Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H13: Drops: Electric Fields |
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Chair: William Ristenpart, University of California, Davis Room: 3020 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H13.00001: Leidenfrost droplets in an electric field Sander Wildeman, Chao Sun, Detlef Lohse In a recent video broadcast dubbed the ``Knitting Needle Experiment,'' astronaut Don Petit aboard the ISS demonstrated how weightless water droplets can be made to orbit a statically charged Teflon rod. We study the earthly analogue of mobile droplets in an electric field, whereby the mobility is ensured by a thin vapor film sustained between the droplet and a hot plate (the Leidenfrost effect). We find that in a strong vertical electric field the droplet starts to bounce progressively higher, defying gravitational attraction. From its trajectory we can deduce the temporal evolution of the charge on the droplet. The measurements show that the charge starts high and then decreases in a step-like manner as the droplet evaporates. The discharge trend is predicted well by treating the droplet as a dielectric sphere in electrical contact with the hot plate, but the mechanism by which definite lumps of charge are transferred through the vapor film is still an open question. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H13.00002: Leidenfrost state suppression at ultrahigh temperatures Arjang Shahriari, Jillian Wurz, Vaibhav Bahadur The Leidenfrost effect is the formation of a vapor layer between a liquid and an underlying hot surface which severely degrades heat transfer and results in surface temperature overshoots. We demonstrate and analyze electrostatic suppression of the Leidenfrost state at ultrahigh surface temperatures. A concentrated electric field across the vapor layer can attract liquid towards the surface and promote wetting. This principle is successful even at ultrahigh temperatures. Elimination of the vapor layer increases heat dissipation capacity by more than one order of magnitude. Heat removal capacities exceeding 500 \textit{W/cm}$^{2}$ are reported, which is a significant advancement in boiling heat transfer. The underlying science can be understood via a multiphysics analytical model which captures the coupled electrical-fluid-heat transport phenomena underlying Leidenfrost state suppression. The physical insights gained are used to devise and demonstrate a novel electrostatic suppression technique which does not need any surface modifications. Overall, this work uncovers the physics underlying dryout prevention and demonstrates electrically tunable boiling heat transfer with ultralow power consumption. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H13.00003: Electrohydrodynamics of a particle-covered drop Malika Ouriemi, Petia Vlahovska We study the dynamics of a drop nearly-completely covered with a particle monolayer in a uniform DC electric field. The weakly conducting fluid system consists of a silicon oil drop suspended in castor oil. A broad range of particle sizes, conductivities, and shapes is explored. In weak electric fields, the presence of particles increases drop deformation compared to a particle-free drop and suppresses the electrohydrodynamic flow. Very good agreement is observed between the measured drop deformation and the small deformation theory derived for surfactant-laden drops (Nganguia et al, 2013). In stronger electric fields, where drops are expected to undergo Quincke rotation (Salipante and Vlahovska, 2010), the presence of the particles greatly decreases the threshold for rotation and the stationary tilted drop configuration observed for clean drop is replaced by a spinning drop with either a wobbling inclination or a very low inclination. These behaviors resemble the predicted response of rigid ellipsoids in uniform electric fields. At even stronger electric fields, the particles can form dynamic wings or the drop implodes. The similar behavior of particle--covered and surfactant--laden drops provides new insights into understanding stability of Pickering emulsions. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H13.00004: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 11:22AM - 11:35AM |
H13.00005: Electric field induced deformation of sessile drops Lindsey Corson, Costas Tsakonas, Brian Duffy, Nigel Mottram, Carl Brown, Stephen Wilson The ability to control the shape of a drop with the application of an electric field has been exploited for many technological applications including measuring surface tension, producing an optical display device, and optimising the optical properties of microlenses. In this work we consider, both theoretically and experimentally, the deformation of pinned sessile drops with contact angles close to either $0^\circ$ or $90^\circ$ resting on the lower substrate inside a parallel plate capacitor due to an A.C. electric field. Using both asymptotic and numerical approaches we obtain predictive equations for the static and dynamic drop shape deformations as functions of the key experimental parameters (drop size, capacitor plate separation, electric field magnitude and contact angle). The asymptotic results agree well with the experimental results for a range of liquids. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H13.00006: Immersed Interface Method for Drop Electrohydrodynamic Herve Nganguia, Yuan-Nan Young, Anita Layton, Wei-Fan Hu, Ming-Chih Lai A numerical scheme based on the immersed interface method (IIM) is developed to simulate the dynamics of an axisymmetric viscous drop under an electric field. In this work, the IIM is used to solve both the fluid velocity field and the electric potential field. Detailed numerical studies on the numerical scheme shows second-order convergence. Moreover, the numerical scheme is further validated by the good agreement with published analytical models, and results from the Boundary Integral method. The IIM code is used to investigate inertia effects and/or time-varying electric field on drop electro-deformation. Results from the simulations illustrate how the inertia effects and time dependence of the electric field affect the electro-deformation of a viscous leaky dielectric drop. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H13.00007: A high throughput droplet based electroporation system ByeongSun Yoo, MyungMo Ahn, DoJin Im, InSeok Kang Delivery of exogenous genetic materials across the cell membrane is a powerful and popular research tool for bioengineering. Among conventional non-viral DNA delivery methods, electroporation (EP) is one of the most widely used technologies and is a standard lab procedure in molecular biology. We developed a novel digital microfluidic electroporation system which has higher efficiency of transgene expression and better cell viability than that of conventional EP techniques. We present the successful performance of digital EP system for transformation of various cell lines by investigating effects of the EP conditions such as electric pulse voltage, number, and duration on the cell viability and transfection efficiency in comparison with a conventional bulk EP system. Through the numerical analysis, we have also calculated the electric field distribution around the cells precisely to verify the effect of the electric field on the high efficiency of the digital EP system. Furthermore, the parallelization of the EP processes has been developed to increase the transformation productivity. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H13.00008: Electric-Field-Assisted Droplet Dispensing on Immiscible Fluids Taewoong Uhm, Jiwoo Hong, Sang Joon Lee, In Seok Kang Dispensing tiny droplets is a basic and crucial process in numerous practical applications, such as printed electronics, DNA microarray, and digital microfluidics. The precise positioning with demanded size of droplets is the main issue of dispensing tiny droplets. Furthermore, capability of dispensing charged droplets on the immiscible fluids could bring out more utilities.~In this work, we demonstrate the droplet dispensing on immiscible fluids by means of electrical charge concentration (ECC). This results from the fact that the droplet is generated by electric force caused by electric induction between the surface of droplet and the immiscible fluid. The temporal evolution of the droplet-dispensing process was observed consecutively with a high-speed camera. In addition, the relationship between the size of dispensed droplet and the parameters, such as physical properties of fluids and electrical field strength, is established. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H13.00009: Measurement of Charge Transfer to Aqueous Droplets In High Voltage Electric Fields Eric Elton, Ethan Rosenberg, William Ristenpart When water droplets contact electrodes in insulating oils, the electrodes impart a net charge to the droplet. Under sufficiently high field strengths, the droplet moves back and forth between electrodes, in effect ``bouncing'' between them. Although the droplets clearly acquire charge, the exact mechanism by which charge transfer occurs remains unclear. Here we present evidence that the charge transfer process for a given droplet varies strongly with the number of previous bounces. Simultaneous high speed video and high resolution electrometry were used to quantify the effect of droplet composition, ionic strength, electrode material, and applied voltage. We show that bounce-to-bounce variation in charge transfer is a strong function of ionic strength and pH, and we establish that the amount of charge transferred systematically drifts in magnitude with time. Taken together, the results suggest that electrochemical reactions play a key role in modulating the charge transferred to the drop. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H13.00010: Levitation of oil droplets over an electrode in oscillatory electric fields Scott Bukosky, William Ristenpart Application of an oscillatory electric field causes immiscible oil droplets in water to aggregate, a phenomenon believed to result from induced electrohydrodynamic (EHD) fluid flows. Recently it has also been shown that rigid colloids exhibit a distinct bifurcation of their equilibrium height over the electrode in response to low frequency electric fields. Here we report that oil droplets also exhibit a bifurcation in their equilibrium height in response to oscillatory fields. Optical and confocal microscopy observations show that at low applied frequencies ($<$ 100 Hz) a large fraction of droplets levitates up to several microns away from the electrode. We investigate the impact of the electric field properties and droplet size on the levitation, and we discuss the implications of this height bifurcation phenomenon for separation of emulsified oils from solution via a contactless electrostatic precipitation process. [Preview Abstract] |
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