Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J09: Microscale Flows: Emulsions (8:00am - 8:45am CST)Interactive On Demand
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J09.00001: Entrainment Regime in Selective Withdrawal with a Tube Zehao Pan, Janine Nunes, Howard Stone Selective withdrawal is applied through a capillary tube oriented perpendicular to a flat gravitationaly-separated liquid-liquid interface. Fluid entrainment occurs when both the top and bottom phases are withdrawn. The use of tube introduces two distinct features to the conditions for entrainment: an early ending to entrainment as the tube moves into the interface and a minimum withdrawal flow rate for entrainment. We show that these phenomena can be understood based on the Reynolds number that governs the flow field around the capillary and the capillary and Weber numbers that regulate the effect of the surface tension. [Preview Abstract] |
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J09.00002: Raydrop: universal monodisperse emulsificator Javier Rivero-Rodriguez, Adrien Dewandre, Youen Vitry, Benjamin Sobac, Benoit Scheid The production of monodisperse micro-droplets has many applications in pharmaceutics, biology and medicine. We have developed a microfluidic device based on capillaries and a 3D printed nozzle, instead of the standard flow-focusing configuration fabricated by soft-lithography, hence avoiding wettability treatments. This device works in the dripping regime in which the production of droplets is monodisperse. We have numerically characterised this regime in the quasi-static limit for negligible flow rate of the disperse phase. For small flow rate of the continuous phase, a folding bifurcation predicts the occurrence of drop formation. However, for larger flow rates the folding bifurcation is inhibited and a jetting regime establishes. To the best of our knowledge, it is the first time a sharp transition between the dripping to jetting regime is computed in the context of the quasi-static approximation. Additionally, this transition is smoothed out by non-negligible flow rate of the disperse phase. Numerical simulations very well reproduce the experiments, allowing for the optimisation of the geometry and the determination of the complete phase diagram. [Preview Abstract] |
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J09.00003: Time Evolution and Effect of Dispersant on the Morphology and Viscosity of Water-in-Crude Oil Emulsions Diego F. Muriel, Joseph Katz This study examines the time evolution and effects of adding dispersant (Corexit 9500) on the microscopic morphology and bulk viscosity of salt water-in-crude oil (Louisiana) emulsions. Methods include rheology and microscopy, followed by a machine-learning-based analysis of the emulsion structure. Initially, the water droplets appear as a multi-scale lattice with mean diameter of 2.7 $\mu m$ and polydispersity of 0.44, with small droplets aggregating around large ones. The bulk viscosity is one to two orders of magnitude higher than that of the crude oil. After 7 days, the number of submicron droplets increases and the nearest neighbor distance decreases, indicating preferential aggregation. At high shear rates (5-100 s$^{-1}$), the viscosity increases by 60-130\% compared to the initial values. After 14 and 21 days, as the droplets coalesce and many of the clusters merge, the bulk viscosity decreases. These trends suggest that aggregation contributes to the increase in viscosity. Mixing the emulsion with dispersant accelerates the phase separation. The removed water fraction increases with dispersant concentration, reaching 77\% for a 10$^{-3}$ concentration. The remaining emulsion consists of fine droplets with Newtonian viscosity four times higher than that of the crude oil. [Preview Abstract] |
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J09.00004: Morphology and Viscosity of Water-in-Crude Oil Emulsions Formed by Light, Medium, and Heavy Crudes Diana Bershadsky, Diego F. Muriel, Joseph Katz Rheology and microscopy are used to examine the effect of oil properties and adding dispersant on the time evolution of the morphology and viscosity of salt water-in-crude oil emulsions. The emulsion viscosity from Platform Henry, a medium viscosity oil (0.06 $Pa\:s$), decreases with increasing shear rates from 10$^4$ to 15 times that of the crude oil, with water droplet diameters in the 0.4 to 4 $\mu m$ range. Cold lake and Platform Gina, both heavy crudes (0.27 and 1.3 $Pa\:s$, respectively) also entrain water despite their high viscosity, with droplets in the 0.3 to 7 $\mu m$ range, and a maximum viscosity increase of 350 and 13 times that of the respective oil. The emulsion viscosity of heavy oils increases with time, presumably due to aggregation. Unlike light oils, where dispersant accelerates phase separation, only 11\% of the water is extracted from these emulsions after 21 days. The remaining emulsions are non-Newtonian and have different droplet sizes and spatial distribution. The corresponding mean diameters are larger and their distributions have higher polydispersity. The associated maximum viscosities decrease by 10 times for the medium oil, and by 15-16\% for the heavy oils. Hence, dispersants may be used for decanting of light oil emulsions, but not of heavy ones. [Preview Abstract] |
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J09.00005: Phase dependent surfactant transport and micro-scale droplet coalescence in liquid-liquid systems Yun Chen, Shweta Narayan, Cari Dutcher Emulsions are ubiquitous in various applications such as oily bilgewater and water-entrained diesel fuel. The dispersed droplets in emulsions are often stabilized by surfactants and difficult to be separated due to the lowered interfacial tension (IFT) that inhibits the droplet coalescence. The thin film drainage time between two approaching droplets is used to quantify the droplet coalescence, which is affected by the IFT. Recent studies found different IFT decaying rate when the surfactant appears inside (dispersed) vs outside (continuous), or o/w vs w/o systems, which implies phase dependent surfactant transport to curved interfaces that leads to different film drainage behavior. In addition, the film drainage is also affected by the viscosity ratio between dispersed and continuous phases, and Marangoni stress. In this work, droplet coalescence will be investigated experimentally via microfluidic Stokes trap device to understand the impact of factors on the emulsion stability. Four systems are studied: o/w and w/o systems with oil-soluble surfactant in the oil phase or the same systems with water-soluble surfactants in the water-phase. The possibility of the droplet coalescence and, for coalescing systems, the film drainage time will be discussed. [Preview Abstract] |
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J09.00006: Particle and fluid transport during imbibition of strongly confined emulsions in parallel-wall channels. Masoud Norouzi Darabad, Sagnik Singha, Jerzy Blawzdziewicz, Siva A. Vanapalli, Mark W. Vaughn We investigate capillary imbibition of a monodisperse emulsion into a high-aspect ratio microfluidic channel with the height h comparable to the droplet diameter d. For the confinement ratio d/h $=$ 1.2, the tightly confined droplets in the channel move more slowly compared to the average suspension velocity. Behind the meniscus that drives the imbibition there is a clear fluid region, separated from the suspension region by a sharp concentration front. The suspension exhibits strong density and particle velocity fluctuations, but on average the suspension domain remains uniform. For weaker confinement, d/h $=$ 0.65, the drop phase moves faster than the average suspension flow, resulting in the formation of a dynamically unstable high-concentration region near the meniscus. We describe the macroscopic suspension dynamics using linear transport equations for the particle-phase flux and suspension flux that are driven by the local pressure gradient. A dipolar particle interaction model explains the observed large density and velocity fluctuations in terms of the dynamics of elongated particle clusters with different orientation. [Preview Abstract] |
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