Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F12: Drops: General II |
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Chair: Lydia Bourouiba, Massachusetts Institute of Technology Room: Georgia World Congress Center B217 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F12.00001: Numerical study of double emulsions flowing through a tapered nozzle in axi-symmetric Stokes flow. Monika Nitsche A boundary integral method is applied to simulate the motion of double emulsions through a tapered nozzle in axi-symmetric Stokes flow. Double emulsions are a mode of choice to encapsulate and release active materials in applications such as drug delivery, food processing and cosmetics. Motivated by experiments of Chen et al (Softmatter 2011, vol 7, no 6, 2345) we simulate the motion of two drops, one within the other, carrying distinct fluids, as they exit |
Monday, November 19, 2018 8:13AM - 8:26AM |
F12.00002: Fog harvesting on a vertical wire Youhua Jiang, Shaan Savarirayan, Yuehan Yao, Kyoo-Chul Park In spite of extensive studies on fog harvesting using arrays of vertical wires and woven meshes, the fundamental correlation between fog-harvesting rate on a single wire and the characteristics of the wire under different fog flow conditions is largely underexplored. In this study, we investigate fog harvesting using a single vertical wire with systematically varying wire dimensions and fog harvesting conditions. In particular, a superhydrophilic aluminum wire was vertically suspended from the top of a wind tunnel with controlled air flow perpendicular to the axial direction of the vertical wire. A horizontal superhydrophobic plate was attached to the bottom end of the wire so that the fog-harvesting rate can be quantified by optically measuring the dimensions of a droplet on the plate growing by water flowed from the vertical wire over time. Results show that the fog-harvesting rate decreases with the decrease in wire diameter. By contrast, the harvesting rate per unit surface area increases with the decrease in wire diameter, which is roughly consistent with an empirically obtained deposition rate St/(St+π/2). |
Monday, November 19, 2018 8:26AM - 8:39AM |
F12.00003: Atomization of brine for zero liquid discharge Katherine A. Cai, Youhua Jiang, Yuehan Yao, Maya Kurup, Kyoo-Chul Park Solar thermal evaporation of brine is one of the most energetically-efficient approaches for achieving zero liquid discharge, which can solve the environmental problems associated with various desalination technologies. To accelerate the evaporation process, brine can be atomized to microdroplets (i.e., fog) by ultrasonic atomization. In this study, brine with systematically varying concentrations of salt and surfactants is atomized to investigate the effects of salinity and surfactants on the rate of fog generation and the size of generated fog droplets. The fog-generation rate is calculated by measuring the decrease in brine mass on a microbalance and the droplet size is directly visualized using a high-speed camera with a microscopic lens. At a fixed atomization condition, results show that the rate of fog generation decreases and the size of generated fog droplets increases with the increase in the brine salinity. With the addition of surfactants to the brine, the fog generation rate is enhanced and the fog droplet size is reduced. The physico-chemical properties of brine such as surface tension are found to be the main factors that determine the generation rate and the size of droplets. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F12.00004: Physical property measurement of primary reference fuels and blends using a droplet generator and high-speed imaging Wanjun Dang, Shyam Menon A liquid fuel delivery system for a fuel screening device has been developed using a piezoelectric droplet generator. Besides fuel delivery, it is desired to use the setup to characterize fuel physical properties relevant to spray atomization, evaporation, and mixing in internal combustion engines. This has motivated the development of a technique for real-time measurement of fuel viscosity and surface tension using shape oscillation dynamics. Shape oscillation theory relates the frequency of drop oscillation to fluid surface tension and its decay to fluid viscosity. This technique is applied to measure viscosity and surface tension of droplets in real-time by imaging the droplets and determining the amplitude and frequency of the droplet shape oscillation as a function of time. Measurements are compared with literature data and predictions of physical properties based on mixing rules. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F12.00005: Investigation of Droplet Behaviour Under the Influence of an Oblique Impinging Jet Xueqing Zhang, Serhiy Yarusevych, Sean D. Peterson Liquid droplets residing on a solid substrate unpin when the external gas-phase flow reaches a critical velocity. Previous studies have focused on a droplet subjected to a steady flow (e.g., Schmucker et al., 2017APS..DFDQ11006W). The present investigation considers droplet behaviour under the influence of a starting slot impinging jet. Single droplets of distilled water are subjected to a two-dimensional slot jet which is accelerated at three different rates, dUj /dt= 1.2, 2.2 and 4.4 m/s2. The tests are performed for water droplets at four different volumes, V = 20, 35, 50, and 65 µL located 2, 4 and 7 slot widths downstream of the impingement point. The results indicate that in the immediate vicinity of the impingement region, the critical velocity shows minor sensitivity to droplet volume and jet ramp rate. However, the influence of these parameters becomes more significant when droplets are located further downstream from the impingement region. The observed trends are related to characteristics of the quasi-steady flow fields assessed with particle image velocimetry. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F12.00006: Drop Generation Using Cross-Flow in Rigid Body Rotation Haipeng Zhang, Sangjin Ryu Inspired by crossflow membrane droplet generation and microfluidic droplet generation, we propose an easy method to generate monodisperse drops using cross-flow caused by rigid body rotation. In our approach, a dispersed phase liquid was injected through a stationary vertical needle into an immiscible continuous phase liquid which was in rigid body rotation. A dispersed phase drop growing at the end of the needle experienced fluid dynamic forces from the horizontal continuous phase flow, and it was detached from the needle when it had grown to a certain size. This study investigated the relationship between the resultant size and generation frequency of drops and the controllable experimental factors including the flow velocity of the continuous phase at the needle end and the flow rate of the dispersed phase through the needle. As an example of possible application, the method was utilized to generate monodisperse hydrogel beads. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F12.00007: Growth dynamics of surface nanodroplets Brendan Dyett, Akihito Kiyama, Yoshiyuki Tagawa, Detlef Lohse, Xuehua Zhang Solvent exchange is a simple solution-based process to produce surface nanodroplets over a large area. The final size of the droplets is determined by both the flow and solution conditions for a given substrate. In this work, we investigate the growth dynamics of surface nanodroplets during solvent exchange by using total internal reflection fluorescence microscopy (TIRF). The results show that during the solvent exchange, the time for the number density and surface coverage of the droplets to reach their respective plateau values is determined by the flow rate. From the observed evolution of the droplet volume and of the size of individual growing droplets, we are able to determine that the growth time of the droplets scales with the Peclet number Pe with a power law ∝Pe−1/2. This is consistent with Taylor–Aris dispersion. These findings provide a valuable insight toward controlling droplet size and spatial distribution. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F12.00008: Statistics of compound droplets generated by fragmentation of immiscible buoyant oil jet in the water Xinzhi Xue, Joseph Katz Very little experimental data exists on the droplet formation and morphology in the near field of a crude oil jet fragmenting in water because of the inability to probe dense droplet cloud. PIV and PLIF measurements are made possible by using a surrogate refractive index matched liquid pair: silicone oil in sugar water. The oil jet fragmentation is visualized by fluorescently tagging the oil. Compound oil ligaments and droplets, which contain smaller water droplets, form regularly during the fragmentation process. In some cases, the droplets consist of multiple layers of oil and water, creating a “Russian Doll” like phenomenon. They affect the buoyancy and interfacial areas, which might affect associated biochemical interactions. A random forest-based trainable model is applied to distinguish between phases within the droplets and to quantify the resulting volume fractions and interfacial areas. While rarely occurring in a low Reynolds number jet (Re=594), the fraction of compound droplets increases to 21% and 26% as the jet Reynolds number increases to Re=1358 and 2122, respectively. The probability distributions of water fraction indicate a characteristic 4% decrease in droplet buoyancy and 12% increase in the interfacial area. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F12.00009: Extraction of Droplet Genealogies from High-Fidelity Atomization Simulations Roland F.C Rubel, Mark F Owkes Many research groups are performing high-fidelity simulations of atomizing jets that are taking advantage of the continually increasing computational resources and advances in numerical methods. These high-fidelity simulations produce extremely large data-sets characterizing the flow and giving the ability to gather a better understanding of atomization. One of the main challenges with these large data sets is the ability to extract relevant physics from them. With this physics extraction technique the genealogy of droplets created as the coherent liquid core continually breaks into droplets and ligaments which may proceed to break up further are provided. Along with the ancestry of the droplets information such as droplet size, shape, flow field characteristics, etc. will be stored in a database. Allowing the information to be readily queried to assist in the development and testing of low-fidelity atomization models that agree with the physics predicted by high-fidelity simulations. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F12.00010: Coalescence rate of polydisperse sub-Kolmogorov droplets settling in a turbulent gas Johnson Dhanasekaran, Anubhab Roy, Donald L Koch Cloud droplets coalesce due to the coupled effects of differential sedimentation and turbulent shear. Droplets of diameter 10 to 50 mm represent the bottleneck to precipitation and exhibit little deformation, interfacial mobility, or colloidal force so that the non-continuum nature of the gas is critical to allow coalescence. We evaluate the collision rate of inertia-less droplets settling in a local linear flow field and interacting by a uniformly valid hydrodynamic mobility function that captures non-continuum lubrication at small separations and full continuum hydrodynamic interactions at larger separations. Calculations are first performed for steady biaxial extensional flows with different orientations with respect to gravity providing a “persistent” model of turbulence similar to the classic work of Saffman and Turner. Next, the local linear flow seen by the drops is evolved using Girimaji and Pope’s Lagrangian stochastic model. The collision efficiency is determined for a broad range of droplet size ratios, Knudsen numbers (ratio of gas mean-free path to mean drop radius), and ratios of settling velocity to turbulent shear induced interparticle velocities. |
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