Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D33: Drops III: Electric Field Effects |
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Chair: Aditya Khair, Carnegie Mellon University Room: 404 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D33.00001: Electrohydrodynamic interactions in Quincke rotation: from pair dynamics to collective motion Debasish Das, David Saintillan Weakly conducting dielectric particles suspended in a dielectric liquid can undergo spontaneous sustained rotation when placed in a sufficiently strong dc electric field. This phenomenon of Quincke rotation has interesting implications for the rheology of these suspensions whose effective viscosity can be reduced by application of an external field. While previous models based on the rotation of isolated particles have provided accurate estimates for this viscosity reduction in dilute suspensions discrepancies have been reported in more concentrated systems where particle-particle interactions are likely significant. Motivated by this observation we extend the classic description of Quincke rotation based on the Taylor-Melcher leaky dielectric model to account for pair electrohydrodynamic interactions between identical spheres using method of reflections. We also consider the case of spherical particles undergoing Quincke rotation next to a planar electrode, where hydrodynamic interactions with the no-slip boundary lead to a self-propelled velocity. The interactions between such Quincke rollers are analyzed, and a transition to collective motion is predicted in sufficiently dense collections of many rollers, in agreement with recent experiments. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D33.00002: Electrorotation of a viscous droplet in a uniform direct current electric field Hui He, Paul Salipante, Petia Vlahovska We study both analytically and numerically the experimentally observed nonaxisymmetric droplet deformation and orientation in a uniform DC electric field [1]. The theoretical model shows that above a threshold electric field a rotational flow is induced about the droplet. As a result, drop shape becomes a general ellipsoid with major axis obliquely oriented to the applied field direction. The theory is in excellent agreement with the experimental data for high viscosity drops [2]. Low viscosity drops undergo significant deformation during rotation, which is captured by numerical simulations using the boundary integral method. \\[4pt] [1] Salipante and Vlahovska, Physics of Fluids, 22:112110 (2010)\\[0pt] [2] He, Salipante, and Vlahovska, Physics of Fluids,25:032106 (2013) [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D33.00003: The Influence of Inertia and Charge Relaxation on Electrohydrodynamic Drop Deformation Javier Lanauze, Lynn Walker, Aditya Khair An analytical model is presented for the electrohydrodynamic deformation of a drop in a leaky-dielectric fluid under a uniform electric field, taking into account transient fluid inertia and a finite electrical relaxation time. Capillary forces are assumed to be sufficiently large that the drop only slightly deviates from its equilibrium spherical shape. The temporal droplet deformation is governed by two dimensionless groups: (i) the ratio of capillary to momentum diffusion time scales: an Ohnesorge number $Oh$; and (ii) the ratio of charge relaxation to momentum diffusion time scales, which we denote by $Sa$. If charge and momentum relaxation occur quickly compared to interface deformation, $Sa \ll 1$ and $Oh \gg 1$ for the droplet and medium, a monotonic deformation is acquired. In contrast, $Sa > 1$ and $Oh < 1$ for either phase can lead to a non-monotonic deformation. In the latter case, the droplet and medium behave as perfect dielectrics at early times, which always favors an initial prolate deformation. Consequently, for a final oblate deformation, there is a shape transition from prolate to oblate at intermediate times. After the transition there may be an overshoot in the deformation, which is proceeded by an algebraic tail describing the arrival towards steady state. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D33.00004: Deformation of leaky-dielectric fluid globules under strong electric fields: Boundary layers and jets at large Reynolds numbers Ory Schnitzer, Itzchak Frankel, Ehud Yariv In Taylor's theory of electrohydrodynamic drop deformation (Proc. R. Soc. Lond. A, vol. 291, 1966, pp. 159-166), inertia is neglected at the outset, resulting in fluid velocity that scales as the square of the applied-field magnitude. For large drops, with increasing field strength the Reynolds number predicted by this scaling may actually become large, suggesting the need for a complementary large-Reynolds-number investigation. Balancing viscous stresses and electrical shear forces in this limit reveals a different velocity scaling, with the 4/3-power of the applied-field magnitude. We focus here on the flow over a gas bubble. It is essentially confined to two boundary layers propagating from the poles to the equator, where they collide to form a radial jet. At leading order in the Capillary number, the bubble deforms due to (i) Maxwell stresses; (ii) the hydrodynamic boundary-layer pressure associated with centripetal acceleration; and (iii) the intense pressure distribution acting over the narrow equatorial deflection zone, appearing as a concentrated load. Remarkably, the unique flow topology and associated scalings allow to obtain a closed-form expression for this deformation through application of integral mass and momentum balances. On the bubble scale, the concentrated pressure load is manifested in the appearance of a non-smooth equatorial dimple. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D33.00005: Retreating behavior of a charged ionic liquid droplet in a dielectric liquid under electric field Myung Mo Ahn, Do Jin Im, In Seok Kang Ionic liquids show great promise as excellent solvents or catalysts in energy and biological fields due to their unique chemical and physical properties. The ionic liquid droplets in microfluidic systems can also be used as a potential platform for chemical biological reactions. In order to control electrically the ionic liquid droplets in a microfluidic device, the charging characteristics of ionic liquid droplets need to be understood. In this work, the charging characteristics of various ionic liquids are investigated by using the parallel plate electrodes system. Under normal situation, a charged droplet shows bouncing motion between electrodes continuously. However, for some special ionic liquids, interesting retreating behavior of charged ionic liquid droplet has been observed. This retreating behavior of ionic liquid droplet has been analyzed experimentally by the image analysis and the electrometer signal analysis. Based on the hypothesis of charge leakage of the retreating ionic liquid droplets, FT-IR spectroscopy analysis has also been performed. The retreating behavior of ionic liquid droplet is discussed from the intermolecular point of view according to the species of ionic liquids. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D33.00006: A numerical method for electro-kinetic flow with deformable fluid interfaces Michael Booty, Manman Ma, Michael Siegel We consider two-phase flow of ionic fluids whose motion is driven by an imposed electric field. At a fluid interface, a screening cloud of ions develops and forms an electro-chemical double layer or Debye layer. The imposed field acts on this induced charge distribution, resulting in a strong slip flow near the interface. We formulate a ``hybrid'' or multiscale numerical method in the thin Debye layer limit that incorporates an asymptotic analysis of the electrostatic potential and fluid dynamics in the Debye layer into a boundary integral solution of the full moving boundary problem. Results of the method are presented that show time-dependent deformation and steady state drop interface shapes when the timescale for charge-up of the Debye layer is either much less than or comparable to the timescale of the flow. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D33.00007: Electrohydrodynamics of drops covered with small particles Malika Ouriemi, Petia Vlahovska A weakly conductive drop immersed in a more conductive liquid first undergoes an oblate deformation, and then experiences a rotation similar to Quincke rotation when submitted to an increasing DC uniform electrical field. We present an experimental study of a drop with an interface partially or completely covered with microscopic particles. Depending on the field intensity, the surface coverage, and the characteristics of the particles, the drop exhibits: (i) prolate deformation, (ii) emergence of pattern of sustained particle motions, or (iii) decrease of the electrical field that induces rotation. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D33.00008: Numerical simulations of a ferrofluid drop on a substrate under an applied magnetic field Ivana Seric, Shahriar Afkhami, Lou Kondic Understanding the behavior of a ferrofluid drop on a surface is of direct relevance to applications such as adaptive liquid lens optics. Here, we consider a ferrofluid drop on a horizontal substrate subjected to an applied uniform magnetic field with a non-magnetizable passive gas atop. Governing equations are the static Maxwell equations coupled with the incompressible Navier-Stokes equations. We use the long wave approximation to derive the equation that governs the non-linear evolution of the drop interface. Contact angles are imposed through the disjoining pressure model. The evolution equation is solved numerically and the results are compared with data from experiments. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D33.00009: Electrowetting-driven spreading and jumping of drops in oil Jiwoo Hong, Sang Joon Lee Electrowetting-based practical applications include digital microfluidics, liquid lenses, and reflective displays. Most of them are performed in water/oil system, because oil medium reduces the contact-angle hysteresis and prevents drop evaporation. In this study, the effects of drop volume, oil viscosity, and applied voltage on the dynamic behaviors of spreading drops, such as transition of spreading pattern and response time, are investigated. Interestingly, jumping phenomena of drops are observed in oil when the applied voltage is turned off after reaching the electrowetted equilibrium radius of drops. A numerical model to predict the transient behavior of jumping drops is formulated based on the phase-field method. The numerical results for the transient deformation of jumping drops show quantitative agreement with the experimental results. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D33.00010: Wetting of sessile water drop under an external electrical field Valerie Vancauwenberghe, Paolo Di Marco, David Brutin The enhancement of heat and mass transfer using a static electric field is an interesting process for industrial applications, due to its low energy consumption and potentially high level of evaporation rate enhancement. However, to date, this phenomenon is still not understood in the context of the evaporation of sessile drops. We previously synthesized the state of the art concerning the effect of an electric field on sessile drops with a focus on the change of contact angle and shape and the influence of the evaporation rate [1]. We present here the preliminary results of an new experiment set-up. The novelty of the set-up is the drop injection from the bottom that allows to generate safety the droplet under the electrostatic field. The evaporation at room temperature of water drops having three different volumes has been investigated under an electric field up to 10.5 kV/cm. The time evolutions of the contact angles, volumes and diameters have been analysed. As reported in the literature, the drop elongate along the direction of the electric field. Despite the hysteresis effect of the contact angle, the receding contact angle increases with the strength of the electric field. This is clearly observable for the small drops for which the gravity effect can be neglected. [Preview Abstract] |
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