Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H25: Microscale Flows: Drops and Bubbles-II |
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Chair: Siva Vanapalli, Texas Tech University Room: E145 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H25.00001: Dynamics of droplet entrapment in a constricted microchannel Mehdi Nekouei, Swastika Bithi, Siva Vanapalli Droplet migration and clogging in confined geometries is a problem of fundamental importance in oil recovery and droplet microfluidics. A confined droplet flowing through a conduit can either be arrested at the constriction or squeeze through it. The dynamics of the trapped and squeezed states are expected to depend on capillary number, drop size, viscosity ratio. Although there have been a number of studies on the dynamics of droplets passing through a constriction, investigations of dynamics of trapped droplets in constricted microchannels is lacking. In this work, we performed three-dimensional simulations of droplet trapping and squeezing process in a constricted microchannel. We also conducted experiments to validate the key results of the simulations. We investigated the impact of different system parameters on the onset of droplet immobilization at the constriction. We found that the continuous phase flows through the corners of the droplet, i.e. gutter flows to play an important role in determining the transition between trapping and squeezing. Therefore we evaluated the effect of different system parameters on gutter flows and found that the hydrodynamic resistance of gutters depends on the viscosity, size and confinement of the droplet. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H25.00002: Bubble nucleation in superhydrophobic microchannels due to subcritical heating Adam Cowley, Daniel Maynes, Julie Crockett, Brian Iverson We report on experiments that investigate the effects of heating on laminar flow in superhydrophobic (SH) microchannels. The parallel plate microchannels (180 $\mathrm{\mu m}$ spacing) consist of two surfaces: a rib/cavity structured SH surface and a smooth glass surface. The back of the SH surface is in contact with an aluminum strip that is heated and a camera is used to image through the glass surface to visualize the flow. Thermocouples embedded in the aluminum obtain the temperature profile along the length of the channel. The friction factor-Reynolds product (fRe) is obtained via pressure drop and volumetric flow rate measurements. 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. Both degassed and air-saturated water are used. When air-saturated water is used, the cavities of the SH surfaces act as nucleation sites and air is desorbed out of the water. Depending on the surface type/configuration, large bubbles can form and result in a large increase in fRe and channel surface temperatures. When degassed water is used no bubble nucleation is observed, however, the air trapped in the cavities of the SH surfaces is quickly absorbed and the surfaces transition to a wetted state. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H25.00003: Droplet breakup dynamics of weakly viscoelastic fluids Kristin Marshall, Travis Walker The addition of macromolecules to solvent, even in dilute quantities, can alter a fluid’s response in an extensional flow. For low-viscosity fluids, the presence of elasticity may not be apparent when measured using a standard rotational rheometer, yet it may still alter the response of a fluid when undergoing an extensional deformation, especially at small length scales where elastic effects are enhanced. Applications such as microfluidics necessitate investigating the dynamics of fluids with elastic properties that are not pronounced at large length scales. In the present work, a microfluidic cross-slot configuration is used to study the effects of elasticity on droplet breakup. Droplet breakup and the subsequent iterated-stretching – where beads form along a filament connecting two primary droplets – were observed for a variety of material and flow conditions. We present a relationship on the modes of bead formation and how and when these modes will form based on key parameters such as the properties of the outer continuous-phase fluid. The results are vital not only for simulating the droplet breakup of weakly viscoelastic fluids but also for understanding how the droplet breakup event can be used for characterizing the extensional properties of weakly-viscoelastic fluids. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H25.00004: In-air microfluidics: Drop and jet coalescence enables rapid multi-phase 3D printing Claas Willem Visser, Tom Kamperman, Detlef Lohse, Marcel Karperien For the first time, we connect and integrate the fields of microfluidics and additive manufacturing, by presenting a unifying technology that we call In-air microfluidics (IAMF). We impact two liquid jets or a jet and a droplet train while flying in-air, and control their coalescence and solidification. This approach enables producing monodisperse emulsions, particles, and fibers with controlled shape and size (10 to 300 \textmu m) and production rates 100x higher than droplet microfluidics. A single device is sufficient to process a variety of materials, and to produce different particle or fiber shapes, in marked contrast to current microfluidic devices or printers. In-air microfluidics also enables rapid deposition onto substrates, for example to form 3D printed (bio)materials which are partly-liquid but still shape-stable. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H25.00005: An Integrated microfluidic platform for liquid droplet in gas flow generation with in liquid flow collection and manipulation Pooyan Tirandazi, Carlos H. Hidrovo Discretization of biological samples and chemical reactions within digital droplets is a powerful technique which has rapidly emerged in many biochemical syntheses. The ability to generate, manipulate, and monitor millions of microdroplets in a short time provides great potential for high throughput screening and detection in microbiology. Here we report a microfluidic device for the formation of uniform microdroplets (50$\mu $m-100$\mu $m) using a high speed gas as the continuous phase. Gas-borne droplets are generated in a chip-based flow-focusing device fabricated in PDMS, and travel along the gaseous microchannel and are subsequently captured within a second liquid phase. The droplets are then transferred and collected in a minichamber and move into the manipulation section for further processing operations on the drops. All these steps are performed automatically in a single multilayer chip. This integrated microfluidic platform for generation, collection, and manipulation of the droplets provides great opportunities for monitoring and detection of gas-analytes. Utilizing the generated picoliter airborne droplets feature lower reaction times and higher transfer rates as compared to conventional air sampling techniques. Thus, it can greatly facilitate the investigation of airborne analytes by interrogation of the digital droplets using different analytical techniques. Furthermore, the presented liquid-in-gas generation method can be utilized for production of oil-free microparticles and microcapsules used in the food industry and for drug delivery. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H25.00006: Confinement and viscosity ratio effect on droplet break-up in a concentrated emulsion flowing through a narrow constriction Jian Wei Khor, Ya Gai, Sindy Tang We describe the dimensionless groups that determine the break-up probability of droplets in a concentrated emulsion during its flow in a tapered microchannel consisting of a narrow constriction. Such channel geometry is commonly used in droplet microfluidics to investigate the content of droplets from a concentrated emulsion. In contrast to solid wells in multi-well plates, drops are metastable, and are prone to break-up which compromises the accuracy and the throughput of the assay. Unlike single drops, the break-up process in a concentrated emulsion is stochastic. Analysis of the behavior of a large number of drops ($N$~\textgreater 5000) shows that the probability of break-up increases with applied flow rate, the size of the drops relative to the size of the constriction, and the viscosity ratio of the emulsion. We found that the break-up probability collapses into a single curve when plotted as a function of the product of capillary number, viscosity ratio, and confinement factor defined as the un-deformed radius of the drop relative to the hydraulic radius of the constriction. The results represent a critical step towards the understanding of the physics governing instability in concentrated emulsions. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H25.00007: Phase field investigation of droplet formation in flow focusing devices Feng Bai, Xiaoming He, Xiaofeng Yang, Cheng Wang The phase field (P-F) method has been increasingly utilized to simulate multiphase fluids in microscale devices. However, most of the current models are based on mathematical analysis; and the physical meaning and validation of phase field method are still unclear. We demonstrate a phase field model in a flow focusing device to investigate the physical application of P-F method and clarify the role of diffusive term in Cahn-Hillard equation. A characteristic mobility is defined as the product of the mobility tuning parameter and the square of interfacial thickness. The characteristic mobility reflects the correct relaxation time of the interface. Through systematic numerical investigations, we find that the characteristic mobility should be kept as a constant in order to correctly capture the physical process of droplet formation. This criterion has been verified with simulations by employing different interfacial thicknesses. In addition, this phase field model is validated by experiments with comparison of the size of droplet, the velocity of droplet and the period of droplet formation process. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H25.00008: Flow-induced differential lateral migration of deformable particles by inner/outer viscosity ratio Yeng-Long Chen, Shih-Hao Wang, Wei-Ting Yeh We investigate the practicality of flow-driven separation of deformable particles (DP) such as cells, droplets, and capsules in microfluidic flow. We use lattice Boltzmann-immersed boundary method to model the hydrodynamic coupling between DP and the fluid. We find that whether a DP migrates towards the wall or to the center at steady state depends strongly on particle Reynolds number \textit{Re}, capillary numbers \textit{Ca}, and viscosity ratio $\lambda $. The lateral steady state position $d$* and velocity is determined by the competition between the inertia- and deformation-driven forces. In the deformation-dominated regime (\textit{Ca} \textgreater \textgreater \textit{Re}), DP migrates towards the channel centerline and flow faster for sufficiently small $\lambda $. In the inertia-dominated regime (\textit{Ca} \textless \textless \textit{Re}), $d$* is between the channel center and the wall and flow slower for small $\lambda $. For sufficiently large $\lambda $, DP migrates towards the wall as the inertia-driven lift effects increase and the particle velocity decreases. In the intermediate regime (\textit{Ca} \textasciitilde \textit{Re}), we find that $d$* has non-monotonic dependence on $\lambda $, leading to complicated dependence of particle velocity. We find that the non-monotonic trend is a consequence of inertia-deformation coupling, and only occurs if the inertia- and deformation-driven lift effects are comparable. This result could provide be further utilized for separating soft particles with different internal fluid property. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H25.00009: Micro-droplet formation via 3D printed micro channel Zhen Jian, Jiaming Zhang, Erqiang Li, Sigurdur T. Thoroddsen Low cost, fast-designed and fast-fabricated 3D micro channel was used to create micro-droplets. Capillary with an outer diameter of 1.5 $mm$ and an inner diameter of 150 $\mu m$ was inserted into a 3D printed cylindrical channel with a diameter of 2 $mm$. Flow rate of the two inlets, insert depth, liquid (density, viscosity and surface tension) and solid (roughness, contact angle) properties all play a role in the droplet formation. Different regimes - dripping, jetting, unstable state - were observed in the micro-channel on varying these parameters. With certain parameter combinations, successive formation of micro-droplets with equal size was observed and its size can be much smaller than the smallest channel size. Based on our experimental results, the droplet formation via 3D printed micro T-junction was investigated through direct numerical simulations with a code called Gerris. Reynolds numbers $Re=\rho U L/\mu$ and Weber numbers $We=\rho U^2 L/\sigma$ of the two liquids were introduced to measure the liquid effect. The parameter regime where different physical dynamics occur was studied and the regime transition was observed with certain threshold values. Qualitative and quantitative analysis were performed as well between simulations and experiments. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H25.00010: Droplets coalescence at microchannel intersection chambers with different shapes Zhaomiao Liu, Xiang Wang, Rentuo Cao, Yan Pang The~influence~of~microchannel~intersection~chamber~shape~on~droplets~coalescence~process~is~investigated~in~this~study.~Three~kinds~of~chamber~shapes~(half-round,~triangle~and~camber)~are~designed~to~realize~head-on~droplets~coalescence.~The~coalescence~processes~are~visualized~with~high-speed~camera~system~and~the~internal~flow~patterns~are~resolved~with~micro-PIV~system.~Experimental~analyses~on~droplets~coalescence~position,~coalescence~time~and~the~critical~conditions~are~discussed.~Both~direct~coalescence~and~late~coalescence~can~be~observed~in~the~camber~junction~while~only~the~late~coalescence~is~present~for~the~half-round~and~the~triangle~junction. The~critical~capillary~number~Ca*~varies~for~different~working~systems~or~intersection~shapes.~Ca*~in~the~camber~junction~is~larger~than~that~in~the~other~two~junctions~for~each~working~system~and~it~decreases~with~the~increase~of~the~viscosity~ratios~for~each~intersection~shape.~Moreover,~the~characteristics~of~the~velocity~fields~for~different~coalescence~cases~are~analyzed~for~in-depth~understanding~of~the~process. [Preview Abstract] |
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