Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G31: Drops: Electrowetting and Charge Effects |
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Chair: William Ristenpart, University of California at Davis Room: 312 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G31.00001: Ion adsorption-induced wetting transition in oil-water-mineral systems Bijoyendra Bera, Andrea Cavalli, Dirk van den Ende, Frieder Mugele The relative wettability of a rock substrate to oil and water is a central issue in many technological applications, especially in the field of enhanced oil recovery. We here consider a salty water droplet deposited on a mica substrate inside an oil bath. By adding specific ions to the water phase, a wetting transition can be induced. The water solution completely wet the mica substrate if it only contains monovalent cations (K$^{\mathrm{+\thinspace }}$,Na$^{\mathrm{+}})$. However, when divalent (Ca$^{\mathrm{2+}}$ , Mg$^{\mathrm{2+}})$ cations are added to the water phase, a finite contact angle (around 10$^{\mathrm{o}})$ can be observed. We explain this phenomenon in the scope of a Poisson-Boltzmann model. The absorption of divalent ions at the mica interface generates a positive surface charge, and induces an attractive interaction to the negatively charged oil-water interface, which triggers the transition. We also observe that different cations can be arranged in an Hofmeister-like sequence, based on their effectiveness in changing the wettability of the mineral substrate. Finally, we show that adding small amounts of a polar surfactant to the oil phase synergistically enhances the wetting transition. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G31.00002: Liquid bridges in complex geometries: Equilibrium shape metamorphosis using electrowetting Davood Baratian, Andrea Cavalli, Dirk van den Ende, Frieder Mugele The equilibrium morphology of liquid drops exposed to geometric constraints can be rather complex. Even for simple geometries, analytical solutions are scarce. We investigate the equilibrium shape and position of liquid drops confined in the wedge between two solid surfaces. Using electrowetting, we control the contact angle and thereby manipulate the shape and the equilibrium position of aqueous drops in ambient oil. In the absence of contact angle hysteresis and buoyancy, we find that the equilibrium shape is given by a truncated sphere, prior to filling the wedge corner, at a position that is determined by the drop volume and the contact angle. At this position, the net force between drop and the surfaces vanishes. The effect of buoyancy gives rise to substantial deviations from this equilibrium configuration which we discuss it as well. We elegantly show how the geometric constraint and electrowetting can be used to position droplets inside a wedge in a controlled way, without mechanical actuation. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G31.00003: Spreading of thin droplets of perfect and leaky dielectric liquids on inclined surfaces Andrew Corbett, Bo Kyung Ryu, Si Hyung Lee, Satish Kumar The spreading of droplets may be influenced by electric fields, a situation that is relevant to applications such as coating, printing, and microfluidics. In this work we study the effects of an electric field on the gravity-driven spreading of droplets down an inclined plane. We consider both perfect and leaky dielectric liquids, as well as perfectly and partially wetting systems. Lubrication theory is applied to generate a set of coupled partial differential equations for interfacial height and charge, which are then solved numerically with a finite- difference method. Electric fields tend to increase the height of the capillary ridge in both perfect and leaky dielectric droplets. In partially wetting liquids, the presence of large concentrations of surface charge can cause the main droplet to split into several smaller droplets. Although the model predicts that electric fields do not significantly change the long- time spreading rate, flow visualization experiments reveal that electric fields can significantly alter the dynamics of droplet spreading outside of the regime in which the model is valid. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G31.00004: Dynamic electrowetting on microstructured surfaces Satoshi Nita, Jiayu Wang, Minh Do-Quang, Yu-Chung Chen, Yuji Suzuki, Gustav Amberg, Junichiro Shiomi Surface modification such as surface charging or microstructuring has been shown as an effective method to control static wetting, but its influence on dynamic wetting is still unclear. Previously, we found that the initial stage of droplet spreading can be significantly hindered by surface microstructures [1, 2], while previous experiments showed that the effect of surface charge on dynamic wetting on a flat surface is minor. Here, we combine microstructuring and electrowetting to further enhance the controllability of the dynamic wetting. Microstructures are fabricated on silicon wafers and the spontaneous spreading of a droplet is imaged with a high-speed camera. We reveal that the spreading rate sensitivity to surface charge increases in the presence of microstructures. Furthermore, numerical simulations solving Cahn-Hilliard/Navier-Stokes equations are performed and the effect of surface modification is quantified in terms of the contact-line friction. \newline [1] J. Wang, et al., Surface structure determines dynamic wetting. \textit{Scientific reports} \textbf{5}, 8474 (2015). \newline [2] M. Do-Quang, et al., When and how surface structure determines the dynamics of partial wetting. \textit{Europhysics letters} \textbf{110}, 46002 (2015). [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G31.00005: Analysis of the triple contact point in electrowetting Sam Brzezicki, Darren Crowdy, Elena Louca In electrowetting applications, it is often necessary to consider static conducting droplets sitting on substrates. In this talk we perform a detailed study of the triple contact point at which the droplet, the ambient medium and the substrate touch. A local analysis is performed to understand the nature of the field singularities at the contact point. We also present some global solutions obtained using a novel transform formulation and gain insights into how those solutions depend on the system parameters. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G31.00006: Oscillating dynamics of a bubble immersed in an electrical fie Herve Caps, Jerome Hardouin From the pioneer work of Millikan, it is known that adding electrical charges in a droplet causes both volume and surface forces. The first force allowed Millikan to determine the electrical charge of the electron, while Taylor paved the way into the second effect by evidencing capillary pressure variations. In the present study, we focus on the dynamics of a hemispheric bubble (1cm in diameter) deposed onto one of the two conducting plates of a capacitor. The bubble is observed to experience periodic electrical breakdowns followed by more quite periods. One of the important facts is that the bubble never explodes even electrical sparks are generated. The bubble dynamics has been followed by mean of a high-speed camera and allowed us to grasp the deformations of the bubble due to the electric field, from rest to the electrical breakdown. We propose a rather simple model accounting for the experimental parameters such as the applied electric field. By balancing the surface tension, viscous and electrical forces, this model mimics the bubble dynamics and evidences the charge dynamics inside the bubble. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G31.00007: Nano-crater Formation on Electrodes during the Electrical Charging of Aqueous Droplets Eric Elton, Ethan Rosenberg, William Ristenpart A water drop in an insulating fluid acquires charge when it contacts an electrode, but the exact mechanism of charge transfer has remained obscure. Previous work, dating back to Maxwell, has implicitly assumed that the electrode remains unaltered by the charging process. Here we demonstrate that, contrary to this assumption, water drops and other conducting objects create ``nano-craters'' on the electrode surface during the charging process. We used optical microscopy, SEM, and atomic force microscopy to characterize the electrode surfaces before and after water drops were electrically bounced on them. We show that each drop contact creates an approximately micron wide and 30-nm deep crater to form on the electrode surface. Given enough time, the drop will form enough nano-craters to effectively `eat through' a sufficiently thin electrode. We discuss possible physical mechanisms for the nano-crater formation, including localized melting caused by Joule heating during the charge transfer event. The observations reported here are of particular interest in the development of microfluidic devices that use thin film electrodes to control the motion of aqueous drops. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G31.00008: Interfacial Charge Effects on Sticky Bubble Morphology in a Microchannel Jonathan Hui, Peter Huang Many multiphase fluidic processes in small conduits, such as petroleum extraction and biochemical analysis, can encounter disastrous flow blockages due to the lodging of immiscible bubbles or droplets. The complete drainage of a thin-film lubrication layer surrounding an adhered bubble demands a significantly higher threshold pressure gradient in order to reinitiate bulk flows. In this work, we investigate bubble morphology due to the lubrication layer drainage process that results in bubble adhesion and study how an electrostatically charged bubble interface and charged channel wall may affect bubble morphology in preventing bubble adhesion. We report on our multiphysics computational analysis of an oversized gas bubble in a water-filled microchannel under the influence of surface tension and interfacial electrostatic forces. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G31.00009: The effect of electric field on the stability and breakup of liquid nano-thread. Darshak Bhuptani, Sarith Sathian The stability behavior of nano-scale stationary polar liquid (water) thread in vacuum and in environment under the action of an external constant electric filed is investigated using molecular dynamics (MD) simulations. The Rayleigh instability predicts the non-dimensional wave number for nano-thread to be in the range of 0.2 to 0.5 whereas for a macroscale it is around 0.697. The classical viscid theory predict the first breakup time of nanoscale liquid thread accurately. At nanoscale, thermal fluctuation and surface tension forces plays a critical role in the breakup mechanism. The main objective is to investigate the effect of an additional external force in the form of an electric field on the stability and breakup. The effect of surface tension and the role of thermal fluctuation on the breakup in such cases are investigated. A uniform electric field is applied along the axial direction of thread. For lower values of field strength (0.1V/nm), no breakup is observed as the surface tension force is completely balanced by the electrical force experienced by the surface molecules. With increase in the electrical field strength, different phenomenon such as Taylor's cone formation, molecule orientation, whipping instability and splaying are observed. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G31.00010: Launching droplets from a super-hydrophobic surface using electrowetting Zhantao Wang, Dirk van den Ende, Andrea Cavalli, Daniel Wijnperle, Frieder Mugele Electrowetting (EW) on super-hydrophobic surfaces in ambient air has been reported to be mostly irreversible due to the transition from the Cassie to the Wenzel state. By applying short voltage pulses using interdigitated electrodes, embedded in the substrate we demonstrate a reversible contact angle variation up to 70 degrees on a single-tier super-hydrophobic surface, which is much higher than previously reported. For a range of voltages and pulse durations the droplet can be launched from the substrate due to conversion of interfacial energy to kinetic energy of the center of mass. We have studied the jumping height as a function of the applied voltage and pulse duration and identified the parameters to maximize this height. The energy dissipation during the droplet detachment and subsequent bouncing was also analyzed by analyzing the drop shape and position from the side and bottom view recordings of the jumping drop. We also investigate the role of the ambient phase by considering the EW-actuated detachment of water drops in oils of different viscosities. [Preview Abstract] |
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