Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D10: Microscale Flows: Mixing and Reactions in Droplets |
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Chair: Shimon Rubin, Israel Institute of Technology Room: 3005 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D10.00001: Convection-diffusion driven concentration gradients in nanolitre droplets for microfluidic screening applications Raviraj Thakur, Ahmed Amin, Steven Wereley Ability to generate a concentration gradients in emulsified aqueous droplets is a highly desired feature for several lab-on-chip applications. Numerous schemes exists for generating concentration gradients in continuous flow devices such as Y junctions, split-and-recombine techniques, etc. However, varying the sample concentration in emulsified droplets is quite challenging. In this work, we have developed a scheme for generating and controlling concentration gradients in programmable multi-layer PDMS microfluidic chips. Briefly, a high concentration sample is injected into a steady stream of buffer. The buffer with the sample pulse and an immiscible oil phase are flowed through a T-junction in an alternate manner. As the sample pulse advances, the combined effect of diffusion and convection produced dispersion of sample pulse in streamwise direction. This continuous gradient stream is split into discrete droplets at the T-junction. Pulsatile flow condition are maintained using on-chip diaphragm peristaltic pumps. The problem can be thought of an extension of Taylor-Aris dispersion with laminar pulsatile flow in rectangular channels. The concentration profile is found to be dependent upon the frequency of pulsatile flow and thus can be fine-tuned according to application needs. Theoretical framework is established for pump regimes that correlates the diffusion coefficients of the input samples with the resultant concentration profiles. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D10.00002: Intra-phase mixing in a bi-component translating drop Thomas Ward The intra-phase mass transport in a translating spherical drop containing two species will be studied numerically in the zero capillary number limit. The problem is relevant to microfluidic systems where it is common to form two drops of unequal or nearly equal volume in a microfluidic channel where they subsequently merge and then translate. The mixing process in this system is controlled by diffusion due to the small length scales despite the relatively large velocities and low diffusivities. The species conservation equation are discretized using a $4th$ order finite difference scheme in space with an adaptive explicit Runge-Kutta-Merson scheme to advance in time. With this scheme the solutions conserve mass throughout the numerical integration cycle. Numerical data for Peclet numbers ranging between 1000-10000 will be used to estimate the deviation from the equilibrium concentration as a function of time. Initial species concentration range from ratios of 1:9 to 1:1. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D10.00003: Modeling the Dilution of Static Droplet Arrays with Moving Plugs William Wang, Siva Vanapalli Generation of arrays of immobilized microfluidic droplets with variation in reagent concentration from drop-to-drop is important for a variety of biochemical and screening assays. Recently our laboratory (Sun et al., Lab Chip, 2011) showed that such gradients in chemical concentration can be achieved by coalescing diluting plugs with drops immobilized in a microfluidic parking network. In this study, we investigate the key hydrodynamic mechanisms responsible for generation of concentration gradients in static droplet arrays, with the goal of predicting the dilution profiles observed in experiments. We conduct simulations based on a phenomenological model that includes diffusion, advection due to circulating flow within moving plugs, enhanced material transfer due to coalescence and break-up events, and geometry. Consistent with experiments, we find that the concentration profiles can exhibit segmentation between rows of parked droplets due to coalescence events occurring on alternating sides of the diluting plug. Tail-sweeping of wall material can increase concentrations in the plug tail. Also, coalescence and break-up events can significantly enhance dilution rates and ranges. Our results impact the design of SDAs for creating broad and predictable concentration gradients. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D10.00004: Self-assembly and novel planetary motion of ferrofluid drops in a rotational magnetic field Ching-Yao Chen, Hao-Chung Hsueh We experimentally investigate the motion of ferrodrops in a rotating magnetic field. Magnetized and driven by the external field, the ferrodrops are stretched and self-align to form a drop array along the field orientation. An interesting planet-like dual rotation, including local self-spins of individual drops and a global revolution of the drop array, is newly identified. While the drops spin nearly synchronized with the external field, the revolution always lags behind the field and appears a forth and back movement. Prominence of the net revolutionary movement depends on the strength and uniformity of the overall field as well as the number of drops containing in the array. In general, more uniform and stronger rotating field lead to a more prominent global revolution. Phenomenon of such planetary motion can be applied to mix two fluids more effectively than self-spin drops. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D10.00005: Migration of deformable droplets caused by microfluidic inertial effects Guoqing Hu, Chundong Xue, Xiaodong Chen The inertial effect is an effective way of focusing and sorting droplets suspended in microchannels. Here we conduct numerical simulations and experiments on the droplet motion and deformation in a straight microchannel. In contrast to most existing literature, the present simulations are three-dimensional and full length in the streamwise direction. The migration dynamics and equilibrium positions of the droplets are obtained for different fluid velocities and droplet sizes. Droplets with diameters larger than half of the channel height migrate to the centerline in the height direction and two equilibrium positions are observed between the centerline and the wall in the width direction. In addition to the well-known Segre-Silberberg equilibrium positions, new equilibrium positions closer to the centerline are observed. This finding is validated by preliminary experiments that are designed to introduce droplets at different initial lateral positions. Small droplets also migrate to two equilibrium positions in the quarter of the channel cross section, but with the coordinates between the centerline and the wall. The distributions of the lift forces, angular velocities and the deformation parameters of droplets along the two confinement direction are also investigated in details. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D10.00006: Micro-droplets lubrication film thickness dynamics Axel Huerre, Olivier Theodoly, Isabelle Cantat, Alexander Leshansky, Marie-Pierre Valignat, 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 between meniscus and wall. However, this film may become thin enough that intermolecular forces come into play and affect classical scalings. We report here the first experimental evidence of the disjoining pressure effect on confined droplets by measuring droplet lubrication film thicknesses in a microfluidic Hele-Shaw cell. We find and characterize two distinct dynamical regimes, dominated respectively by capillary and intermolecular forces. In the former case {\it rolling} boundary conditions at the interface are evidenced through film thickness dynamics, interface velocity measurement and film thickness profile. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D10.00007: Droplet formation and lateral migration via solvent shifting in a microfluidic setup Ramin Hajian, Steffen Hardt When a non-solvent is added to a solvent/solute mixture and if the solvent and the non-solvent are miscible, a part of the solute transforms to tiny (i.e. micron-/submicron-sized) droplets when the solvent concentration reduces. This phenomenon, resulting from supersaturation, is termed solvent shifting or Ouzo effect. Here we investigate this process in a co-flow microfluidic device. Thanks to the laminar nature of the flow, the mass transfer is mainly diffusive and can be analyzed employing (semi)analytical models. Using the resulting concentration profiles along with the ternary phase diagram (TPD) we analyze droplet formation and their lateral migration in the channel. The ternary system consists of a binary mixture (0.5wt{\%} divinyle benzene (DVB) $+$ 95.5wt{\%} ethanol) and deionized water (non-solvent). Plotting concentration trajectories in the TPD we show that they hit the binodal curve in a region in which droplets of DVB form via nucleation, as opposed to spinodal decomposition. The lateral migration of droplets is partially attributed to the Marangoni effect induced by concentration gradients. However, the main effect governing droplet migration appears to be the phase-separation front (separating the one-phase and two-phase regions) moving toward the center of the channel. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D10.00008: Theory of microfluidic step-emulsification Alexander Leshansky, Zhenzhen Li, Samuel Metais, Len Pismen, Patrick Tabeling We present a comprehensive study of the microfluidic step-emulsification process for high-throughput production of monodisperse colloidal droplets. The ``microfluidic step emulsifier'' combines a shallow microchannel operating with two co-flowing immiscible fluids and an abrupt (step-like) opening to a deep and wide reservoir. Based on Hele-Shaw hydrodynamics, we determine the quasi-static shape of the fluid interface prior to transition to oscillatory step-emulsification at low capillary numbers. The transition threshold obtained from scaling arguments yields an excellent agreement with experimental data. A closed-form expression for the size of the droplets generated in the step-emulsification regime and derived using geometric arguments also shows a very good agreement with the experiment. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D10.00009: Numerical simulation of droplet formation regimes and sizes in microfluidic T-junction devices Mehdi Nekouei, Siva Vanapalli The T-junction geometry has been widely used for producing monodisperse droplets in microfluidic devices. Droplet formation regimes and sizes are expected to depend on a variety of conditions including flow rates, capillary number, channel geometry and viscosity ratio. Experiments have investigated drop production at a T-junction in a narrow control parameter space and developed analytical models for specific operating regimes. In this study, we take advantage of numerical simulations based on volume-of-fluid method to explore this broad parameter space systematically, and contrast our results with prior experimental data. We find our simulations predict well the regimes of squeezing, dripping and jetting. We also observe that our drop size data is in good agreement with three different experimental reports. Although our results match experimental data, the analytical models do not agree with each other since they are based on specific operating conditions. We use numerical simulations to elucidate the missing components in the physics of drop formation at a T-junction, with an attempt to reconcile existing analytical models. [Preview Abstract] |
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