Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session LD: Microfluidics X: Complex Fluids |
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Chair: Anuj Chauhan, University of Florida Room: Tampa Marriott Waterside Hotel and Marina Grand Salon CD |
Tuesday, November 21, 2006 8:00AM - 8:13AM |
LD.00001: Dynamics of polymeric drop breakup in microchannels Paulo Arratia, Jerry Gollub, Douglas Durian The dynamics of drop formation of sheared polymeric and Newtonian fluids are investigated in a 50 \textit{$\mu $}m microchannel. Inverse emulsions are obtained in a cross-like geometry by impinging a continuous oil phase (with surfactant) onto either a polymeric or a Newtonian aqueous solution. The viscosity ratio between the continuous and dispersed phases is kept close to unity, and both flow rates are varied. Solutions containing small amounts (100 ppm) of flexible polymers strongly affect the filament and drop breakup processes when compared to a Newtonian solution of similar viscosity. We find that the thinning of the filament for the Newtonian case is characterized by linear decline followed by a rapid approach to breakup. The polymeric case shows an initial Newtonian-like thinning followed by a slower, elasticity- dominated thinning. Consequently, the filament breakup time and length are considerably increased for the polymeric solutions. Also, larger primary drops and beads-on-string phenomena are found for the polymer solutions. [Preview Abstract] |
Tuesday, November 21, 2006 8:13AM - 8:26AM |
LD.00002: Coalescence of Droplets in Microfluidic Channels Gordon Christopher, Joel Bergstein, Nicolas End, Shelley Anna The use of droplets as tiny reactors and sample transporters in lab on a chip devices is a concept that has been explored recently due to its potential advantage in precisely controlling discrete, tiny sample volumes. However, to realize the full potential of these devices, other chip operations need to be developed, for example the merging of multiple droplets for mixing or reacting disparate samples.. Coalescence in microfluidics is reported to be difficult, and indeed we observe that droplet collisions are more likely to lead to splitting of droplets rather than merging. We present a phase diagram in terms of the capillary number and droplet size indicating conditions under which coalescence will occur. We also examine the scaling behavior of contact time of the drops, relating this to timescales for film drainage in conventional experiments and timescales for propagation of interfacial instabilities. Finally, we examine the influence of collision angle on the ability for drops to coalesce. [Preview Abstract] |
Tuesday, November 21, 2006 8:26AM - 8:39AM |
LD.00003: Molecular Modeling of Transport across Surfactant Covered Oil-Water Interface Dmitry Kopelevich, Anuj Chauhan, Ashish Gupta Mass transport across densely packed surfactant covered oil-water interfaces in microemulsions plays a key role in numerous applications such as separations, reactions, drug delivery, and detoxification. In this talk we present results of molecular modeling of transport of solute molecules across hexadecane-water interfaces covered by Brij surfactants and development of a theoretical model for the transport mechanism. We discuss effects of such parameters as solute sizes and degrees of hydrophobicity, as well as the length of surfactant molecules on the transport properties. We obtain a generalized Langevin equation for the solute transport using molecular dynamics simulations with the solute center of mass constrained in the direction normal to the interface. We observe non-trivial behavior of the stochastic force acting on the solute: the autocorrelation time of this force is extremely sensitive to the solute position within the interface and the force relaxation times differ by two orders of magnitude within a narrow region of the interface. This phenomenon is related to the density fluctuations of the surfactant as well as water and oil molecules around the solute. We further discuss implications of this phenomenon on the transport properties. [Preview Abstract] |
Tuesday, November 21, 2006 8:39AM - 8:52AM |
LD.00004: On the Effects of Channel Geometry and Long Scale Time Dependence on Microfluidic Drop Breakup Nadia Noharuddin, Gordon Christopher, Shelley Anna The use of microfluidic devices to create monodisperse emulsions and solid particles has grown in interest recently due to its potential impact on lab on a chip applications. In these devices, the drop size is typically reported as a function of the continuous and dispersed phase volume flow rates, although some recent studies indicate that pressure is a more relevant quantity. In addition, our experiments show that the overall device geometry plays a significant role in determining the resulting drop size even when similar capillary numbers and flow rate ratios are imposed. Furthermore, we observe long term time variation in drop size, frequency, and polydispersity that is orders of magnitude longer than the period of drop formation or the pump stepper period. We interpret these observations in terms of simple analytical models that demonstrate the importance of geometry and flow parameters in these processes. [Preview Abstract] |
Tuesday, November 21, 2006 8:52AM - 9:05AM |
LD.00005: Droplets break-up in junctions with and without electric field Laure Menetrier, Herve Willaime, Patrick Tabeling, Dan E. Angelescu Experiments are performed on droplet break up in microfluidic junctions of arbitrary angles, with Capillary numbers below 10-2. By studying the droplet dynamics under various geometries and flow conditions, with different fluids, with and without electric field, we found the existence of two break-up regimes: direct break-up (the droplet invades the two branches of the junction before splitting up in two parts) and retarded break-up (a finger develops in the secondary branch, retreats and eventually breaks up). In all flow conditions, the diagram length of the finger velocity in the secondary channel represents well the conditions of existence of these regimes. It has the same structure, with and without an electric field, throughout the range of conditions we considered. Remarkably, direct break-up is controled by a critical length that depends exclusively on the geometry of the junction and the applied electric field. These characteristics are surprisingly well described at a semi quantitative level - by a theory assuming small capillary numbers. We succeeded in particular to describe the effect of the geometry and, at a more qualitative level, the influrence of the electric field on the breakup conditions. [Preview Abstract] |
Tuesday, November 21, 2006 9:05AM - 9:18AM |
LD.00006: Drop manipulation and surgery using electric fields Leslie Yeo, Richard Craster, Omar Matar We study the dynamics of a slender drop sandwiched between two electrodes. A coupled system of evolution equations for the film thickness and interfacial charge density is derived using lubrication theory in the limit of large liquid conductivity. The contact line singularity is alleviated by postulating the existence of a wetting precursor film, which is stabilised by intermolecular forces. We examine the motion of the drop as a function of system parameters: the electrode separation, an electric capillary number and a spatio-temporally varying bottom electrode potential. The possibility of drop manipulation and surgery is demonstrated; this includes drop spreading, translation, splitting and recombination, using appropriate tuning of the properties of the bottom potential. For relatively small electrode separations and/or large electric capillary numbers, the drop assumes cone- like structures as it approaches the top electrode; the latter stages of this approach are found to be self-similar and a power-law exponent has been determined for this case. These results may have potential implications for drop manipulation schemes in various microfluidic applications. [Preview Abstract] |
Tuesday, November 21, 2006 9:18AM - 9:31AM |
LD.00007: Effects of geometry and fluid elasticity during polymeric droplet pinch-off in microfluidic environments Ben Steinhaus, Amy Shen, Radhakrishna Sureshkumar We investigate the effects of fluid elasticity and channel geometry on polymeric droplet pinch-off by performing systematic experiments using viscoelastic polymer solutions which possess practically shear rate-independent viscosity (Boger fluids). Four different geometric sizes (width and depth are scaled up proportionally at the ratio of 0.5, 1, 2, 20) are used to study the effect of the length scale, which in turn influences the ratio of elastic to viscous forces as well as the Rayleigh time scale associated with the interfacial instability of a cylindrical column of liquid. We observe a power law relationship between the dimensionless (scaled with respect to the Rayleigh time scale) capillary pinch-off time, T, and the elasticity number, E, defined as the ratio of the fluid relaxation time to the time scale of viscous diffusion. In general, T increases dramatically with increasing E. The inhibition of ``bead-on-a-string'' formation is observed for flows with effective Deborah number, De, defined as the ratio of the fluid relaxation time to the Rayleigh time scale becomes greater than 10. For sufficiently large values of De, the Rayleigh instability may be modified substantially by fluid elasticity. [Preview Abstract] |
Tuesday, November 21, 2006 9:31AM - 9:44AM |
LD.00008: Liquid crystal droplet production in a microfluidic device Ben Hamlington, James Feng, Darren Link, Michael Shelley, Amy Shen Liquid crystal drops dispersed in a continuous phase of silicone oil are generated with a narrow distribution in droplet size in microfluidic devices both above and below the nematic to isotropic transition temperature. We observe different dynamics in liquid crystal droplet generation, coalescence, and distinct droplet morphology by altering the microchannel surface energy. The effect of the defect structures of the nematic liquid crystal can lead to distinctly different scaling of droplet size in comparison to the Newtonian system. Capillary instabilities in thin nematic liquid crystal filament has additional contribution from anisotropic effects such as surface gradients of bending stress which can provide extra instability modes compared to that of isotropic fluids. [Preview Abstract] |
Tuesday, November 21, 2006 9:44AM - 9:57AM |
LD.00009: Microfluidic bubble logic Manu Prakash, Neil Gershenfeld We present a new all-fluidic logic family based on two-phase flow in micro-scale geometries. Hydrodynamic interactions are exploited as a primary mechanism to introduce nonlinearity. Presence or absence of a bubble represents a bit. A bubble can thus carry both information and a material payload at the same time. Microfluidic bubble logic gates (AND/OR/NOT), memory and cascaded boolean circuits will be presented. Applications of such a control scheme to large-scale integrated bio-chemical processors will be highlighted. [Preview Abstract] |
Tuesday, November 21, 2006 9:57AM - 10:10AM |
LD.00010: Development of functional microbubble generator using a microchannel Akiko Fujiwara, Shu Takagi, Yoichiro Matsumoto Recently, microbubbles with the diameter of less than several um are utilized in medical field such as contrast agents to ultrasound diagnostics and micro capsules for drag delivery systems. These microbubbles have been already used as contrast agents, although the knowledge of the suitable combination of constituents of bubbles, which are the shells and gas component for a stable bubble in liquid, and generating techniques of microbubbles have not been established yet. We propose a simple microbubbles generator using a microchannel. A microchannel is constructed with main- and sub- channel. Sub-channel is connected perpendicular to the main channel, both of which have the width and the depth of several tens of um. Liquid phase flows in a main channel and the gas does in a sub-channel. Microbubbles are chopped off from the gas phase by the liquid flow at the T-junction of microchannel. By changing the channel size and the flow rates, diameter of micro bubbles were tried to be controlled. The preliminary results will be discussed. [Preview Abstract] |
Tuesday, November 21, 2006 10:10AM - 10:23AM |
LD.00011: Theory of microbubble streaming in confined geometries David Hansen, J.C. Tsai, Sascha Hilgenfeldt Bubbles attached to the walls of microfluidic devices are usually seen as a nuisance. When driven by ultrasound, however, the oscillating microbubbles generate steady streaming flows with large speeds and large shear forces. The principle of bubble streaming and the control of streaming speed and direction by substrate patterning have been demonstrated in a semi-infinite geometry, with a bulk liquid next to a wall. Here we treat a case more relevant for applications, where a second wall at a small distance from the first confines the fluid. We present analytical studies of directional flow through such a device, analyzing both its potential to mix the fluid in the gap and to transport it in a desired direction. Depending on the amplitude of the ultrasonic driving and the geometry of the device, the resulting flow can be controlled to provide either efficient fluid transport or effective fluid mixing. [Preview Abstract] |
Tuesday, November 21, 2006 10:23AM - 10:36AM |
LD.00012: Two Phase Flow Visualization and Flow Regime Characteristics in Gas Diffusion Layer Integrated Microchannels Eon Soo Lee, Julie Steinbrenner, Fu-Min Wang, Carlos Hidrovo, Kenneth Goodson, John Eaton Management of liquid water transported through the reactant-gas-supply channels is a significant problem in proton exchange membrane fuel cells. Typically these channels have three smooth, impermeable walls and a fourth wall consisting of a porous carbon paper called the gas-diffusion layer (GDL). This research addresses microchannel fuel cells where the channel height is comparable to the GDL thickness and both gas and liquid may move longitudinally through either the channel or the porous layer. Transparent test structures were built with 500 $\mu $m wide by 200 $\mu $m deep by 15 cm long channels with one of the 500 $\mu $m wide channel walls bounded by a 190 or 390 $\mu $m thick GDL layer. A controlled flow of water was injected into the backside of the GDL at 4 equally distributed points along the channel. Two phase flow regimes in the channel were observed to be a function of the water and gas flow rates, GDL hydrophobicity, GDL thickness and channel wall hydrophobicity. Generally the flow regime evolves from plug flow, to wavy annular flow, to stratified flow as the gas flow rate increases. The transition points are determined by the system parameters and the hysteresis of the flow regime is observed with the direction of gas flow variation. Modeling of the system requires correct accounting for a significant gas flow passing longitudinally through a partially-saturated GDL. [Preview Abstract] |
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