the dynamics of colloidal particles in dielectric liquids is poorly understood compared to that in aqueous solutions. In the present paper, we report a novel electrohydrodynamic (EHD) flow which occurs near the objects immersed in dielectric liquids containing small amounts of polar additives. We suggested that the EHD flow is generated due to a electrical conductivity gradient induced by a non-uniform electric-field. Analytical and numerical solutions are obtained and verified by comparision with experimental results. We discuss the effect of electric-field strength, particle size, and ac frequency on velocity and pattern of the EHD flow. [Preview Abstract] |
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