Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G36: Microfluids: Drops/Bubbles |
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Chair: Taehun Lee, City College of New York Room: 407 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G36.00001: A co-flow-focusing monodisperse microbubble generator Jiaming Zhang, Erqiang Li, Sigurdur Sigurdur Here we report the design and fabrication of a simple and inexpensive microfluidic device based on micro-scope glass slides and two tapered glass capillaries, for generating monodisperse microbubbles. The first capillary that used for transporting gas, was heated and pulled to have a sharp tapered tip around 2 microns, and was inserted into the second capillary, with its sharp tip aligned to the converging-diverging throat of the second capillary. This configuration provides a smooth, small gas flow rate, as well as a high velocity gradient at the gas outlet. By varying liquid flow rates and viscosities, highly monodisperse microbubbles with diameter range from 3.5 to 50 microns have been successfully produced, at a rate up to 50 kHz. A simple scaling law based on capillary number and flow rate ratios, used to predict bubble size, is also proposed. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G36.00002: Critical behavior of droplet breakup in T-junction microchannels Volkert van Steijn, Duong Hoang, Luis Portela, Chris Kleijn, Michiel Kreutzer The critical behavior of droplet breakup in T-junction mirochannels is studied using three-dimensional numerical simulations. Two scenarios can happen when a droplet flows into a T-junction: (i) if the flow is strong enough, it breaks into two daughter droplets and (ii) otherwise, it drifts away into one branch of the T-junction owing to flow perturbations. Whether a droplet breaks or not is determined by the ratio between two timescales: breakup time and drifting time. Symmetric-boundary-condition simulations allow us to study the breakup time without any flow perturbations, thus to accurately compute the critical capillary number below which the droplet does not break. We study the drifting using full-T-junction simulations, identifying three phases in drifting process: (i) an exponential drifting, (ii) a transition phase and (iii) a linear drifting. Combining the understanding of the breakup and drifting behavior, we found that the critical capillary number below which the droplet drifts away increases more than 10\% with respect to the one obtained in free-perturbation flow systems. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G36.00003: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 8:39AM - 8:52AM |
G36.00004: Tipstreaming from the rear of surfactant laden droplets traveling through a microchannel Todd Moyle, Lynn Walker, Shelley Anna Microscale tipstreaming is a hydrodynamic phenomenon able to generate submicron sized droplets in a microfluidic device. The tipstreaming process results in the generation of a thin thread from a highly curved interface. In this work, we present observations of tipstreaming occurring at the rear of droplets translating along a microchannel. Drops are formed in a flow focusing geometry at geometry-controlled formation conditions. The drops then accelerate in the exit channel due to the addition of continuous phase liquid from two intersecting channels. Upon acceleration, the droplets form a highly curved tip at the rear and begin to shed a stream of tiny drops. The distance between the acceleration point and the location downstream at which tipstreaming occurs depends on surfactant concentration, drop size, and flow rate of the added continuous phase liquid. We examine the effect of these parameters on the tipstreaming process. Because tipstreaming occurs downstream of the acceleration point, after the drop has attained a new steady state velocity, we hypothesize that the adsorption of additional surfactant on the interface is the primary factor driving the onset of tipstreaming. We use these measurements to probe the timescale for surfactant adsorption to the droplet interface. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G36.00005: Gas bubble formation and its pressure signature in T-junction of a microreactor Shahram Pouya, Manoochehr Koochesfahani The segmented gas-liquid flow is of particular interest in microreactors used for high throughput material synthesis with enhanced mixing and more efficient reaction. A typical geometry to introduce gas plugs into the reactor is a T-junction where the dispersed liquid is squeezed and pinched by the continuous fluid in the main branch of the junction. We present experimental data of time resolved pressure along with synchronous imaging of the drop formation at the junction to show the transient behavior of the process. The stability of the slug regime and the regularity of the slug/plug pattern are investigated in this study. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G36.00006: Early microfluidic dissolution regime of CO$_{2}$ bubbles in viscous oils Martin Sauzade, Thomas Cubaud We investigate the initial dynamical behavior of dissolving micro-bubbles composed of carbon dioxide gas in highly viscous silicone oils over a range of flow rates and pressure conditions. Microfluidic periodic trains of monodisperse CO$_{2}$ bubbles are used to probe the interrelation between bubble dissolution and high-viscosity multiphase flows in microgeometries. The effective mass diffusion flux across the interface is measured by tracking individual bubbles and monitoring their shape as they experience a size reduction. The initial steady mass flux is characterized using a dissolution coefficient that depends on the fluids physicochemical properties. Our findings show the possibility to control and exploit the interplay between capillary and mass transfer phenomena with highly viscous fluids in small-scale systems. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G36.00007: The Physical Mechanisms Governing Drop Coalescence: Models vs Experiments James Sprittles, Yulii Shikhmurzaev The dominant physical mechanisms in the coalescence of liquid drops are identified by utilizing recent advances in experimental and computational techniques that resolve unprecedentedly small spatio-temporal scales. To do so, the predictions of both the ``conventional'' model and the (singularity-free) interface formation model, where the dynamics of an ``internal interface'' trapped between the two bulk phases takes a finite time to disappear, before the conventional model takes over, are compared to experimental measurements on microfluidic scales of the very initial moments of coalescence. Using the full numerical solution of the problem in the framework of each of the two models, we show that the recently reported electrical measurements are better described by the interface formation model. As a by product of our results, the range of validity of scaling laws proposed for the phenomenon is established, with inconsistencies in previous works rectified and particular attention paid to quantifying the external gas' effect on the dynamics. Finally, a new scaling law developed for the inertial regime is shown to capture experimental data from the literature remarkably well. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G36.00008: Optofluidic droplet coalescence on a microfluidic chip Jin Ho Jung, Kyung Heon Lee, Kang Soo Lee, Hyunjun Cho, Byung Hang Ha, Ghulam Destgeer, Hyung Jin Sung Coalescence is the procedure that two or more droplets fuse during contact to form a larger droplet. Optofluidic droplet coalescence on a microfluidic chip was demonstrated with theoretical and experimental approaches. Droplets were produced in a T-junction geometry and their velocities and sizes were adjusted by flow rate. In order to bring them in a direct contact of coalescence, optical gradient force was used to trap the droplets. A theoretical modeling of the coalescence was derived by combining the optical force and drag force on the droplet. The analytical expression of the optical force on a sphere droplet was employed to estimate the trapping efficiency in the ray optics regime. The drag force acting on the droplet was calculated in terms of the fluid velocity, viscosity and the geometrical parameters of a microfluidic channel. The droplet coalescence was conducted in a microfluidic setup equipped with a 1064 CW laser, focusing optics, a syringe pump, a custom-made stage and a sCMOS camera. The droplets were successfully coalesced using the optical gradient force. The experimental data of coalescence were in good agreement with the prediction. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G36.00009: Constrained Energy Minimization of a Pinned Droplet on an Inclined Plate Michiel Musterd, Volkert van Steijn, Chris R. Kleijn, Michiel T. Kreutzer A long standing problem is the prediction of the maximum volume of a droplet that can hang on an inclined plate without rolling off. A key issue in this prediction is to understand the deformation of the droplet. We show that the common assumptions of a fixed droplet base or a shape at global energy minimum result in significant errors. We study droplets on a inline using locally constrained energy minimization. The initial shape of the droplet and maximum and minimum attainable contact angles hereby put constraints on the energy minimization. This results in a history dependence of the droplet behavior before roll-off, but surprisingly, a universal behaviour of the front-to-back baselength of the droplet at roll-off. This universal behavior can be predicted from equilibrium droplet shapes on a horizontal surface and understood from energy landscapes for a 2D droplet. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G36.00010: The way to reduce electrical charge of a droplet dispensed from a pipette tip Dongwhi Choi, Horim Lee, Do Jin Im, Dong Sung Kim Recently, we reported that a conventional pipetting always makes a charged droplet by spontaneous electrical charging process. The charge amount depends on the constituents of the droplet, on coating material of pipette tip and on atmospheric humidity. We clarified that this natural electrification of a droplet is originated from the charge separation between a droplet and pipette tip surface. The electrical interaction between charged droplet hanging on the end of the pipette tip and the pipette tip inner surface makes the droplet hard to detach from the pipette tip. To suggest the way to suppress the electrification phenomenon, we investigate the influence of the polymer composition on the amount of the charge of the droplet. The Faraday cup method is performed to measure the charge amount of the droplet. The result can be used to reduce charge amount of a droplet dispensed from the micropipette tip effectively. [Preview Abstract] |
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