Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q12: Drops: Electric Field Effects |
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Chair: Yuan-Nan Young, New Jersey Institute of Technology Room: Georgia World Congress Center B217 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q12.00001: Double emulsion droplet breakup under DC electric field Ryungeun Song, Hyoungsoo Kim, Jinkee Lee, Muhammad Salman Abbasi We studied experimentally, theoretically, and numerically how electrohydrodynamic deformation and breakup of double emulsion droplets occurs under DC electric field. Experiments show that there are four distinct modes of breakup depending on the viscosity ratio, electrical conductivity ratio, permittivity ratio, and volume ratio of the core to shell liquid. The breakup modes are classified such as a unidirectional breakup mode, two different bidirectional breakup modes, and a tip-streaming breakup mode. In order to investigate the dynamics of core droplet migration and the thin film drainage of shell liquid, theoretical studies are performed aided by numerical simulation. For droplet breakup, sufficiently large Maxwell stresses (∼εE2) are essential in comparison with the capillary pressure (∼γ/rc) of the core. During breakup, the Plateau’s criteria and geometry effect are critical factor determining the breakup modes. We believe that this study could provide impacts for the comprehension of double emulsion droplet functionalities in various applications including drug delivery, material science, biological and chemical engineering. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q12.00002: Oscillations of an inviscid ring-constrained charged drop Brayden W Wagoner, Doraiswami Ramkrishna, Michael T Harris, Osman A Basaran Dynamics of constrained liquid/gas interfaces are important in various applications e.g. capillary switches and liquid lenses. Here, linear oscillations of an inviscid conducting drop constrained by a ring of negligible thickness are studied using normal mode analysis. Similar to linear oscillations of a charged, inviscid free drop (Rayleigh 1882), theoretical analysis of the oscillations of a constrained drop yields an eigenvalue problem. The free and constrained drop problems, however, differ because of an additional boundary condition in the latter compared to the former. Specifically, vanishing of the shape perturbation at the constraint in the latter case results in a constrained optimization problem. Oscillation frequencies (eigenvalues) and drop shapes (eigenfunctions) are determined as a function of constraint location and total drop charge for the lowest modes of oscillation. Interestingly, for certain sets of parameters, the eigenvalues of certain distinct modes become nearly identical and their eigenfunctions interchange. The correctness of the analytical results are demonstrated by simulations in which the free boundary problem comprised of the Navier-Stokes, continuity, and Laplace equations is solved numerically in the limit of small viscosities. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q12.00003: Electro-capillary Induced Droplet Wetting on Structured Surfaces Vartika Parihar, Soumen Das, Sunando DasGupta Electrowetting on microstructured surfaces enhances the mobility of the three-phase contact line to a greater extent due to the higher initial contact angle encountered on such surfaces. We have observed that a greater maneuverability is possible only through directional electrowetting and the initial wetting state of the droplet has an important role to play. We have obtained a transition in the initial wetting state by varying the feature size of the surface microstructures. The initial Cassie state of wetting has a limited directionality but a greater mobility of the contact line due to the higher initial contact angle. However, even though the initial Wenzel state of wetting has a lower mobility of the contact line, it has a higher directional wetting property. This is due to the additional electro-capillary effect due to the presence of micro-ridge like structures, which manifests themselves as open micro-capillaries beneath the droplet. This observation can be useful in designing droplet-based microfluidic systems for better control over the directionality of the droplet movement. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q12.00004: Driving Liquid Barrels with Electrowetting Elfego Ruiz Gutierrez, Davood Baratian, Rodrigo Ledesma Aguilar, Frieder G Mugele Liquid barrels---droplets trapped in a wedge geometry---appear in biological physics, granular media and microfluidics. Recent electrowetting experiments show that the equilibrium configuration of a liquid barrel is a truncated sphere that intersects the wedge walls with an equilibrium contact angle adjusted by the applied voltage (D. Baratian, Soft Matter, 2015). The ability to control the motion of liquid barrels promises applications of droplet manipulation in microfluidic channels; however, the dynamics to new equilibria induced by sudden changes in voltage has not been studied in detail. In this talk, we present experiments and simulations of the dynamics of liquid barrels driven by electric fields. To model the liquid-barrel dynamics, we carried out lattice-Boltzmann simulations of the coupled Navier-Stokes and Cahn-Hilliard equations (T. Krüger, Springer, 2016). To account for electrowetting, we equipped our lattice-Boltzmann algorithm with a solver of the electric potential field; this allows us to analyse in detail the competition of viscous, capillary and electrostatic forces that act on the shape of the liquid-barrel. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q12.00005: Augmented Droplet Evaporation by Electric Field Stimulus Vivek Jaiswal, Purbarun Dhar The presence of ionic inclusions is known to trigger solutal advection, which enhances the evaporation rate of the pendant droplet. The present work experimentally investigates the effect of a transverse alternating electric field on the evaporation dynamics of the saline droplet. The changes in evaporation characteristics are probed as functions of field strength and frequency. The evaporation rate is observed to follow the D2 law. However, enhancement in evaporation rate is observed under field stimulus. The classical diffusion driven evaporation model fails to predict the improved evaporation rate. The involvement of electrohydrodynamic circulation within a droplet in addition to solutal advection is proposed. Flow visualization was performed to quantify the internal circulation and augmented electrohydrodynamic advection and its dominance is observed. The improved internal advection is responsible for the improved evaporation by interfacial shear driven replenishment of the diffusion layer shrouding the droplet. The present study improves the understanding of electric field modulated fluid flow, thermal and mass transfer within a droplet. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q12.00006: Abstract Withdrawn
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Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q12.00007: Wall-induced migration of a drop in a uniform electric field Jeremy Koch, Petia M Vlahovska A uniform electric field induces an axisymmetric flow about a drop suspended in an unbounded fluid. Accordingly, there is no net force on the drop -- the drop deforms but does not move. A nearby wall, parallel to the applied field, breaks the symmetry and repels the drop, provided R/S < 1, where R/S is the ratio of the bulk charge relaxation times of the drop and suspending fluids. Under the same conditions, a rigid sphere does not migrate which implies that the lift is of electrohydrodynamic (EHD) origin. We experimentally study the EHD drop migration for different fluid and wall properties using leaky dielectrics: silicone oil drop in castor oil. We develop a theoretical model for the drop migration normal to the wall using an image stresslet related the drop polarization. If the applied electric field is perpendicular to the direction of gravity the balance of EHD lift and drop sedimentation leads to an equilibrium separation between the drop and the wall. Our study highlights the importance of confinement in electrohydrodynamic applications. |
Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q12.00008: Influence of Capillary Number and Substrate Curvature on Taylor Cone Formation in Thin Conductive Viscous Films Theodore G. Albertson, Sandra Troian We previously demonstrated by direct numerical simulation that Taylor cone formation in liquid metals confined between parallel substrates held at constant voltage difference proceeds by a self-similar process irrespective of the Reynolds number Re. The power law exponents characterizing the Maxwell and capillary pressure at the conical tip vary smoothly with Re at fixed capillary number Ca. The observed behavior smoothly bridges the inviscid prediction by Zubarev (2001) to the Stokes flow prediction by Fontelos, Kindelan and Vantzos (2008). In this work, we focus on the thin film limit in axisymmetric geometry and explore two additional aspects, namely the influence of Ca on surface excitation and growth and the influence of substrate curvature on single and multimode protrusions. Generally speaking, we find that increasing Ca at fixed Re for flow on curved surfaces generates undulations whose wavelength closely approximate the fastest growing mode associated with the fundamental planar linear instability. More interestingly, certain types of substrate curvature trigger sinusoidal traveling waves with the potential for both on- and off-axis emission. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q12.00009: Influence of electric field on the dynamics of drop formation from submerged orifices Binita Nath, Manash Pratim Borthakur, Gautam Biswas, Amaresh Dalal We investigate the influence of an external electric field on the dynamics of drop formation from a submerged orifice in a liquid-liquid system by performing direct numerical simulations. A Volume-of-Fluid solver coupled with the electric field equations is used for simulating the electro-hydrodynamic system. The simulations reveal that the ratio of fluid permittivity and conductivity plays a pivotal role in deciding the volume of the detached drops as well as the limiting length at breakup. For a system wherein the injected fluid has a lower permittivity and higher conductivity than the surrounding fluid (system A), the volume of the drops formed decreases with electric field strength and the breakup length increases. In contrast, when the injected fluid has higher permittivity and lower conductivity than the surrounding fluid, the volume of the detached drops decreases with increasing field strength. The aspect ratio of the detached drops also depends on the relative magnitude of the fluid permittivity and conductivity. The contradictory nature of charge accumulation at the interfaces and the relative difference of electric field strength inside and outside the drop are found to be the root cause for this contrasting behavior between the two systems considered in our study. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q12.00010: Abstract Withdrawn
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Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q12.00011: No contact-manipulation of drops on a liquid-infused surface using electric fields Nico Sinn, Maximilian Timothy Schür, Steffen Hardt We propose a simple way to actuate an aqueous droplet sitting on a hydrophobic grounded substrate using a pin electrode placed above it. The highly non-uniform electric field causes the droplet to move towards the pin electrode or follow its movement relative to the substrate, respectively. Drop velocities in excess of 1 cm/s can be achieved. The main challenge in that context was to provide an appropriate hydrophobic surface with low contact-angle hysteresis. This has been addressed by using silicon-oil based liquid infused surfaces that exhibit a contact angle (CA) of around 100° for water and a CA hysteresis below 5°. Using water-glycerol mixtures for the droplets we can vary the viscosity while retaining an almost constant permittivity and minimizing evaporation. The experiments are supplemented by numerical computations of the electrostatic force on a droplet, assuming a perfectly spherical surface. The numerical results show good agreement with the experimental data, from which we conclude that the underlying mechanism is dielectrophoresis. Furthermore, we propose a simplified (closed-form) model for the electric force on the droplet based on the induced dipole moment that fits the experimental data very well. |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q12.00012: Effects of surfactant transport on the electro-deformation of viscous drops Herve Nganguia, On Shun Pak, Yuan-nan Young In this work we report quantification of effects of surfactant transport on a viscous drop under a DC electric field, focusing on characterization through the dimensionless Peclet number. Our findings reveal distinct equilibrium deformations that depend on the type of drops (leaky versus conducting) and the transport regime (convection or diffusion). |
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