Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S24: Drops: Electric Field Effects II |
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Chair: Priya Gambhire, Indian Institute of Science Room: 606 |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S24.00001: Sorption-controlled electrohydrodynamics of a surfactant-covered viscous drop Herve Nganguia, Wei-Fan Hu, Ming-Chih Lai, Yuan-Nan Young Surfactants physico-chemistry is often exploited to control the dynamics of viscous drops and bubbles. For instance, the adsorption-desorption kinetics plays a critical role in the deformation of drops in extensional and shear flows. By contrast these kinetics effects have yet to be accounted for drops in an electric field to the best of our knowledge. In this talk we present work on the effects of sorption kinetics on a surfactant-covered viscous drop in an electric field (both dc and ac). Specifically, we look at the dependence of drops deformation on three dimensionless numbers: the electric capillary number $\text{Ca}_E$, Biot number $\text{Bi}$, and Peclet number $\text{Pe}$. We present the numerical methods employed for simulations, and discuss preliminary results from our findings as well as future extensions. We also illustrate how our results may be applied to explain recent experiments on extreme drop deformation under an electric field (Brosseau and Vlahovska, PRL 2017). [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S24.00002: Electrocoalescence behavior exhibited by anchored aqueous droplets in air Raunaq Hasib, Rochish Thaokar Electrocoalescence of droplets is the preferred method in phase separation owing to its low energy consumption. In this work, effect of droplet conductivity on electrocoalescence phenomena is studied for an anchored droplet-in-air system under the influence of direct current electric field. Phase diagrams representing the coalescence/non-coalescence behaviour are constructed for three different droplet conductivities. Droplets of deionized water, 0.01 M NaCl solution, and 1 M NaCl solution are used for low, moderate and high conductivity experiments, respectively. Subsequent events after first non-coalescence event are also studied and an effort is made to explain and categorize the behaviour under different regimes. Beyond the critical electrocapillary number (estimated to be $\sim 0.25)$, the stabilizing capillary force cannot balance the destabilizing electric force which leads to contact of droplets. Variation in drop conductivity, cone angle, and separation distance between droplets do not influence the critical electrocapillary number. Low light experiments were conducted to detect presence of sparks during the non-coalescence events. The non-coalescence mechanism is observed to differ with change in droplet conductivity. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S24.00003: A study of non-coalescence of aqueous droplets suspended in castor oil under electric field Subhankar Roy, Rochish Thaokar Experimental and numerical investigations are carried out to study the effect of electric field on coalescence and non-coalescence behaviour of two water droplets suspended in an insulating oil (castor oil). Unlike immediate breakup of the bridge, as reported in earlier studies like Ristenpart et al. (2009) [Non-coalescence of oppositely charged drops. Nature 461 (7262), 377--380], the non-coalescence behaviour observed in our experiments show that at higher than critical electric fields the droplets form a bridge which starts thickening thereby exhibiting a tendency to coalesce. However, soon this phenomenon slows and comes to a stop, and the thickened bridge starts thinning dramatically, initiating the phenomenon of eventual non-coalescence. Numerical simulations using boundary integral method are able to explain the physics behind the thickening of this bridge, followed by thinning and non-coalescence. The fundamental reason is the competing meridional and azimuthal curvatures of the bridge under the effect of electric field induced Maxwell stresses which determine the Laplace pressure inside the bridge to become either positive or negative thereby determining the direction of fluid flow towards or away from the centre of the bridge. Velocity and pressure profiles confirm this postulation, thus enabling us to predict this behaviour of transitory coalescence followed by non-coalescence. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S24.00004: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S24.00005: Shaping of Charged Sprays Paul W. Vesely, Rudolf J. Schick Spraying oil by conventional hydraulic and air-assisted nozzles presents issues with overspray and uneven application. Electrostatic atomization provides an alternative method to spray oils and other electrically insulating fluids, utilizing the repulsive force of like charges instead of high fluid and air pressure. A spray plume of charged droplets provides a very high transfer efficiency when spraying onto grounded conductive substrates. A plane-to-plane electrostatic atomization nozzle produces a full cone spray plume, but for many conveyor coating processes, a flat spray is required to provide a uniform coating across a substrate. Flat plate electrodes were added outside of the electrostatic atomization nozzle near the orifice to generate an electric field that acts on the negatively charged spray plume shaping it into a flat fan plume. Variously shaped and sized electrodes operating at a range of voltages of negative polarity were investigated for their effectiveness in forming the full cone spray into a flat fan spray plume. High-speed imaging, phase Doppler interferometry, and particle imaging velocimetry systems were used to investigate the effect of the external electrode configurations tested. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S24.00006: Resolving anomalies in contrasting droplet dynamics under direct and alternating current electrical forcing Kirti Sahu, Suman Chakraborty Electrically driven dynamics of droplets has given rise to several apparent anomalies, as attributable to complex interconnections between the underlying physical forces and geometrical dimension driven morpho-dynamic topology. In sharp contrast to reported theory on electro-mechanics of droplets that trivially predicts shape oscillations of a droplet subjected to alternating current electric field about the steady state deformation under an equivalent root mean square direct current electric field under all possible electrical conductivity and permittivity contrasts of the droplet and the carrier phase, here we bring out a novel dimensionality driven physical paradigm under which the same does not necessarily hold true. Our results reveal a dramatic reversal in shape transition to an elongated profile in a direction orthogonal to the electric field, in contrast to the classically postulated elongated shape in the direction of the electric field, as the electric field is changed from alternating to a direct one, for contrasting permittivity and electrical conductivity ratios. We attribute these findings to dimensionality driven topological transitions and validate the same with reported experiments. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S24.00007: Electrical impedance of a deforming conducting drop Priya Gambhire, V Kumaran Deformation of drops under electric field, both direct and alternating, has been a subject of study for nearly 5 decades now. Yet, most theoretical models describing the deformation assume fluids to be either perfect or leaky dielectrics to simplify the analysis. In a real scenario, fluids are rarely dielectric and therefore a complete theoretical description of a conducting drop deforming under an alternating electric field is lacking. Additionally, to validate the models, deformation of drops is estimated in practice via optical methods which can involve expensive piece of equipment such as high-speed cameras. If the deformation can be correlated to drop properties such as its electrical impedance, it can potentially be developed into an electronic method of quantifying the deformation extending the utility of experiments. In this presentation, our attempts to find solutions to both the aforementioned points will be discussed. To describe the deformation, a Debye-Falkenhagen theoretical approach is used which describes ionic double layers. Also, an attempt made to correlate changes in the electrical impedance of the drop and its deformation will be discussed. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S24.00008: A lattice-Boltzmann model of Electrocapillarity Elfego Ruiz Gutierrez, Rodrigo Ledesma Aguilar, Gary G. Wells, Glen McHale, Andrew M. J. Edwards, Carl V. Brown, Michael I. Newton Dielectrophoresis and electrowetting have become widely used techniques for controlling and manipulating small amounts of liquids. Applications of these include the transport, and separation of liquids and other particles of different electric permittivity, electronic paper displays, adjustable lenses, and lab-on-a-chip devices. The underlying phenomena can be encompassed under electrocapillarity, the interaction of electric fields with multiphase systems where the effects of surface tension are comparable with electrostatic forces. Fundamental aspects in electrocapillarity are still open for investigation, for example, the motion of contact lines and the shape that the liquid interface acquire in the presence of electric stresses. Here, we propose a simple lattice-Boltzmann method that is capable of simulating electrocapillarity. We use a binary fluid model that includes capillary phenomena and extend the algorithm to include the forces produced by electric fields. We first validate our method by comparison against the experimental observations. Then, we examine the morphology of droplets under dielectrophoretic stresses. [Preview Abstract] |
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