Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L14: Microscale Flows: Drops and BubblesDrops Micro
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Chair: Mehdi Nekouei, Texas Tech University Room: 507 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L14.00001: ABSTRACT WITHDRAWN |
Monday, November 20, 2017 4:18PM - 4:31PM |
L14.00002: Experimental study of the flow pattern around a bubble confined in a microfluidic Hele-Shaw cell Yannis Tsoumpas, Christophe Fajolles, Florent Malloggi The flow field around a bubble moving with respect to a surrounding liquid in a Hele-Shaw cell can usually be characterized by a recirculating flow, which is typically attributed to a Marangoni effect due to surface tension gradients generated by a non-uniform distribution of surfactants (or temperature) along the liquid-gas interface. In the present study, we try to visualize such a flow employing 3D micro-particle tracking velocimetry. We perform experiments on an immobile flattened air bubble that is surrounded by a flow of aqueous solution of surfactant (SDS), in a microfluidic chamber described in the work of Sungyon Lee et al. (Soft Matter, 2012, 8, 10750). The suspending fluid is seeded with spherical micro-particles, with those captured by the recirculating flow orbiting in a three-dimensional trajectory in the vicinity of the liquid-air interface. We address the effect of velocity of the surrounding fluid, surfactant concentration and bubble radius on the recirculating flow pattern. The case of a liquid-liquid interface, with a hexadecane drop as the dispersed phase, is also discussed. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L14.00003: Cell-encapsulating droplet formation in a flow-focusing configuration Mohammad Nooranidoost, Majid Haghshenas, Metin Muradoglu, Ranganathan Kumar Cell encapsulation in a flow-focusing microchannel is computationally studied using a three- phase front-tracking method. A series of cells with predefined size and frequency are encapsulated by the disperse phase forming compound droplets suspended in an outer fluid. Compound droplet formations are examined for a wide range of parameters including cell size, viscosity ratio ($\beta =\mu_{\mathrm{out}}$/$\mu_{\mathrm{in}})$ and capillary number of the continuous (Ca$_{\mathrm{out}})$ and disperse (Ca$_{\mathrm{in}})$ phases. An extensive simulation on Ca$_{\mathrm{out\thinspace }}$and Ca$_{\mathrm{in}}$ for different viscosity ratios ($\beta =$0.5,1,2 and 4) reveals a region in which a uniform compound droplet production is most likely to occur. Depending on the fluid and flow parameters, compound droplets can consist of one or multiple cells, while for some cases no cells are found in a compound droplet. For a fixed Ca$_{\mathrm{out\thinspace }}$and Ca$_{\mathrm{in}}$ in the favorable region, effects of viscosity ratio and cell size are studied to control cell encapsulation dynamics. It is found that decreasing viscosity of the outer phase results in higher rate of cell encapsulation, while decreasing cell size adversely impacts compound droplet generation. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L14.00004: The effect of channel height on bubble nucleation in superhydrophobic microchannels due to subcritical heating Adam Cowley, Daniel Maynes, Julie Crockett, Brian Iverson This work experimentally investigates the effects of heating on laminar flow in high aspect ratio superhydrophobic (SH) microchannels. When water that is saturated with dissolved air is used, the unwetted cavities of the SH surfaces act as nucleation sites and air effervesces out of solution onto the surfaces. The microchannels consist of a rib/cavity structured SH surface, that is heated, and a glass surface that is utilized for flow visualization. Two channel heights of nominally 183 and 366 $\mu $m are considered. The friction factor-Reynolds product (fRe) is obtained via pressure drop and volumetric flow rate measurements and the temperature profile along the channel is obtained via thermocouples embedded in an aluminum block below the SH surface. Five surface types/configurations are investigated: smooth hydrophilic, smooth hydrophobic, SH with ribs perpendicular to the flow, SH with ribs parallel to the flow, and SH with both ribs parallel to the flow and sparse ribs perpendicular to the flow. Depending on the surface type/configuration, large bubbles can form and adversely affect fRe and lead to higher temperatures along the channel. Once bubbles grow large enough, they are expelled from the channel. The channel size greatly effects the residence time of the bubbles and consequently fRe and the channel temperature. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L14.00005: Microparticle separation in moving droplets using traveling surface acoustic wave Kwangseok Park, Jinsoo Park, Jin Ho Jung, Ghulam Destgeer, Husnain Ahmed, Raheel Ahmad, Hyung Jin Sung In droplet-based microfluidics systems, in-droplet separation of sample mixture is one of strategic prerequisites for purification, enrichment and detection signal improvement of targets in droplet-based microfluidic systems. There have been a few studies on separation in a moving droplet; however, they have limitations such as a time-consuming mechanism and a need of targets with electric or magnetic properties. In this study, we demonstrated the lateral position control of 5 and 10 $\mu $m polystyrene particles inside a moving water-in-oil droplet using traveling surface acoustic waves (TSAWs). Microparticles affected by an acoustic radiation force (ARF) of TSAW are laterally moved in the direction of TSAW propagation. The ARF of first 135 MHz TSAW acts on both particles inside the droplet while that of 95 MHz TSAW in the opposite propagating direction to that of 135 MHz TSAW affects the bigger size. Thus, lateral positions of the microparticles inside the droplet differ depending on the particle size. In-droplet separation is complete after droplet splitting at a bifurcation zone. Compared to previous studies, our system enables a label-free and on-demand control. Furthermore, it is the first time to realize the separation of different-sized microobjects in the moving droplets. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L14.00006: Break-up dynamics of a confined droplet in a microchannel containing obstacle Mehdi Nekouei, Siva Vanapalli Understanding the break-up of confined drops passing through obstacle-laden microchannels is important for a variety of applications ranging from oil recovery to blood flows to microfluidics. A confined droplet impacting an obstacle can either break-up or bypass the obstacle. Although there have been a number of studies on the dynamics of droplet break-up at microfluidic T-junctions, investigations of dynamics of droplet break-up against an obstacle in a microchannel is limited. Using volume-of-fluid three-dimensional simulations, we investigated the influence of different system parameters on the break-up of a droplet. We observed that by increasing the viscosity of the droplet, break-up occurs at smaller imposed flow rate, i.e. highly viscous droplets break-up more easily. This observation is in contrast to unconfined droplets in shear flows, where both low ($\lambda $ \textless 0.1) and high viscosity ($\lambda $ \textgreater 10) droplets are more difficult to fragment. We found that the hydrodynamic resistance of the droplets plays an important role in dictating the break-up. In addition, we performed stop-flow simulations, to investigate the role of curved interface and internal flow in autonomous pinch-off of the droplet. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L14.00007: Influence of thermal fluctuations on ligament break-up: a fluctuating lattice Boltzmann study Xiao Xue, Luca Biferale, Mauro Sbragaglia, Federico Toschi Thermal fluctuations are essential ingredients in a nanoscale system, driving Brownian motion of particles and capillary waves at non-ideal interfaces. Here we study the influence of thermal fluctuations on the breakup of liquid ligaments at the nanoscale. We offer quantitative characterization of the effects of thermal fluctuations on the Plateau-Rayleigh mechanism that drives the breakup process of ligaments. Due to thermal fluctuations, the droplet sizes after break-up need to be analyzed in terms of their distribution over an ensemble made of repeated experiments. To this aim, we make use of numerical simulations based on the fluctuating lattice Boltzmann method (FLBM) for multicomponent mixtures. The method allows an accurate and efficient simulation of the fluctuating hydrodynamics equations of a binary mixture, where both stochastic viscous stresses and diffusion fluxes are introduced. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L14.00008: A micro-PIV study on the internal flow within concentrated emulsion droplets flowing in a microchannel. Chia Min Leong, Ya Gai, Sindy K. Y. Tang Droplet microfluidics has enabled a wide range of high throughput applications through the use of monodisperse droplets. Previous studies on the internal flow pattern of droplet microfluidics have mainly focused on a single drop. The study of concentrated emulsions is important for increasing the throughput, but the fluid dynamics of such emulsions in confined channels is not well understood. In this work, we use micro-PIV to probe the two-dimensional, mid-height flow inside individual drops within a concentrated emulsion. The emulsion has 85{\%} volume fraction and flows as a monolayer in a microchannel. The effects of confinement and viscosity ratio on the internal flow patterns inside the drops were studied. The results show rotational structures inside the drops always exist and is dependent on the emulsion confinement and the droplet location in the channel. In addition, we show droplets involved in rearrangement event exhibit transient internal vortical structures, which arise due to the nature of concentrated emulsion and cannot be predicted by the flow in single droplet or diluted emulsion. To our best knowledge, no work has probed the flow field inside concentrated emulsion droplets at high volume fractions in confined channels. Current work is in progress to measure the three-dimensional flow field in such system. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L14.00009: Dynamics of viscous drops confined in a rough medium Ludovic Keiser, Armelle Gas, Khalil Jaafar, Jose Bico, Etienne Reyssat We focus on the dynamics of viscous and non-wetting "pancake" droplets of oil confined in a vertical Hele-Shaw cell filled with a less viscous surfactant solution. These dense drops settle at constant velocity driven by gravity. The surfactant solution completely wets the walls, and a thin lubrication film separates the drops from the walls. With smooth walls, two main dynamical regimes are characterized as the gap between the walls is varied. Viscous dissipation is found to dominate either in the droplet or in the lubrication film, depending on the ratio of viscosities and length scales. A sharp transition between both regimes is observed and successfully captured by asymptotic models. With rough walls, that transition is dramatically altered. Drops are generally much slower in a rough Hele-Shaw cell, in comparison with a similar smooth cell. Building up on the seminal works of Seiwert et al. (J.F.M. 2011) on film deposition by dip coating on a rough surface, we shed light on the non-trivial friction processes resulting from the interplay of viscous dissipation at the front of the drop, in the lubrication film and in the bulk of the drop. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L14.00010: Mixing efficiency inside micro-droplets coalesced by two components in cross-structure Yanlin Ren, Zhaomiao Liu, Yan Pang The mixing of micro-droplets is used in analytical chemistry, medicine production and material synthesis owing to its advantages including the encapsulation and narrow time residence distribution. In this work, droplets are coalesced by two dispersed phase with different flow rates, generated in cross-structure and mixed in planar serpentine structure. The mixing efficiency of micro-droplets under control characters including the width of entrance and the flow rate of dispersed phases have been investigated by experiments and numerical simulations. The UDS (user-defined scalar) as dimensionless concentration of the solution is adopted in simulation, and is used to calculate the concentration and the mixing effect. By changing the flow rates and the entrances` width, the changing rules of the mixing characters have been obtained. The asymmetry distributions of components make rapid mixing process in half part of each droplet when travel through a straight channel. Increasing of the ratio of entrance width result into larger droplet and weaken the chaotic mixing effect. Meanwhile, the coalesced mechanism can be performed by ranging the ratio of flow rates, the ranges are also determined by the widths of entrances. [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L14.00011: A passive approach for upgrading the uniformmity of droplet array in the microfluidic trapping network Longxiang Zhang, Zhaomiao Liu, Yan Pang, Mengqi Li Droplet-based microfluidics has shown much promise in protein crystallization, material synthesis and blood detection, due to its unique advantages including no cross-contaminations and reduced amounts of reagents. To monitor the kinetic characteristics of biochemical reactions inside the sample chambers, it is necessary to store the moving drop train at a certain position within the device. A noval approach, which utilizes the unique physical properties of bubble, is proposed to trap the drops in a microfluidic trapping network. The trapping mode diagram of drop train with and without a guiding bubble is specifically concentrated on in this work. The trapping mode of drops with the bubble leading transitions from sequentially uniform trapping for smaller drops in a narrow range, to nonuniform trapping induced by breakup and collision for larger drops. Disordered trapping is heavily attributed to the instability of drop speed and spacing in the bypass, while the flow region of desired uniform trapping is broadened by introducing the bubble. This investigation is beneficial to enhance the applicability of microfluidic chips for passively trapping drops. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L14.00012: Investigation of a piezoelectric droplet delivery method for fuel injection and physical property evaluation Wei Zhao, Shyam Menon A piezoelectric droplet generator is investigated to deliver liquid hydrocarbon fuels to a micro-combustor application. Besides fuel delivery, the setup is intended to measure fuel physical properties such as viscosity and surface tension. These properties are highly relevant to spray generation in internal combustion engines. Accordingly, a drop-on-demand piezoelectric dispenser is used to generate fuel droplet trains, which are studied using imaging and Phase Doppler Particle Anemometry (PDPA). The diagnostics provide information regarding droplet size and velocity and their evolution over time. The measurements are correlated with results from one-dimensional (1D) models that incorporate sub-models for piezo-electric actuation and droplet vaporization. By validating the 1D models for fuels with known physical properties, a technique is developed that has the capability to meter low-vapor pressure liquid fuels to the microcombustor and use information from the droplet train to calculate physical properties of novel fuels. [Preview Abstract] |
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