Electrohydrodynamic Manipulation Of Liquid Droplet Emulsions In A Microfluidic Channel

This work specifically aims to provide a fundamental framework, with some experimental validation, for understanding droplet emulsion dynamics in a microfluidic channel with an applied electric field. Electrification of fluids can result in several different modes of electrohydrodynamics (EHD). Several studies to date have provided theoretical, experimental, and numerical results for stationary droplet deformations and some flowing droplet configurations, but none have reported a method by which droplets of different diameters can be separated, binned and routed through the use of electric fields. It is therefore the goal of this work to fill that void and report a comprehensive understanding of how the electric field can affect flowing droplet dynamics. This work deals with two primary models used in electrohydrodynamics: the leaky dielectric model and the perfect dielectric model. The perfect dielectric model assumes that fluids with low conductivities do not react to any effects from the small amount of free charge they contain, and can be assumed as dielectrics, or electrical insulators. The leaky dielectric model suggests that even though the free charge is minimal in fluids with low conductivities, it is still is enough to affect droplet deformations. Finite element numerical results of stationary droplet deformations, implemented using the level set method, compare well both qualitatively (prolate/oblate and vortex directions), and quantitatively with results published by other researchers. Errors of less than 7.5% are found when comparing three-dimensional (3D) numerical results of this study to results predicted by the 3D leaky dielectric model, for a stationary high conductivity drop suspended in a slightly lower conductivity suspending medium. Droplet formations in a T-junction with no applied electric field are adequately predicted numerically using the level set finite element technique, as demonstrated by other researchers and verified in this study. For 3D models, droplet size is within 6%, and droplet production frequency is within 2.4% of experimental values found in the microfluidic Tjunction device. In order to reduce computational complexity, a larger scale model was solved first iii to obtain electrical potential distributions localized at the channel walls for the electrode placement configurations. Droplet deceleration and pinning is demonstrated, both experimentally and numerically, by applying steep gradients of electrical potential to the microchannel walls. As droplets flow over these electrical potential “steps,” they are pinned to the channel walls if the resulting electric forces are large enough to overcome the hydrodynamic forces. A balance between four dimensionless force ratios, the electric Euler number (Eue – ratio of inertial to electric forces), Mason number (M a – ratio of viscous to electric forces), electric pressure (P s – ratio of upstream pressure forces to electric forces), and the electric capillary number (Cae – ratio of electric to capillary forces) are used to quantify the magnitudes of each of these forces required to pin a droplet, and is consistent with a cubic dependency on the drop diameter. For larger drop diameters, effects of hydrodynamic forces become more prominent, and for smaller droplets, a greater electric forces is required due to the proximity of the droplet boundary with reference to the electrified channel wall. Droplet deceleration and pinning can be exploited to route droplets into different branches of a microfluidic T-junction. In addition, using steep electrical potential gradients placed strategically along a microchannel, droplets can even be passively binned by size into separate branches of the microfluidic device. These characteristics have been identified and demonstrated in this work

Numerical Modeling of Deformation, Oscillation, Spreading and Collision Characteristics of Droplets in an Electric Field

Electric field induced flows, or electrohydrodynamics (EHD), have been promising in many fast-growing technologies, where droplet movement and deformation can be controlled to enhance heat transfer and mass transport. Several complex EHD problems existing in many applications were investigated in this thesis. Firstly, this thesis presents the results of numerical simulations of the deformation, oscillation and breakup of a weakly conducting droplet suspended in an ambient medium with higher conductivity. It is the first time that the deformation of such a droplet was investigated numerically in a 3D configuration. We have determined three types of behavior for the droplets, which are less conducting than ambient fluid: 1) oblate deformation (which can be predicted from the small perturbation theory), 2) oscillatory oblate-prolate deformation and 3) breakup of the droplet. Secondly, a numerical study of droplet oscillation placed on different hydrophobic surfaces under the effect of applied AC voltage including the effect of ambient gas was investigated. The presented algorithm could reproduce droplet oscillations on a surface considering different contact angles. It has been found that the resonance frequency of the water droplet depends on the surface property of the hydrophobic materials and the electrostatic force. Thirdly, a new design of an electrowetting mixer using the rotating electric field was proposed which offers a new method to effectively mix two droplets over a different range of AC frequencies. Two regimes were observed for droplet coalescence: 1) coalescence due to the high droplet deformation, 2) coalescence due to the interaction of electrically induced dipoles. Fourthly, the spreading and retraction control of millimetric water droplets impacting on dry surfaces have been investigated to examine the effect of the surface charge density and electric field intensity. The effect of the surface charge on the spreading of droplets placed gently on surfaces was investigated in the first part. It was found that the maximum spreading diameter increases with an increasing charge. In the second part, the impact of a droplet on a ground electrode was considered. It was also found that in order to keep the maximum diameter after the impact, less charge is needed for surfaces with lower contact angle. Finally, the interaction between two identical charged droplets was investigated numerically. The effects of the impact velocity, drop size ratio and electric charge on the behavior of the combined droplet were investigated. It was shown that two conducting droplets carrying charges of the same polarity under some conditions may be electrically attracted. The formation of charged daughter droplets has been investigated and it was found that the number of the satellite droplets after collision appears to increase with an increase in the droplet charge

Estudos numéricos em eletrohidrodinâmica

Orientador: Marcos Akira D'ÁvilaTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: A eletrohidrodinâmica (EHD) descreve o movimento do fluido induzido por tensões elétricas. Sob o efeito de um campo elétrico, as moléculas de fluido podem polarizar e uma migração de íons carregados ou cargas livres através do fluido é induzida. Estes fenômenos dão origem a forças elétricas que atuam sobre a superfície do fluido colocando-a em movimento até que a tensão superficial e as tensões viscosas proporcionem o equilíbrio necessário. Aplicações como atomização de líquidos, transferência de calor e massa, dispersão de polímeros e tecnologias microfluídicas fizeram com que a eletrohidrodinâmica fosse extensivamente estudada ao longo dos anos no intuito de compreender as respostas de sistemas de fluidos submetidos a campos elétricos e de desenvolver novos processos. No entanto, a natureza complexa dos processos EHD limita as explorações tanto experimentais como de desenvolvimento. Portanto, para obter resultados mais rápidos e com custos menores, são frequentemente utilizados estudos envolvendo modelagem e simulações numéricas. Neste trabalho, utilizando um solver eletrohidrodinâmico baseado no modelo leaky dielectric, analisamos dois problemas diferentes relacionados à EHD. O primeiro consiste em investigar o efeito da viscoelasticidade na deformação e quebra de gotículas inseridas em um campo elétrico uniforme. Este é um dos primeiros e mais fundamentais problemas em EHD, porém nunca foi avaliado através de simulações numéricas utilizando fluidos não-Newtonianos. Assim, com os resultados aqui apresentados, pretendemos elucidar alguns dos principais aspectos da deformação viscoelástica em problemas de EHD. O segundo caso é um problema aplicado tem atraído um crescente interesse nos últimos anos. Consiste na deformação de lentes líquidas pela aplicação de um campo elétrico. Por meio de simulações numéricas, investigamos a influência do formato do eletrodo na deformação da lente e analisamos seu desempenho usando uma plataforma de design óptico. Esta abordagem nunca foi feita antes e sugere uma nova visão sobre os sistemas micro-ópticos adaptáveisAbstract: Electrohydrodynamics (EHD) describes the fluid motion induced by electric stresses. Under the effect of an electric field the fluid molecules may get polarized and a migration of charged ions or free charges through the fluid is induced. These phenomena give rise to electric forces that act on the fluid surface putting it into motion until the surface tension and viscous stresses provide the necessary balance. Applications such as liquid atomization, heat and mass transfer, polymer dispersion and microfluidic technologies have made electrohydrodynamics to be extensively studied over the years in order to understand the responses of fluids systems subjected to electric fields and to develop new processes. However, the complex nature of EHD processes limits both experimental and development explorations. Therefore, in order to obtain faster results and at lower costs, studies involving modeling and numerical simulations are frequently used. In this work, using an EHD solver based on the leaky dielectric model, we analyze two different problems related to electrohydrodynamics. The first one consists on the investigation of the effect of viscoelasticity on the deformation and breakup of droplets inserted in a uniform electric field. This is one of the first and most fundamental problems in EHD. However it has never been evaluated through numerical simulations using non-Newtonian fluids. Thus, with the results presented here we aim to elucidate some of the main aspects of the viscoelastic deformation in EHD problems. The second case is an applied problem that has drawn increasing interest in the past few years. It consists in the deformation of liquid lenses by the application of an electric field. By means of numerical simulations we investigate the influence of the electrode shape on the lens deformation and we analyze its performance using an optical design platform. This approach has never been done before and it suggests a new insight into the adaptive micro-optical systemsDoutoradoMateriais e Processos de FabricaçãoDoutor em Engenharia Mecânica233361/2014-6CNP

Numerical modeling of capillary electrophoresis : electrospray mass spectrometry interface design

Capillary electrophoresis hyphenated with electrospray mass spectrometry (CE-ESI-MS) has emerged in the past decade as one of the most powerful bioanalytical techniques. As the sensitivity and efficiency of new CE-ESI-MS interface designs are continuously improving, numerical modeling can play important role during their development. In this review, different aspects of computer modeling and simulation of CE-ESI-MS interfaces are comprehensively discussed. Relevant essentials of hydrodynamics as well as state-of-the-art modeling techniques are critically evaluated. Sheath liquid-, sheathless-, and liquid-junction interfaces are reviewed from the viewpoint of multidisciplinary numerical modeling along with details of single and multiphase models together with electric field mediated flows, electrohydrodynamics, and free fluid-surface methods. Practical examples are given to help non-specialists to understand the basic principles and applications. Finally, alternative approaches like air amplifiers are also included

Dynamics of Falling Droplet Under Effects of Electric Fields

Physical properties and especially the size of drops are important parameters in many industrial and medical applications. High voltage electric field is one of the effective means to control the final size of drops during the fabrication process which could greatly influence the final size of the product. Therefore a detailed study of electric field effect on a liquid drop is very important. In this work deformation and fragmentation of a falling droplet under gravity and electric force have been studied numerically. The electric force is used as an effective external controlling mechanism to influence the deformation of a drop. The three-dimensional deformation of a falling droplet is studied numerically using the open-source volume of-fluid solver, Gerris with dynamic adaptive grid refinement. The numerical results are compared with previous analytical, experimental and numerical data and excellent agreements between the results are obtained. The results are presented for a broad range of Bond numbers (Bo) from low Bond number (drop with small deformation) to large Bond number (drop breakup and fragmentation). The results revealed that the electric field can be used as a powerful controlling tool in delaying and expediting the falling drop breakup process. The results also showed that falling drop deforms severely by increasing Bo number which leads to the breakup and fragmentation compared to the cases of low Bo number in which the drop deforms mildly without breakup. Moreover, analytical solutions of drop’s deformation are presented in detail and then the outcomes were compared with numerical results. The numerical results are presented for various values of density ratios and electrical conductivity and permittivity. The comparison of the results shows a great agreement between the analytical solutions and the direct numerical simulation (DNS) results

Electro-deformation of a moving boundary: a drop interface and a lipid bilayer membrane

This dissertation focuses on the deformation of a viscous drop and a vesicle immersed in a (leaky) dielectric fluid under an electric field. A number of mathematical tools, both analytical and numerical, are developed for these investigations. The dissertation is divided into three parts. First, a large-deformation model is developed to capture the equilibrium deformation of a viscous spheroidal drop covered with non-diffusing insoluble surfactant under a uniform direct current (DC) electric field. The large- deformation model predicts the dependence of equilibrium spheroidal drop shape on the permittivity ratio, conductivity ratio, surfactant coverage, and the elasticity number. Results from the model are carefully compared against the small-deformation (quasispherical) analysis, experimental data and numerical simulation results in the literature. Moreover, surfactant effects, such as tip stretching and surface dilution effects, are greatly amplified at large surfactant coverage and high electric capillary number. These effects are well captured by the spheroidal model, but cannot be described in the second-order small-deformation theory. The large-deformation spheroidal model is then extended to study the equilibrium deformation of a giant unilamellar vesicle (GUV) under an alternating current (AC) electric field. The vesicle membrane is modeled as a thin capacitive spheroidal shell and the equilibrium vesicle shape is computed from balancing the mechanical forces between the fluid, the membrane and the imposed electric field. Detailed comparison against both experiments and small-deformation theory shows that the spheroidal model gives better agreement with experiments in terms of the dependence on fluid conductivity ratio, electric field strength and frequency, and vesicle size. Asymptotic analysis is conducted to compute the crossover frequency where a prolate vesicle crosses over to an oblate shape, and comparisons show the spheroidal model gives better agreement with experimental observations. Finally, a numerical scheme based on immersed interface method for two-phase fluids is developed to simulate the time-dependent dynamics of an axisymmetric drop in an electric field. The second-order immersed interface method is applied to solving both the fluid velocity field and the electric field. To date this has not been done before in the literature. Detailed numerical studies on this new numerical scheme shows numerical convergence and good agreement with the large-deformation model. Dynamics of an axisymmetric viscous drop under an electric field is being simulated using this novel numerical code