Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session EF: Microfluidics: Devices III |
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Chair: Amir Hirsa, Rensselaer Polytechnic Institute Room: 101F |
Sunday, November 22, 2009 4:15PM - 4:28PM |
EF.00001: Taylor-Aris Dispersion in Retentive, Ordered Pillar Array Columns Xiaohong Yan, Qiuwang Wang, Haim Bau The method of volume averaging is applied to estimate the dispersion coefficient of solute advected in ordered pillar array columns with wall retention of the type used in chromatographic separation. The mass transfer resistance of the stationary phase is accounted for. The appropriate closure equations are solved in a unit cell to obtain the dispersion tensor as a function of the Peclet number, pillar pattern and size, and solute properties. The contributions of the flow and the wall adsorption to the dispersion are identified and discussed. The model is verified by comparing its predictions and obtaining favorable agreement with the results of direct numerical simulations and with available experimental data for columns containing pillars. The model is then used to optimize the pillars' shape and pattern to minimize longitudinal dispersion. It is demonstrated that by judicious selection of the pillars' shape and pattern, one can achieve significant reduction in the longitudinal dispersion coefficient. [Preview Abstract] |
Sunday, November 22, 2009 4:28PM - 4:41PM |
EF.00002: Axial Dispersion in Segmented Gas-Liquid Flow: Effects of the Channel Curvature Metin Muradoglu The effects of channel curvature on the axial dispersion in segmented gas-liquid flows have been studied computationally in a two-dimensional setting using a front-tracking/finite-volume method. Passive tracer particles are used to visualize and quantify the axial dispersion. The molecular diffusion is modeled by random walk of tracer particles. It is found that there is significant axial dispersion in serpentine channels even in the absence of molecular diffusion and dispersion increases with channel curvature. It is known that there is no dispersion in straight channels since a lubricating thin liquid layer persists on the wall. However this lubricating liquid layer is periodically broken in the curved channel case leading to enhanced axial dispersion. It is found that the dispersion increases as the Peclet number $(Pe$) decreases both in straight and curved channels. Difference between the straight and curved channel decreases continuously as the Peclet number decreases and virtually disappears at low Peclet numbers, i.e., $Pe<10$ in the present study. A model is proposed based on the difference between the liquid film thicknesses on the inner and outer side of the bend in the limit as $Pe\rightarrow\infty$. Good agreement is found between the computational results and the model when the liquid slug is well mixed by the chaotic advection. [Preview Abstract] |
Sunday, November 22, 2009 4:41PM - 4:54PM |
EF.00003: A Micro-PIV Study of the Pulsed Micro-Flows Driven by an Insulin Pump Bing Wang, Ayodeji Demuren, Eric Gyuricsko, Hui Hu In recent years, there is a surge in the popularity of using insulin pump or continuous subcutaneous insulin infusion therapy, as opposed to multiple daily injections by insulin syringe or an insulin pen. Some case studies have suggested that insulin delivery failure may be caused by precipitation of insulin within the infusion set. Speculation also exists that the flow of insulin through an insulin infusion set may be reduced or inhibited by air bubbles entrained into the micro-sized tubing system since there are chances that air be introduced into the insulin reservoir during the filling process. In the present study, a microscopic Particle Image Velocimtry (micro-PIV) system was used to characterize the transient behavior of the pulsed micro-flows inside the micro-sized tubing system of an insulin infusion set with insulin pump operating in basal mode (i.e., pulsed insulin pumping). The effects of the air bubbles entrained into the micro-sized tubing system on the insulin delivery process were assessed based on the micro-PIV measurements. [Preview Abstract] |
Sunday, November 22, 2009 4:54PM - 5:07PM |
EF.00004: A Methodology for Time-Resolved microDPIV Jaime Schmieg, Adric Eckstein, John Charonko, Pavlos Vlachos Micro Digital Particle Image Velocimetry (uDPIV) measurements are often limited to time averaged analyses due to low signal to noise ratios, high background illumination, and low particle seeding. As a result, the measurement of transient microscale flows is difficult to achieve through conventional DPIV correlation methods. Eckstein and Vlachos (2009), presented the Robust Phase Correlation (RPC) method which utilizes a series of digital filters to mitigate the effects of background noise. This study further explores the potential of RPC using experimentally derived, time-resolved uPIV images taken within three different microchannels of various geometries. Performance comparisons were based on RMS error, as well as percent of erroneous vectors, as determined by the mode-ratio bootstrapping method (Pun et. al. 2007). Results displayed a significant reduction of RMS error and erroneous vectors for the RPC method in comparison to standard techniques. 1. Eckstein and Vlachos. Meas. Sci. Technol. (2009). 2. Pun et al. Meas. Sci. Technol. (2007). [Preview Abstract] |
Sunday, November 22, 2009 5:07PM - 5:20PM |
EF.00005: Inertial and channel confinement effects on laminar flow in microchannels with superhydrophobic surfaces Yongpan Cheng, Chiangjuay Teo, Boocheong Khoo In order to reduce the pressure drop for flow through microchannels, the superhydrophobic surfaces which consist of micro-grooves, posts or holes are widely adopted. In this paper, the effective slip performances of transverse grooves, longitudinal grooves, posts and holes are investigated numerically. The numerical results show that the effective slip lengths of square posts, square holes and transverse grooves decrease with increasing Reynolds number, except those corresponding to longitudinal grooves. For small pattern width to channel height ratios, at low shear-free fractions, the effective slip length corresponding to square posts is equivalent of that of transverse grooves, and their slip lengths are lower than those of square holes and longitudinal grooves. With increasing shear-free fractions, the effective slip length of square posts surpasses those of square holes and longitudinal grooves. Square posts exhibit the highest effective slip length at extremely high shear-free fractions. This study may be useful for applications pertaining to the reduction of flow resistance in microchannels employing superhydrophobic surfaces. [Preview Abstract] |
Sunday, November 22, 2009 5:20PM - 5:33PM |
EF.00006: Designing ridged microchannels for continuous separation of cells based upon stiffness Alexander Alexeev, John P. Arata The mechanical stiffness of human cells can be a key parameter that reveals the disease state of the cell, for example, in various cancers and in malaria. However, for stiffness to be utilized in diagnostic settings, we will require methods for continuous monitoring of the stiffness of cells in high throughput. Using three-dimensional computational modeling, we show that a pressure-driven microfluidic flow in a channel with solid diagonal ridges can effectively separate compliant microscopic particles, which represent biological cells and synthetic microcapsules. The flow separation is driven by an energy minimization process associated with periodical deformations of elastic particles in narrow constrictions. As a result of this process soft and stiff particles disperse to the opposite walls of microfluidic channel, thus enabling continuous flow separation of mechanically distinct particles. This simple microfluidic method could prove useful in various biomedical applications for continuous stiffness-based separation of biological cell. [Preview Abstract] |
Sunday, November 22, 2009 5:33PM - 5:46PM |
EF.00007: Interfacial rheology in two-phase microchannel flows Steven Hudson, Jeffrey Martin Droplet dynamics are studied experimentally by particle tracer velocimetry and shape analysis to measure interfacial retardation in various flow fields, surfactant concentrations and interfacial sorption dynamics. Aqueous drops in mineral or silicone oils are investigated in Poiseuille flow in rectangular channels. The drop size (tens of microns) is small compared to the channel height and width. The drop size is also small compared to an intrinsic length scale defined by surfactant mass transport coefficients, so that the experiments may probe a regime where interfacial sorption dynamics are relevant. The interfacial mobility of the drop is compared with fully mobile predictions and is found to range from a few percent to nearly full mobility, as surfactant concentration is adjusted. The effect of channel asymmetry on drop circulation is also tested. The droplet shape relaxation rates are measured, indicating the local surfactant concentration and interfacial tension. The interfacial tension and Marangoni effects are thus measured in a single experiment. [Preview Abstract] |
Sunday, November 22, 2009 5:46PM - 5:59PM |
EF.00008: Separation and Stabilization of Deformable Drops in Microfluidics Wingki Lee, Katharina Schrank, Lynn Walker, Shelley Anna Microfluidic processes are effective for producing highly monodisperse droplet streams, but some desired processes inherently require the formation of polydisperse droplet populations. As an example, tipstreaming produces micron scale droplets along with larger drops that are 10-100 microns in size. Separation of these sizes is needed in order for the process to be useful. We have designed a microfluidic separator allowing fractionation of droplet sizes on-the-fly along with the injection of a surfactant-laden stream for further downstream stabilization of the fractionated emulsion. The behavior of the device is different for deformable emulsions than rigid particles, although the design can work for either. We report on the performance of the device as well as size distributions resulting from the fractionated populations. [Preview Abstract] |
Sunday, November 22, 2009 5:59PM - 6:12PM |
EF.00009: Pumpless femtoliter drop-on-demand generation via satellite formation Dustin Moon, Dong Wook Lee, In Seok Kang Manipulation of discrete or digital droplets can be a key process in chemical, pharmaceutical process in micro scale. This paper provides a drop-on-demand generation method of such micro droplets. Many drop generation techniques have been developed, from flow focusing to electrospray, dispensing droplets into different phases. But due to relatively long microchannel length and the use of syringe pumps, these methods show some limitations for single drop generation at a wanted time. Satellites, an unwanted droplet generated from liquid bridge breakup, have rather stable size distribution resulting from its rapid formation mechanism. In this experimental work, satellites are formed between metallic capillaries of outer diameter 910, 460, 260 micron respectively, and inside a PDMS channel of 100 micron-depth. To form a liquid bridge and break it, capillaries can be moved, or pulsed electric field is applied to deform, elongate droplets, and to form an unstable liquid bridge, and break. Several fluids from DI water to PEG with and without polystyrene particles and E.Coli. were used to form satellite drops in air and in oil. Details of each behavior were captured using Photron PCI 1024X high speed camera and analyzed accordingly. [Preview Abstract] |
Sunday, November 22, 2009 6:12PM - 6:25PM |
EF.00010: Droplet Dynamics in Two Phase Microflows Brian Carroll, Carlos Hidrovo An experimental investigation is presented that addresses the engineering challenges of a two-phase, inertial-based micromixer. Results indicate a predictable pattern between the Reynolds number in the microchannel and the detached droplet size and geometric features, with increasing gas velocities leading to a high aspect ratio (elongated) detached slugs. Full droplet detachment and entrainment into the gaseous flow, where the droplet shares no interface with the solid wall, was not observed in the range of parameters tested. The influence of the solid wall during the collision and mixing process remains unresolved, although it is evident that increasing the surface energy of the solid boundary facilitates the coalescence and mixing process. Using Laser Induced Flourescence, the extent of mixing is quantified experimentally and compared for different detached droplet sizes, velocities, and microchannel geometries. This data, together with the conditions required for detachment and entrainment, will provide a more complete picture of an inertial-based, droplet collision micromixer. [Preview Abstract] |
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