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 L38: Microscale Flows: Drops, Bubbles |
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Chair: Carlos Hidrovo, Northeastern University Room: Sheraton Back Bay B |
Monday, November 23, 2015 4:05PM - 4:18PM |
L38.00001: Generation of Monodisperse Liquid Droplets in a Microfluidic Chip Using a High-Speed Gaseous Microflow Pooyan Tirandazi, Carlos Hidrovo Over the last few years, microfluidic systems known as Lab-on-a-Chip (LOC) and micro total analysis systems ($\mu $TAS) have been increasingly developed as essential components for numerous biochemical applications. Droplet microfluidics, however, provides a distinctive attribute for delivering and processing discrete as well as ultrasmall volumes of fluid, which make droplet-based systems more beneficial over their continuous-phase counterparts. Droplet generation in its conventional scheme usually incorporates the injection of a liquid (water) into a continuous immiscible liquid (oil) medium. In this study we demonstrate a novel scheme for controlled generation of monodisperse droplets in confined gas-liquid microflows. We experimentally investigate the manipulation of water droplets in flow-focusing configurations using a high inertial air stream. Different flow regimes are observed by varying the gas and liquid flow rates, among which, the ``dripping regime'' where monodisperse droplets are generated is of great importance. The controlled size and generation rate of droplets in this region provide the capability for precise and contaminant-free delivery of microliter to nanoliter volumes of fluid. Furthermore, the high speed droplets generated in this method represent the basis for a new approach based on droplet pair collisions for fast efficient micromixing which provides a significant development in modern LOC and $\mu $TAS devices. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L38.00002: Planar Microfluidic Drop Splitting and Merging Sean Collignon, James Friend, Leslie Yeo Open drop microfluidic platforms offer attractive alternatives to closed microchannel devices, however, to be effective they require efficient schemes for planar drop transport and manipulation. While there are many methods that have been reported for drop transport, it is far more difficult to carry out drop operations of dispensing, merging and splitting. In this work, we introduce a novel alternative to merge and split drops using laterally-offset modulated surface acoustic waves (SAWs). To do so, the energy delivery into the drop is modulated to induce drop stretching. Upon removal of the SAW energy, capillary forces at the center of the elongated drop drain the capillary bridge region towards both ends, resulting in its collapse and consequential splitting of the drop. This occurs only below a critical Ohnesorge number, a balance between the viscous forces that retard the drainage and the sufficiently large capillary forces that cause the liquid bridge to pinch. By this scheme we show the possibility of both reliable symettric splitting of a drop with an average deviation in droplet volumes of only around 4\%, and no greater than 10\%, as well as asymmetric splitting, by tuning the input energy to the device---thus presenting a comparable alternative to electrowetting. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L38.00003: Droplet velocity in a micrometric Hele-Shaw Cell Benjamin Reichert, Axel Huerre, Olivier Theodoly, Isabelle Cantat, Marie-Caroline Jullien Droplet-based microfluidics is a growing field often requiring an accurate synchronization for automated systems. The question we address is the prediction of a viscous droplet velocity pushed by a surrounding liquid set at a fixed mean velocity. In a previous work, we showed that the level of confinement plays a crucial role by investigating the lubrication film thickness. Two regimes have been observed [Huerre et al., PRL accepted 2015]: at low capillary number the film is so thin that intermolecular forces come into play setting the film thickness at a constant value whatever the capillary number, at higher capillary number a scaling law is observed following Hodges et al. model [Hodges et al. JFM 2004]. As the properties of the lubrication film impacts the dissipation mechanisms, we expect that the level of confinement also plays a crucial role in setting the droplet velocity. We have performed rational experiments (investigating viscosity ratio, droplet confinement). We show that two regimes of droplet velocity as a function of capillary number are also observed and, in the capillary regime the droplets go faster than the one estimated from models. We propose a refined model taking into account a modified droplet dissipation that should be useful for the community. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L38.00004: Interaction between microfluidic droplets in a Hele-Shaw cell Itai Sarig, Yuli Starosvetsky, Amir Gat Various fluidic systems, such as chemical and biological lab-on-a-chip devices, involve motion of multiple droplets within an immersing fluid in narrow micro-channels. Modeling the dynamics of such systems requires calculation of the forces of interaction between the moving droplets. These forces are commonly approximated by superposition of dipoles solutions, which requires an assumption of sufficiently large distance between the droplets. In this work we obtain exact solutions for two droplets, and a droplet within a droplet, located within a moving immersing fluid and without limitation on the distance between the droplets. This is achieved by solution of the Laplace equation for the pressure in a bi-polar coordinate system, Fourier method and transformation and calculation of the force in a Cartesian coordinate system. Our results are validated with numerical computations, experimental data and with the existing dipole-based models. We utilize the results to calculate the dynamics of a droplet within a droplet, and of two close droplets, located within an immersing fluid with oscillating speed. The obtained results may be used to study the dynamics of dense droplet lattices, common to many current micro-fluidic systems. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L38.00005: Nonlinear Dynamics of Droplets in a Hele-Shaw Cell: Short-Lived Solitary Waves in a 1D Lattice Amir Gat, Danila Meimukhin, Yuli Starosvetsky We study the nonlinear dynamics of a one-dimensional lattice consisting of shallow droplets, immersed in an immiscible liquid flowing within a Hele-Shaw cell. Such configurations are commonly used in micro-fluidic devices for chemical and biological applications. We apply regular multi-scale expansions constructed for the asymptotic limit of low energy excitations. The expansions yield Korteweg de Vries and linear Schrodinger equations governing the system dynamics, which is remarkable for configurations without inertial effects. Solutions of the governing equation are shown to include a special class of short-living solitary waves. The analytical findings are validated by the numerical computations. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L38.00006: The mysterious droplet birth in a microfluidic cross junction Stephanie van Loo, Tristan Gilet In microfluidics flow focusing is widely used to produce water-in-oil droplets in microchannels at high frequency. Nevertheless, the scaling laws associated to droplet length, speed and frequency could not be identified yet, owing to the large number of parameters involved (incl. complex geometry). We here present an experimental study of droplet formation in a microfluidic cross-junction with a minimum number of geometrical parameters. We mostly focus on the dripping regime. The formation sequence is decomposed in two steps, inflation and squeezing, that vary differently according to both water and oil flow rates. These variations reveal several insights about the fluid flows in both phases. From there we infer the scaling law that relates droplet volume and frequency to the Capillary number associated to each inlet flow rate. This law involves a minimum of fitting parameters. We finally discuss the influence of inlet control (flow rate vs. pressure) and surfactants on the formation dynamics. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L38.00007: Flow regimes in a T-mixer operating with a binary mixture Simone Camarri, Lorenzo Siconolfi, Chiara Galletti, Maria Vittoria Salvetti Efficient mixing in small volumes is a key target in many processes. Among the most common micro-devices, passive T-shaped micro-mixers are widely used. For this reason, T-mixers have been studied in the literature and its working flow regimes have been identified. However, in most of the available theoretical studies it is assumed that only one working fluid is used, i.e. that the same fluid at the same thermodynamic conditions is entering the two inlet conduits of the mixer. Conversely, the practical use of micro-devices often involves the mixing of two different fluids or of the same fluid at different thermodynamic conditions. In this case flow regimes significantly different than those observed for a single working fluid may occur. The present work aims at investigating the flow regimes in a T-mixers when water at two different temperatures, i.e. having different viscosity and density, is entering the mixer. The effect of the temperature difference on the flow regimes in a 3D T-mixer is investigated by DNS and stability analysis and the results are compared to the case in which a single working fluid is employed. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L38.00008: Droplet migration toward and away from wall in micro-flow Yeng-Long Chen, Shih-Hao Wang, Wei-Ting Yeh The hydrodynamically-induced particle migration phenomenon in microfluidic flow has been applied for cell isolation and particle separation. First-order analysis has been able to predict the migration velocity due to particle surface inertial stress and particle deformation, for small Reynolds $Re$ and Capillary ($Ca$) numbers [1]. However, at moderate flow rates, non-linear dependences of particle migration on flow rate are found [2]. We employed lattice Boltzmann-immersed boundary method to examine the dependence of droplet migration on $Re$, $Ca$, and the droplet inner/outer viscosity ratio $\lambda$. We found that whether a droplet migrates towards or away from the wall at steady state depends strongly on $\lambda$. At high flow rates, droplets with lower inner viscosity migrate towards the center. At low flow rates, there is an optimal $\lambda$ at which the droplet steady state position is closest to the channel center. This result agrees with prior experimental observations for oil in water droplets [3]. The consequences for particle separation will be discussed.\\[4pt] [1] P. C. H. Chan and L. G. Leal, J. Fluid Mech., 1979, 92, 131. \newline [2] Y.-L. Chen, RSC Advances, 2014, 4, 17908 \newline [3] S. C. Hur et al., Lab Chip, 2011, 11, 912 [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L38.00009: Droplet Trajectory Control Using Light-Induced Thermocapillary Effects in a Microchannel June Won, Seungmin Kang, Simon Song Controlling droplets is one of the important functions on a microfluidic chip. Marangoni effects induced by interfacial tension gradient has been paid attention due to its strong driving force on a droplet by means of droplet control. Solutalcapillary effects occurs when the interfacial tension gradient is induced due to the transport of surfactant molecules. We aim to investigate light-induced solutalcapillary effects on a droplet trajectory. Unlike few previous studies, we illuminate a continuous phase with a laser beam, in order to minimize possible damage or property change to target molecules contained in droplets. A mixture solution of black metallic ink and oleic acid is used for the continuous phase fluid. DI-water is the disperse phase. As a result, we found that the trajectory shifting increases with increasing the laser power and the droplet diameter and decreasing the droplet velocity. The magnitude of Marangoni force was estimated to be about 100 nN by assuming quasi-equilibrium between drag force and Marangoni force. As an application of this technique, we successfully routed droplets toward one of three outlets at higher than 95\% success rate on demand. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L38.00010: Droplets in microchannels: dynamical properties of the lubrication film Axel Huerre, Olivier Theodoly, Alexander Leshansky, Marie-Pierre Valignat, Isabelle Cantat, Marie-Caroline Jullien The motion of droplets or bubbles in confined geometries has been extensively studied; showing an intrinsic relationship between the lubrication film thickness and the droplet velocity. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation both in the droplet and between meniscus and wall. However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We find and characterize two distinct dynamical regimes, dominated respectively by capillary and intermolecular forces. In the first regime, we also identified a model with interfacial boundary condition considering only viscous stress continuity that agrees well with film thickness dynamics and interface velocity measurement. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L38.00011: Counter-current thermocapilllary migration of bubbles in microchannels using self-rewetting liquids Robson Nazareth, Pedro Saenz, Prashant Valluri, Khellil Sefiane The study of bubble transport in microchannels is of great interest in evaporative cooling of microdevices technologies. This is because bubble transport under heat-transfer or phase-change causes several flow instabilities that are less understood and hinder informed design of microcooling devices. Bubble motion in microchannels under temperature gradients is highly influenced by thermocapillary forces due surface tension gradients. Most studies until now so far are mainly based on pure liquids which present a linear temperature (inverse) dependence of surface tension. In this work, we consider motion of a bubble (formed of inert gas) in the so-called self-rewetting fluid that presents a parabolic (quadratic) dependence of surface tension on temperature, in a temperature range that includes a surface tension minimum. We particularly investigate the counter-current thermocapillary migration of bubbles in these liquids, as experimentally depicted by Shanahan and Sefiane (2014), by means of direct numerical simulations. We present a model that solves the 3D governing equations of mass, momentum, interface and energy for the two-phase system composed by incompressible, Newtonian and immiscible fluids. We resolve the deformable interface by means of a Volume-of-Fluid method. Our results indicate that there exists a pressure drop limit beyond which there would be no counter-current migration of bubbles. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L38.00012: Numerical Simulations of Droplet Dynamics in PEM Fuel Cell Microchannels Eric Cauble, Mark Owkes Proton exchange membrane (PEM) fuel cells are of beneficial interest due to their capability of producing clean energy with zero emissions. An important design challenge hindering the performance of fuel cells is controlling water removal to maintain a hydrated membrane while avoiding excess water that may lead to channel blockage. Fuel cell water management requires a detailed knowledge of multiphase flow dynamics within microchannels. Direct observation of gas-liquid flows is difficult due to the small scale and viewing obstructions of the channels within the fuel cell. Instead, this work uses a CFD approach to compute the formation and dynamics of droplets in fuel cell channels. The method leverages a conservative volume-of-fluid (VOF) formulation coupled with a novel methodology to track dynamic contact angles. We present details of the numerical approach and simulation results relevant to water management in PEM fuel cells. In particular, it is shown that variation of the contact hysteresis angle influences the wetting properties of the droplet and significantly impacts water transport throughout the a fuel cell channel. [Preview Abstract] |
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