Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A36: Drops: Electric Field Effects |
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Chair: Quentin Brosseau, Brown University Room: Ballroom C |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A36.00001: Deformation and stability of surfactant - or particle - laden drop Quentin Brosseau, Gerardo Pradillo, Andrew Oberlander, Petia Vlahovska We present an experimental study of the behavior of a drop covered with insoluble surfactant or colloidal particles in a uniform DC electric field. Steady drop shapes, drop evolution upon application of the field, and drop relaxation after the field is turned off are observed for leaky dielectric fluids: Polybutadiene (PB), Silicon oil (PDMS), and Castor oil (CO). The surfactant is generated at the drop interface by reaction between end-functionalized PB and PDMS. The experimental data is compared with existing theoretical models for the steady shape of surfactant covered droplet, and adjusted models taking into account the presence of colloidal spheres with range of electrical properties. We will discuss the complex interplay of shape deformation, surfactant elasticity, particle redistribution, and interfacial charging in droplet electrohydrodynamics. Our results are important for understanding electrorheology of emulsions commonly found in the petroleum industry. We acknowledge grant NSF CBET 1437545 for funding. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A36.00002: Water-in-oil emulsification in a non-uniform alternating electric field Suhwan Choi, Alexei Saveliev The emulsification of a water microdroplet placed in castor oil was performed using a non-uniform alternating electric field formed in the pin-to-plate geometry. A non-uniform electric field of $\sim $40 kV/mm alternating with a frequency of 6.7 kHz was generated near the pin electrode. The applied frequency exceeded charge relaxation frequency of castor oil (0.3 Hz) and was below charge relaxation frequency of deionized water (7.8 kHz) used in the experiments. The emulsification process was captured with a CCD camera. The emulsification process started with entrainment of the water droplet in the high electric filed region near the pin electrode under the dielectrophoretic force. Upon touching the pin, the microdroplet was disintegrated in numerous channels and secondary droplets. The process continued by entrainment of secondary droplets and continuous size reduction. Three droplet breakup mechanisms were identified: drop elongation and capillary breakup, ac electrospraying of individual droplets, chain and bridge formation and decay. The quasi-steady narrow size distribution of emulsified water droplets with diameters close to 1 $\mu$m was formed after a few minutes. The generated emulsion was confined near the needle electrode due to the dielectrophoretic force. The emulsion had a well-defined boundary with a shape resembling a pendant drop suspended on the pin electrode. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A36.00003: Interaction between electrically charged droplets in microgravity Martin Brandenbourger, Herve Caps, Jerome Hardouin, Youen Vitry, Bernard Boigelot, Stephane Dorbolo The past ten years, electrically charged droplets have been studied tremendously for their applications in industry (electrospray, electrowetting,...). However, charged droplets are also present in nature. Indeed, it has been shown that the droplets falling from thunderclouds possess an excess of electric charges. Moreover, some research groups try to use the electrical interaction between drops in order to control the coalescence between cloud droplets and control rain generation. The common way to study this kind of system is to make hypothesis on the interaction between two charged drops. Then, these hypothesis are extended to a system of thousands of charged droplets. Thanks to microgravity conditions, we were able to study the interaction between two electrically charged droplets. In practice, the charged droplets were propelled one in front of the other at low speed (less than 1 m/s). The droplets trajectory is studied for various charges and volumes. The repulsion between two charged drops is correctly fitted by a simple Coulomb repulsion law. In the case of attractive interactions, we discuss the collisions observed as a function of the droplets speed, volume and electric charges. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A36.00004: How does electricity make liquid bristle? Boris Khusid, Ezinwa Elele, Yueyang Shen, Donald R. Pettit Electrified fluid forms pointed cones triggering sparks, flashes of light, and ejecting droplets. This phenomenon is encountered in lightning and utilized in a number of technologies. Taylor showed that surface tension and electric forces form a conical meniscus with a semivertex angle of 49.3$^{\circ}$. However, meniscus evolution from a rounded shape to a cone was a long-standing puzzle as it overlaps with spontaneous fluid ejection. We developed a method to control the cone-shaped spikes just shy of droplet ejection (PRL 114, 054501, 2015). Experiments were conducted on deionized (DI) water, DI-water with 0.1M KCl, polyethylene glycol, polymer solution simulating human saliva, lubricant with 0.02wt{\%} graphene. Experiments on DI water under microgravity in International Space Station enabled us to extend the measured cone lengths from 0.5 mm (Earth) to 5 cm. The meniscus evolution to a cone was found to exhibit a universal self-similarity scaled by the fluid surface tension and density and strikingly insensitive to the forcing field while a 50{\%} increase in applied voltage shortens the overall time for the meniscus to rise by more than an order of magnitude. Field induced flow inside the cone offers possibilities for non-contact control of separation and mixing inside tiny droplets. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A36.00005: Effect of The Viscosity Ratio on Equilibrium Shapes and Instability of Liquid Drops in Electric Field Asghar Esmaeeli Electrohydrodynamics of liquid drops is currently the focus of increased attention because of its relevance in a host of processes such as micro- and bio-fluidics. In a weak electric field the drop acquires an equilibrium shape, deforming to an oblate or a prolate spheroid. However, beyond a critical electric field strength it will disintegrate through tip-streaming or bulbous breakup. The modes and mechanisms of drop disintegration has been reasonably well-studied, assuming the drop viscosity to be the same as that of the ambient. However, there are some evidences that suggest the viscosity ratio can dramatically affect the dynamics, even leading to new breakup modes. The goal of this study is to provide further insight regarding this issue through computational simulations. To this end, we use a front tracking/finite difference scheme in conjunction with Taylor leaky-dielectric model to solve the governing electrohydrodynamic equations [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A36.00006: Electrohydrodynamics-driven pattern formation of liquid drops Ali Behjatian, Asghar Esmaeeli Direct Numerical Simulations are performed to explore pattern formation of suspension of leaky dielectric liquid drops in uniform DC electric field. The applied electric field strength is moderate so that the drops do not disintegrate, but they go through tangible deformation. The results show that the drops form columnar structures or horizontal rafts, depending on the ratio of the dielectric properties of the drop liquid and the ambient liquid. Scaling arguments are used to characterize the time scale of column and raft formation and their breakup, when the electric field is disconnected. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A36.00007: Enhanced fog collection with electric fields Maher Damak, Seyed Reza Mahmoudi, Kripa Varanasi Fog harvesting is a promising source of fresh water in remote areas. However, the efficiency of current collectors, consisting in fine meshes standing perpendicularly to the wind, is dramatically low. Fog-laden flows generally have low Stokes numbers, which leads to the deviation of fog droplets in the vicinity of the mesh wires. Here, we propose to overcome this aerodynamic limitation using a combination of electric fields and specific collecting surfaces. We show that our system largely increases the fog collection efficiency. We study the trajectories of individual particles and use the results to derive a model to predict the collection efficiency of the system. We finally identify and quantify the mechanisms that can limit the collection of fog particles. The understanding of these mechanisms leads us to construct a design chart that can be used to determine the optimal design parameters that should be used in fog collection applications as a function of the field conditions. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A36.00008: Dispersion and vaporization of a spray of electrically charged droplets in a coflowing hot gas Angel Perena, Francisco Higuera A numerical model of a dilute spray of electrically charged liquid droplets vaporizing in a hot gas is formulated based on a Eulerian description of the gas and Lagrangian tracking of the droplets. The model is used to simulate the dispersion and vaporization of a spray in a cylindrical chamber where the droplets are axially injected with a coflow of hot gas. The effects of the initial size of the droplets and of the gas-to liquid mass flux and inlet temperature ratios are analyzed, and the conditions under which the droplets fully vaporize in the chamber without impacting on its walls are determined. The ranges of operation where these requirements are met widen when the flow rate of liquid is split into a number of sources and injected through different orifices, which also improves the uniformity of the temperature and vapor mass fraction distributions at the outlet of the chamber. The effects of satellite droplets and of a high voltage applied between the bases of the chamber are considered. [Preview Abstract] |
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