Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q36: Microscale Flows: Drops and Bubbles |
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Chair: Prashant Valluri, University of Edinburgh Room: 618 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q36.00001: Deformability-induced inertial focusing of viscous droplets Soojung Hur, Mehran Abolghasemibizaki Deformability-induced lateral migration of soft objects across the streamlines in microchannels provides a robust and continuous way of manipulating droplets and cells inflow. Theoretical analysis has shown that there exists a force away from channel walls in Poiseuille flow that locates deformable particles closer to the channel center than rigid counterparts. As lateral lift forces acting on flowing objects scale with carrier fluids and suspending particles' properties, the deformability can be extrapolated from the positions of objects with known sizes in the channel. Here, behaviors of nearly-neutrally buoyant microscale droplets of various sizes, interfacial tensions, and viscosities were tested to determine droplet's physical attributes and flow conditions, enhancing their lateral migration. Fluorinated oil solutions ($\mu =$1.7mPas and 5mPas) containing droplets (1mPas\textless $\mu $\textless 1.3Pas) were injected into a microfluidic channel at 8\textless Rc\textless 50. The interfacial tensions were varied by adding a controlled amount of a surfactant. The diameter, deformability, and lateral equilibrium position were determined using high-speed microscopy. Through dimensional analysis, we identified critical conditions to inertially-focus deformable droplets in Poiseuille flow. The results suggest that accurate measurements of lateral positions of deformable droplets will provide a means to establish the fundamental understanding required to estimate apparent deformability of flowing objects in high-speed. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q36.00002: Dynamics of High-speed Droplet Generation in Gas-liquid Microfluidic Systems Pooyan Tirandazi, Julian D. Arroyo, Carlos H. Hidrovo In this work we study the interaction of liquid and gas in flow-focusing microchannels with the goal of reproducible generation of microscale droplets in air. The microfluidic channels are fabricated in Polydimethylsiloxane (PDMS) based on established lithography techniques and feature a non-planar architecture to enable the formation of liquid jets and droplets within air. A comprehensive flow map is developed based on the interaction of liquid and gas for a wide range of flow conditions and different channel sizes. In particular, we focus on the characteristics of the Dripping and Jetting modes of droplet formation and present information regarding droplet and jet sizes and breakup frequencies in this system. We show that the microfluidic chips reproducibly generate droplets with frequencies in the order of 10kHz and droplet sizes between 160$\mu $m and 50$\mu $m in the Dripping mode, whereas, for the Jetting we obtain droplets from 50$\mu $m down to 15$\mu $m at frequencies higher than 100 kHz. Finally, we briefly discuss some applications of high-speed gas-liquid microfluidics, namely for oil-free polymer particle fabrication and gas sensing using sample digitization with microdroplets. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q36.00003: Microfluidic droplet manipulations via modulation of interfacial tension Thomas Cubaud The dynamic response of viscous droplets to variations of interfacial tension with the continuous phase is experimentally investigated in microchannels. Microfluidic segmented flows of oil droplets are continuously generated in methanol or ethanol to produce regular patterns that are injected into a focusing section with another alcohol solvent. To probe the role of fluid properties on droplet deformations, a range of fluid combinations are examined with focus on large droplet elongations for vanishing interfacial tension between droplets and continuous phase. This work shows the possibility to process high-viscosity fluid droplets in a variety of solvents in confined microsystems. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q36.00004: Crystallisation-Induced Flows in Evaporating Aqueous Saline Drops Marina Efstratiou, John Christy, Khellil Sefiane When aqueous saline droplets are left to dry on hydrophilic glass slides, a ring of spaced crystals is formed. However, no experimental work has yet linked the flows arising in these droplets with the final crystal ring deposition. In our work, we have performed micro-PIV (Particle Image Velocimetry) to examine the link between the flow along the base of the drop and the final deposit. We report for the first time the existence of a nucleation-driven flow within the droplets which appears to be responsible for the formation of the crystal ring on the periphery of the drops. Three evaporation stages were observed. During stage I, a generally outward flow is manifested driven by evaporative flux, since the evaporation rate is higher on the periphery. After stage I, a transition stage II is shown during which the flow almost momentarily pauses, that is believed to occur at the point where the concentration near the periphery is that of the incipient nucleation. Stage III is governed by the appearance of strong flow jets, directed at the growing crystal, and vortices to either side of the crystal. This flow regime is driven by concentration gradients occurring due to crystal nucleation, demonstrating for the first time how crystal nucleation affects the flow. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q36.00005: Surface Tension Measurements for Atmospheric Aerosols Using Bubble Deformation Dynamics in Multilayer Microfluidic Channel Shihao Liu, Cari Dutcher Atmospheric aerosols significantly impact climate change by serving as cloud condensation nuclei (CCN). Surface tension, which rapidly evolves with changes in chemical composition, dictates CCN activation through water uptake to form cloud droplets. Here we use microfluidics to directly measure surface tension of atmospherically relevant aqueous solutions based on Taylor's small deformation theory. In previous work, a microfluidic tensiometer of uniform height was designed to measure interfacial tension of liquid-liquid phase-separated aerosol systems. But the single layer channel could not be utilized for liquid-air systems, due to challenges arising from the system's high surface tension and low viscosity. Therefore, a multilayer channel is designed, with which small bubbles can move enough slow to satisfy small-Re assumption. Bubble deformation in an expansion-contraction geometry is quantified using image analysis for surface tension calculations. The tensiometer can measure aqueous solutions with various solutes and concentrations, indicating the fate of aerosol particles as CCN in the atmosphere. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q36.00006: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q36.00007: Synchronizing the droplet breakup in a microfluidic channel Eujin Um, Minjun Kim, Hyoungsoo Kim, Howard Stone, Joonwoo Jeong Synchronization phenomena are ubiquitous, such as flickering fireflies or pendula of nearby clocks. While the mechanism of synchronization in many examples is difficult to understand, we propose that oscillating oil/water interfaces in a microfluidics channel can be a simple experimental model system of hydrodynamic synchronization. In a double T-junction design of the microchannel, the aqueous phase is injected from the two opposite branches into the middle channel filled with the flowing oil phase. By changing parameters such as the flow rates and the distance between the interfaces, we identify various regimes of droplet breakup including the in-phase synchronization, where two aqueous phases protrude simultaneously from the branches, leading to the simultaneous droplet breakup from two interfaces. To elucidate our experimental observations of the emergence of the in-phase synchronization, we introduce a numerical model of droplet breakup, considering the changes in pressure between the interfaces and within the droplets as the hydrodynamic coupling factor. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q36.00008: Droplet dynamics in oil-in-water emulsions in enhanced gravity and temperature Bijoy Bera, Karin Schroen Oil-in-water emulsions occur frequently in our daily life and in many cases the emulsion is subjected to extreme conditions such as high shear or enhanced gravity. Although this system has been used for decades, understanding of the fundamental behavior of such emulsions in extreme conditions remains limited. Krebs et al. (2013) is a rare example where drop dynamics in enhanced gravity was studied in detail. However, drop dynamics at combined enhanced gravity and elevated temperature conditions remains unexplored even though it is essential for many systems in practice. We have built and used a novel microfluidic setup where the emulsion can be subjected simultaneously to higher gravity (using centrifugal force) and higher temperatures. We vary the droplet size of the dispersed phase, the amount and type of emulsifier in addition to gravity and temperature. The results clearly indicate the influence of thermal fluctuations in the thin water film upon the coalescence and compression patterns. We have concomitantly developed a model taking into consideration the various factors affecting the drainage of the aqueous film between droplets of the dispersed phase. Our experimental results are validated against the results obtained from this analytical model. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q36.00009: Bio-inspired soft nanofluidic networks created and controlled by light Naval Singh, Arwen Tyler, Guido Bolognesi, Andrew Ward, Colin Bain, Yuval Elani, Oscar Ces The manufacturing of soft fluidic structures, based on multiple micron-sized compartments interconnected by lipid-stabilised or surfactant-stabilised nanoconduits, are attracting an increasing level of attention. These soft constructs have shown a great potential as minimal cells in synthetic biology, simplified model systems for biophysical and biochemical studies and smart containers for drug delivery and microreactor technologies. However, the creation of such nanofluidic networks can be a relatively difficult task. A well-known fabrication protocol is based on the micro-manipulation of polydisperse giant unilamellar vesicles (GUVs), leading to the creation of lipid nanotubes and daughter vesicles. Here, we present new approaches for the rapid generation of nanofluidic networks based on the optical manipulation of soft structures and particles at liquid interfaces, including i) surfactant-coated ultralow tension droplets and ii) adhesive GUVs. These contactless approaches offer several advantages, including easy implementation, fast (few mins) fabrication of arbitrary complex 2D/3D networks, fine control over network geometry parameters and ability to connect chemically distinct reservoirs (from fL to nL in volume) across distances from 1 $\mu$m to 100s of $\mu$m. [Preview Abstract] |
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