Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Z05: Drops: Electric Field Effects (12:15pm - 1:00pm CST)Interactive On Demand
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Z05.00001: Electrohydrodynamic tip streaming in electro-coflow. Josefa Guerrero Millan, Benjamin Overlie In electrospray, the minimum flow rate is the smallest flow rate at which you can sustain a cone-jet steady in time. Although its nature is not well understood, it has many advantages from the experimental point of view: it generates the smallest drops and they are very monodisperse. When these experiments are performed in a low viscosity bath, it is found that it can be reached a smaller minimum flow rate than that of the classical electrospray. In this work, we explore that minimum bound in electro-coflow using glass-based microfluidic devices. Thus, there are new variables to consider, the viscosity and velocity of the outer medium, and how the liquids are driven. Using a pressure driven setup, we found a parametric window of operation where electrohydrodynamic tip streaming is observed. In this regime, the concept of minimum flow rate is lost. We explored the parametric window of operation where electrohydrodynamic tip streaming can be observed and the type of steady modes (cone-jet or whipping) that are observed. [Preview Abstract] |
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Z05.00002: A three-dimensional small deformation theory for drop electrohydrodynamics Debasish Das, David Saintillan Electrohydrodynamics of drops is a classic fluid mechanical problem where deformations and microscale flows are generated by application of an electric field. In weak fields, electric stresses acting on the drop surface cause the drop to adopt a steady axisymmetric shape. This phenomenon is best explained by the leaky dielectric model under the premise that a net surface charge is present at the interface while the bulk is electroneutral. Increasing the electric field beyond a critical value can cause the drop to start rotating spontaneously and assume a steady tilted shape. This symmetry breaking phenomenon, called Quincke rotation, arises due to the action of the electric torque countering the viscous torque on the drop, giving rise to steady rotation in sufficiently strong fields. Here, we present a small deformation theory for the electrohydrodynamics of dielectric drops for the complete leaky dielectric model in three dimensions that is able to capture the transition to Quincke rotation. Retention of both straining and rotational flow components in the governing equation for charge transport enables us to perform a linear stability analysis and derive a criterion for the electric field strength that must be overcome for the onset of Quincke rotation of a viscous drop. [Preview Abstract] |
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Z05.00003: Drop pair dynamics in a uniform electric field Petia Vlahovska, Chiara Sorgentone, Jeremy Kach, Aditya Khair, Lynn Walker We present a theoretical study of drop pair-wise interactions in an applied uniform DC electric field based the leaky dielectric model. We develop three-dimensional numerical simulations using the boundary integral method and an analytical theory assuming small drop deformations. We apply the simulations and the theory to explore the electrohydrodynamic interactions between two identical drops with arbitrary orientation of the their line of centers relative to the applied field direction. Our results show complex dynamics depending on the conductivities and permittivities of the drops and suspending fluids, and the initial drop pair alignment with the applied electric field. [Preview Abstract] |
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Z05.00004: Axisymmetric Deformation of Droplet with an Adsorbed Surfactant Monolayer in a Steady Electric Field Yu Han, Charles Maldarelli, Joel Koplik Electrocoalescence is the process in which pairs of conducting droplets suspended in a continuous dielectric liquid phase are deformed, drawn together and merge upon the application of an electric field. It is an essential unit operation for separating water droplets in a crude oil. Asphaltenes indigenous to crude oil can adsorb on the interface and form highly elastic layers which resist coalescence. We use the boundary integral method to simulate the axisymmetric deformation of a water droplet in dielectric oil in a steady electric field. The insoluble adsorbed monolayer is considered as a Newtonian surface fluid interface with a dilatational viscosity but negligible shear viscosity. The surface tension is described by a Langmuir equation of state. Higher local surface coverage lowers the surface tension but results in a larger elastic stress which resists deformation. We show that the overall effect of the monolayer is to promote droplet deformation, and that elastic effects are significant at large deformation. The surface dilatational viscosity does not affect the equilibrium shape but dominates the dynamic process over a small bulk viscosity. The critical electric capillary numbers beyond which the droplet will break up are determined as a function of surface coverage. [Preview Abstract] |
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Z05.00005: Ferrofluid Drops Falling in a Non-Uniform Field Frederick Wells, Geoff Willmott Ferrofluids, i.e. fluid suspensions of ferromagnetic particles, can form beautiful and interesting shapes when placed in a magnetic field. Most famously, when a magnet is placed near a ferrofluid drop on a surface, the fluid forms spikes (Rosensweig instabilities) which arise from competition between magnetic forces and surface. When a ferrofluid droplet in air is placed in a uniform magnetic field, it becomes non-spherical [1]. This presentation will analyse experiments in which drops fall into a non-uniform magnetic field generated by a simple bar magnet. Using high-speed photography and image analysis methods, droplet shapes are captured. As a drop falls and the magnetic field increases, surface tension becomes less important and the drop elongates, forming a spike at its base. The dynamic evolution of drop shapes is studied by considering energy conservation and forces acting on the drop. The force approach can predict drop shape most effectively, especially if the bulk magnetic force is included. [1] P. Rowghanian, C. D. Meinhart and O. Camp\’{a}s, J. Fluid Mech. 802, 245262 (2016). [Preview Abstract] |
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Z05.00006: A numerical study of electrospraying: regime maps and the role of operating parameters Amartya Viravalli, Ambati Rajashekar, Nikhita Joy, Satyavrata Samavedi, Harish Dixit Electrospinning/electrospraying is a versatile voltage-driven process used to synthesize fibers/particles in the nano to submicron range from polymer solutions. While different operating regimes (e.g., cone-jet, multi-jet) have been reported before, the effects of system/process conditions on the onset of these regimes and associated transitions are not known. Further, how various process parameters affect cone and jet features of the electrified solution near the needle tip have also not been studied. In this study, electrohydrodynamic simulations are carried out using Comsol Multiphysics for a Newtonian fluid by employing the leaky dielectric model. Parametric studies with these simulations elucidate the effects of solution conductivity, surface tension, flow rate, voltage and throw distance on cone/jet features within specific regimes via simultaneous changes to effective field strength, charge density and field line distribution near the Taylor cone apex. In this talk, we will present simulations and scaling laws in conjunction with experimental data to shed light on electrospinning regimes, associated cone/jet features and operating maps. We believe that these insights can potentially aid real-time control of fiber/particle properties for specific applications. [Preview Abstract] |
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Z05.00007: Anomalous External Charge Accumulation on High Voltage Fluidic Systems Overwhelms the Applied Field Kristen Fawole, William Ristenpart In systems where high voltage electric fields are applied to dielectric fluidic devices, the standard assumption is that the magnitude of the electric field is equal to the applied potential difference over the separation distance between the electrodes. Here, we show that charge accumulation at the external solid-air boundary of the fluidic system increases with time depending on ambient air humidity and electrode metal composition. A scaling analysis indicates the increase in charge at the boundary becomes comparable and even greater than the ostensible charge density of the electrode in very short time periods. We provide measurements of the rate of charge accumulation and discuss potential mechanisms for the anomalous increase in charge in high voltage fluidic systems. [Preview Abstract] |
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Z05.00008: A triboelectric series for liquid-solid contacts Pablo Illing, Joshua Méndez Harper, Justin Burton A triboelectric series is a list of materials ranked according to their tendency to charge negatively or positively upon contact. For two solids, this process is known as triboelectrification and has been known since antiquity and extensively studied. Although less well studied, liquids can also experience triboelectrification during flow through pipes or even through air. This charging has been observed in petroleum pipelines, where potentials can become large enough to produce electrical breakdown and explosions. Liquid-solid tribocharging may also operate on worlds with exotic liquids (such as Titan) and may influence sediment transport in these planetary environments. Here, we present a series of experiment showing how liquids such as water, alcohol, and various oils charge while flowing adjacent to a wide variety of materials. Our experiments consist of dripping the fluids through different tubes into an isolated Faraday cup and measuring the increasing charge. We find a large variation in behavior. For example, dripping only though air consistently produces negatively-charged drops, whereas dripping though a Teflon tube leads to positively-charged drops. A strong dependence on flow rate is also observed, as well as the cleaning preparation of the materials. [Preview Abstract] |
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