Mun Mun Nahar

ORCID Identifier(s)

0000-0001-8108-1129

Graduation Semester and Year

2018

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Hyejin Moon

Abstract

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 upto 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.

Keywords

Electrowetting-on-dielectric (EWOD), Droplet microfluidics, Multiphase droplet, Phase separation, Numerical modeling of droplet motion

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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