Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L4: Drops VIII |
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Chair: Francisco Higuera, E.T.S. Ingenieros Aeronauticos, UPM Room: 23C |
Monday, November 19, 2012 3:35PM - 3:48PM |
L4.00001: Measurement of interfacial tension of ionic liquid-dielectric liquid system using the shape of an electrically deformed droplet Dong Woog Lee, Do Jin Im, In Seok Kang When a charged droplet of an ionic liquid is translated by uniform electric field, it is deformed into a prolate shape. The deformed shape is used for estimation of the interfacial tension of ionic liquid-dielectric liquid system. The typical Debye length of an ionic liquid is of sub-nanometer scale, while the typical radius of a droplet is about 0.5 mm. Therefore, from the electrostatics view point, the droplet can be approximated as a perfect conductor. In addition, the net charge accumulated at the droplet surface does not cause any significant change of the composition of ionic liquid. Thus, no change of interfacial tension is expected to be due to the electrical effect. The experimental configuration of the present study has also other advantages. The first is that the Reynolds number is smaller than unity and the Stokes flow limit can be applied. In this small Reynolds number regime, uniform streaming flow does not cause shape change up to the first order shape deformation. This means that to this order, the shape change is solely due to the electrical effect. The second is that net charge of a conducting droplet does not cause shape change up to the first order deformation. This means that we do not need the information about the amount of net charge of a droplet. In the experiments, the ionic liquids 1-alkyl-3-methylimidazolium alkyl sulfate ([Cn-mim] [CH3SO4] and [Cn-min] [C2H5SO4], n=2, 4, 6) are used as the droplet phase and the silicone oil KF-54(500cS) is used as the continuous phase. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L4.00002: Charge Transfer into Aqueous Droplets via Kilovolt Potentials B.S. Hamlin, E.R. Rosenberg, W.D. Ristenpart When an aqueous droplet immersed in an insulating oil contacts an electrified surface, the droplet acquires net charge. For sufficiently large field strengths, the charged droplet is driven back and forth electrophoretically between the electrodes, in essence ``bouncing'' between them. Although it is clear that the droplet acquires charge, the underlying mechanism controlling the charge transfer process has been unclear. Here we demonstrate that the chemical species present in the droplet strongly affect the charge transfer process into the drop. Using two independent charge measurement techniques, high speed video velocimetry and direct current measurement, we show that the charge acquired during contact is strongly influenced by the droplet pH. We also provide physical evidence that the electrodes undergo electroplating or corrosion for droplets with appropriate chemical species present. Together, the observations strongly suggest that electrochemical reactions govern the charge transfer process into the droplet. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L4.00003: Numerical and asymptotic analysis of an electrified jet Santiago E. Ib\'a\~nez, F.J. Higuera A numerical study is presented of a liquid jet of finite electrical conductivity issuing at a constant flow rate from a metallic capillary set at a high voltage relative to a distant electrode. The electric field due to the applied voltage induces a conduction current in the liquid that accumulates electric charge at its surface and gives rise to electric stresses that stretch the jet. A quasi-unidirectional model of the flow is proposed and boundary elements and finite difference methods are used to compute the electric field, the flow of the liquid and the distribution of surface charge as functions of the applied voltage, the flow rate and the physical properties of the liquid. Asymptotic results for large and small values of the flow rate are worked out. At small values of the flow rate, the electric current increases as the square root of the flow rate and the surface of the liquid resembles a cone followed by a thin jet. The electric field at the surface attains a maximum in a certain current transfer region where convection of the surface charge becomes the dominant contribution to the electric current. At large values of the flow rate, the electric current and the length of the current transfer region become independent of the flow rate. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L4.00004: Periodic emission of droplets from an electrified meniscus A.J. Hijano, I.G. Loscertales, S.E. Ibanez, F.J. Higuera We report an experimental characterization of the periodic emission of mass from a meniscus hanging from the tip of a needle, fed at a constant liquid flow rate, when subjected to a constant high voltage with respect to a grounded plate. For a given flow rate, the potential is swept until a periodic regime is reached. Two very distinct emission modes are observed to coexist. In each pulsation, the meniscus elongates under the electric field until it develops a conical cusp from which a highly charged fine aerosol is continuously emitted at a very high frequency, resembling the cone-jet electrospray. Simultaneously, the meniscus develops a neck which eventually forms a droplet, much larger than those emitted from the unsteady electrospray. The detachment of the large droplet occurs at frequencies of the order of the inverse of the liquid capillary time, whereas the fine droplets are emitted at much higher, unmeasurable frequencies. For the low viscosity, highly conducting liquids used in this study, most of the mass emitted per pulse is carried by the large droplets, whereas the charge is transported by the fine aerosol. Scaling laws for the dimensionless pulsation frequency and large droplet diameter are provided in terms of an electric Bond number and a dimensionless flow rate. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L4.00005: Electro-deformation of a surfactant-laden viscous drop Herve Nganguia, Yuan-Nan Young, Petia Vlahovska, Jia Zhang, Hao Lin In this work we investigate the equilibrium shape and dynamics of a surfactant-laden viscous drop under an electric field. The full Taylor-Melcher leaky dielectric framework is employed. We use both small-deformation theory (for a drop slightly deformed from a spherical shape) and semi-decomposition method for a highly deformed (prolate or oblate) spheroidal drop. Both theoretical approaches are validated by comparing predicted deformation with experimental data by Ha and Yang (1998). The dependence of the critical Capillary number for the equilibrium shape on the surfactant coverage is quantified, and more detailed analysis of the models shed light on the surfactant effects on the drop deformation under an electric field. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L4.00006: Whipping in electrified liquid jets Josefa Guerrero Millan, Venkat Gundabala, Alberto Fernandez-Nieves Whipping is a non-axisymmetric instability that appears in electrified jets. In air, it usually manifests in a chaotic fashion and thus, its structure and properties have been hard quantify experimentally. We use electro-coflow to generate a steady-state whipping structure and quantify its geometry and how it depends on operating parameters, like liquid flow rates and applied voltage. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L4.00007: Magnetic actuation of immersed coupled droplets: Experiments and simulations Joseph Olles, Amir Hirsa, Krishnaraj Sambath, Osman Basaran A system of two droplets connected through a cylindrical hole in a plate, with pinned contact lines, shows promise in several engineering applications including fast adaptive optics, microscale actuators and pumps, and adhesion devices. Such coupled droplets, surrounded by a passive gas medium, have been studied extensively. With the motivation of advancing this technology, here we consider coupled-droplet systems comprised of ferrofluid immersed in an immiscible liquid. Nonlinear characteristics of the system are studied by exciting the ferrofluid using a small electromagnet at various frequencies. The responses, tracked by observing the interface motion through high-speed imaging, are analyzed. Fluid velocities measured using index matched PIV techniques are also characterized. To corroborate the experimental results, the ferrofluid system with a magnetic force on the coupled droplets is simulated with the magnetic force approximated by a uniform body force. The axisymmetric Navier-Stokes system, which governs the flow in both the ferrofluid and surrounding fluid, is solved using the Galerkin finite element method. By aligning the simulations with experimental data, a novel method of extracting interfacial fluid properties is elucidated. Operational parameters where experiments are contradicted by simulations are also discussed. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L4.00008: Electrohydrodynamics of suspension of liquid drops in AC fields Md. Abdul Halim, Asghar Esmaeeli Manipulation of liquid drops by an externally applied electric field is currently the focus of increased attention because of its relevance in a broad range of industrial processes. The effect of a uniform DC electric field on a solitary drop is well studied; however, less is know about the impact of electric field on suspension of liquid drops, and very little information is available on the impact of AC field on a single or a suspension of drops. Here we report the results of Direct Numerical Simulations of electrohydrodynamics of suspension of liquid drops. The governing equations are solved using a front tracking/finite difference technique, in conjunction with Taylor's leaky dielectric model. The imposed electric potential comprises of two parts, a time-independent base and a time-dependent part. The goal is to explore the relative importance of these two components in setting the statistically steady state behavior of the suspension. To this end, we report the results of three sets of simulations, where (i) the time-dependent part act as a perturbation on the base potential, (ii) the two components are of the same order, and (iii) the time-dependent part is much larger than the base potential. The problem is studied as a function of the governing nondimensional parameters. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L4.00009: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 5:32PM - 5:45PM |
L4.00010: Influencing the In-flight Shape and Velocity of a Ferrofluid Drop by a Magnetic Field: Case of a Falling Drop Towards a Surface Alidad Amirfazli, J.N. Wu, Miguel Cabrerizo-Vilchez In this work magnetic field generated either by a solenoid or a permanent magnet was used to manipulate and change the shape of a drop approaching a surface. The magnetic field was also used to change the velocity of the drop approaching the surface. The capability to sculpt the drop shape and change its velocity opens up new ways of manipulating drop impact onto a surface, which can be interesting for printing industry, especially 3D printing for manufacturing parts. EFH1 (Ferrotec, USA), a colloidal dispersion of magnetite in an oil, was used as the ferrofluid, and its drops were generated with a size of $\sim $2.4 mm using a dispensing system. High speed imaging and image processing were the primary tools for this study allowing data acquisition, and analysis, respectively. Results showed that the in-flight drop shape can be changed from spherical (no field applied) to mildly elliptical or even cylindrical depending on the method of magnetic field generation, the strength of the magnetic field, and the duration of application of the magnetic field (when solenoid was used). Drop velocities could also be increased by up to three times of what would have been possible under free fall condition for a drop. Finally a discussion of in-flight drop breakup as an ultimate way to change the drop shape and its potential for applications will be provided. [Preview Abstract] |
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