Numerical and Experimental Study of Droplet Generation and Coalscence Using Microcapillaries in an Emulsification Process PDF Download

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Category :
Languages : en
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Author: Alireza Karbalaei Baba
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Category :
Languages : en
Pages : 106

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Passive and electrically active coalescence and mixing of pairs of trapped squeezed nanodroplets were studied in this work. PDMS-glass microfluidic devices were designed and fabricated using multilevel photolithography technique. Flow-focusing method was used to generate nanoliter droplets of died glycerol inside oleic acid. The effect of factors such as flow rates and their ratio, interfacial tension, and viscosities on the size and frequency of droplet generation was studied and concluded by demonstrating the capillary number effect. A passive droplet trapping technique based on minimizing the surface energy of the droplets was employed to minimize the shear flow effects and increase the accuracy of passive coalescence and mixing experiments. The theoretical platform was presented for the analysis of this multiphase problem and a numerical solver was developed based on the lattice Boltzmann method to simulate the passive and electro-coalescence of the droplet pairs. Mixing of nanodroplets was studied by discussing the contributing mixing time scales and passive mixing of glycerol nanodroplets was experimentally realized. The rate of passive mixing percentage was derived by performing image processing and its exponential asymptotical behavior was presented. This study provided physical perspectives for droplet coalescence and mixing and can be extended in several numerical and experimental aspects.

Author: Mehran Mohammadi Farhangi
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Languages : en
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Author: Katerina Loizou
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Category : Drops
Languages : en
Pages : 466

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Author: Mun Mun Nahar
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Category : Microdroplets
Languages : en
Pages : 121

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In the recent years, significant efforts have been devoted to the development of droplet based lab-on-a-chip devices of which advantages include being programmable and reconfigurable. Among various droplet flow based microsystems [1-5], electrowetting on dielectric (EWOD) digital microfluidics has many advantages, such as rapid switching response, no joule heating, no need for moving parts like pumps and valves, and most importantly low power requirement [6]. Basic droplet handling techniques - droplet dispensing, transporting, merging and splitting - can be done by sequentially activating and deactivating specific electrodes, which allows to address each droplet individually and to perform various unit processes such as encapsulation [7], mixing [8], extraction [9,10] and separation [9] in lab-on-a chip devices. Understanding of the dynamics of droplet motion in an EWOD device is crucial to design and build devices in various applications. To date, many researchers have experimentally and numerically investigated droplet dynamics in EWOD. The review article by Mugele and Baret [6] discussed approaches to understand the electrowetting theory applicable for low voltages. They analyzed the origin of electrostatic forces that reduce the contact angle and induce droplet motion. They also briefly discussed about the droplet dynamics.The present study firstly [11] focuses on characterizing droplet velocity measurement and achieving faster droplet motion which is one of the basic aspects towards achieving high throughput. As we will discuss later, although many studies have been done to understand the relation between EWOD droplet velocity and various system parameters, such as electrode width, channel gap, applied voltage etc., a comprehensive guideline to measure and define the velocity of discrete deforming droplets was still missing. After characterizing the measurement technique, we used it to measure droplet velocity of a novel electrode design with different electrode operation schemes. The novel method was found to enhance the droplet velocity up to 100%. The experimental droplet velocity was compared with a theoretical model and velocity enhancement was analyzed. In the second part of the study, we developed a numerical model for EWOD droplet motion. Navier-stokes equations coupled with the advection equation to track the interface of a water-air two-phase system was solved. Both Level Set and Phase Field method were used for the interface tracking and results from both methods were compared. Evolution of droplet shapes with time from numerical modeling was compared with that found from experiments [12]. Thirdly, we propose a study to characterize separation of phases from a compound droplet consisting of two immiscible liquids. Compound droplets can be used as an isolated unit to perform chemical reactions [13], to suppress evaporation [14], to extract agents [10]. In many of these applications, a critical step is the successful separation of the phases. Due to difference in liquid properties, the two phases of a compound droplet remain under different dynamic conditions, and separating them using the usual technique applied on single phase droplet often does not work. Therefore, complete separation of the phases without having any residue from the other phase is challenging. During separation, the location of the pinch off point is determined by the way the neck is formed. Ideally, the interface and the pinch off point should align with each other in order to ensure minimum residue. We therefor propose to study the effects of different viscosity ratios of fluids on necking. The study will further include effects of surface tension, applied frequency in the investigation. Additionally, we will tune the location of necking before separation by applying different schemes of actuation.

Author: Fei Gao
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Category : Blood
Languages : en
Pages : 67

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Droplet dynamics involves multi-scale forces from inertia body force, interior viscous shear stress to surface tension. The main purpose of this thesis is to simulate the normal impact of a liquid droplet on a hydrophobic surface by solving for the Navier-Stokes equations. The numerical results obtained is used to evaluate the effects of variable parameters on the droplet deformation evolution. The numerical simulation also works as a complimentary part of the experimental exploration, by the group members in the Droplet-Surface Impact Project in Northeastern University. The computational tool used is OpenFOAM, an open source CFD software package licensed and distributed by the OpenFOAM Foundation. The specific solver used is interFoam for this two phase problem. Water is modeled as a representative of the Newtonian fluids, while blood is modeled to represent non-Newtonian fluids. Multiple variables of the droplet-surface impact are investigated numerically: initial velocity, droplet diameter, transport properties, all of which contributes to a variation of the Weber number. The numerical results obtained for water droplets are supported by experimental data and also semi-empirical correlations. The spreading behavior and generation of ripples are in good agreement with that observed in the experimental tests. As the capillary effects become more dominant in the recoiling period, a discrepancy starts to show by a pre-maturely generated secondary droplet in simulation. Possible reasons are speculated for this discrepancy between numerical and experimental results. In the numerical comparison between water and blood droplets, a resemblance of droplet behavior in the initial spreading process is observed, while the recoiling process shows differences: blood droplets rebound in an "irregular" way compared to water droplets. The difference in deformation behaviors caused by varied transport properties leads to a way of distinguishing liquids by simple droplet-surface impact tests.

Author: Katerina Loizou
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Category :
Languages : en
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Languages : en
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This thesis provides a general description of micro droplet generation at low and medium Weber numbers using analytical, numerical and experimental methods in an engineering manner of contemplation. Based on the presented results guidelines are deduced to support the designing and operating any type of micro droplet generator. In the first chapter the basic differential equations are presented which are relevant for the droplet formation in the micro scale. Based on these fundamentals analytical and numerical descriptions of the droplet formation process will be discussed throughout the thesis. Important dimensionless numbers like the Reynolds, the Weber and the Ohnesorge number are introduced to describe the fluid flow in micro dimensions and the droplet formation qualitatively. Simple analytical expressions are derived for fluidic components like fluidic resistance, fluidic inertance, fluidic capacitance, an outflow model of a nozzle, an inlet resistance and junction effects like a contraction or an expansion of the cross section. Such compact models can be applied to build equivalent fluidic networks for more complicated fluidic systems. In this work some of the considered droplet generators are described by such a network approach. The main part of the work is engaged with the fundamentals of droplet formation especially with the necessary criteria for a droplet ejection. Therefore the Weber number respectively the critical Weber number is used to derive sufficient critical parameters for the droplet formation like the critical velocity, the critical pressure, the critical time and the critical power. These parameters are introduced using an energetic approach based on the formulations stated in the previous chapter. These critical values are subsequently used to describe the sufficient boundary conditions for a successful droplet generation with a given setup. Moreover these critical values can also be used to depict the influence of the design and geometrical.

Author: Meghan Ann Cozzens
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ISBN: 9781267132123
Category :
Languages : en
Pages : 60

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The applications and characterization of microfluidic droplets on a centrifugal disc platform are explored leading to the development of a mathematical model predicting droplet size within 8.8% of experimentally observed values. Techniques for droplet production and droplet manipulations that had yet to be explored on the C.D. platform were briefly investigated as a part of this study. These novel applications included T-junction droplet production, particle capture, and the polymerization of hydro-gel droplets. The remainder of the work presented here explores the formation and further characterization of droplet microfluidics on the C.D. platform. The development of a predictive model is presented. Both single parameter linear models and multi-parameter linear models were fit to the data to identify any significant dependence in droplet size on geometric and experimental parameters. These various models were tested for statistical significance, accuracy and then evaluated for their simplicity in relation to their predictive power. The optimum model developed allows an investigator to predict the size range of droplets produced in a centrifugal based system from adjustable parameters in the disc geometry and experimental procedure. A dimensionless number analysis was also performed to gauge the range in which droplet production occurred on the C.D.

Author: Molly K. Mulligan
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Category : Microfluidic devices
Languages : en
Pages : 130

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Microfluidics is the science of processing microliters or less of fluid at a time in a channel with dimensions on the order of microns. The small size of the channels allows fluid properties to be studied in a world dominated by viscosity, surface tension, and diffusion rather than gravity and inertia. Microfluidic droplet generation is a well studied and understood phenomena, which has attracted attention due to its potential applications in biology, medicine, chemistry and a wide range of industries. This dissertation adds to the field of microfluidic droplet studies by studying individual droplet deformation and the process of scaling-up microfluidic devices for industrial use. The study of droplet deformation under extensional and mixed shear and extensional flows was performed within a microfluidic device. Droplets were generated using a flow-focusing device and then sent through a hyperbolic contraction downstream of the droplet generator. The hyperbolic contraction allowed the smallest droplets to be deformed by purely extensional flows and for the larger droplets to experience mixed extensional and shear flows. The shear resulted from the proximity of the droplet to the walls of the microfluidic channel. The continuous phase in all of these devices was oil and the dispersed phase was water, an aqueous surfactant solution, or an aqueous suspension of colloidal particles. Droplet deformation dynamics are affected by the use of surfactants and colloidal particles, which are commonly used to stabilize emulsion droplets again coalescence. Microfluidic droplet generating devices have many potential industrial applications, however, due to the low output of product from a single droplet generating device, their potential has not been realized. Using six parallel flow-focusing droplet generators on a single chip, the process of microfluidic droplet formation can be scaled up, thus resulting in a higher output of droplets. The tuning of droplet size and production frequency can be achieved on chip by varying the outlet tubing lengths, thus allowing for a single device to be used to generate a range of droplet sizes.