ORCID Identifier(s)

0000-0002-3614-5354

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Hyejin Moon

Abstract

Over the past decades, microscale chemical reaction technology has been attractive in diverse areas of chemistry. It allows the precise control of quantified reagents and highly efficient heat and mass transfer, because of a large interface-to-volume ratio–particularly in case of the exothermic reaction and mixing–, reduced consumption of toxic or expensive agents, improved reaction profiles, and enhanced selectivity compared to macro-scale reactions. The mainstream microscale reaction processes were established using continuous microchannel flow systems, which has emerged as a powerful complement to batch chemistry on the laboratory scale due to relative advantages, some of which include: increased safety, more accurate temperature control, ease of reaction scale-up, and amenability to automation. Despite of successful demonstration from prior studies, microchannel-based approaches suffer from several limitations. For example, clogging of the channels by products or byproducts may cause the difficulty of maintaining a constant hydrodynamic pressure, thus stable flow. Requirement of complex flow network and cross-contamination due to unwanted diffusion through channels are also concerned. Moreover, solvent-swapping processes pose very challenging problems in microchannel reactors. Another drawback of microchannel reactors presents in combinatorial chemistry–a powerful tool for lead compound discovery and optimization of new drugs and materials. Since a combinatorial synthesis through either batch or flow reactors requires as many reactors as the number of all possible combinations of reactants, the reactor system tends to be excessively complex. A digital microfluidic platform using electrowetting-on-dielectric (EWOD) principle can be an alternative and/or complement a microchannel reactor. An EWOD digital microfluidic platform eliminates the necessity of predetermined channel network and mechanical pumps and valves. Since it is a droplet-based flow, it can prevent cross-mixing and cross-contamination. Each droplet plays as a batch reactor, which brings the feasibility of performing multi-step reactions that may involve with solvents swapping and combinatorial synthesis. Researchers have taken advantages of these unique features of EWOD microfluidic devices to conduct on-chip chemical reactions, e.g. reactions in ionic liquid droplets, synthesis of radiotracers, and synchronized synthesis of peptide-based macrocycles. Of note, that all these reactions on EWOD chip mentioned above utilized solvent fluids that are movable by EWOD actuation. However, contemporary organic synthesis generally requires non-polar or polar aprotic solvents, and their poor movability in an EWOD chip has been a long-standing problem. First, this study focuses on movability of different types of fluids other than aqueous solutions in the EWOD microfluidics to be a versatile platform for various applications. An electromechanical model using a simple RC circuit has been used to predict the mechanical force exerted on a liquid droplet upon voltage application. In this study, two important features missed in previous works are addressed. Energy dissipation by contact line friction is considered in the new model as the form of resistor. The phase angle is taken into account in the analysis of the AC circuit. The new electromechanical model and computation results are validated with experimental measurements of forces on two different liquids. The model is then used to explain influences of contact angle hysteresis, surface tension, conductivity, and dielectric constant of fluids to the mechanical force on a liquid droplet. In the second part of the study, we introduce a novel technique of an “engine-and-cargo” system that enables use of non-movable fluids (e.g., organic solvents) on an EWOD device. With esterification as the model reaction, on-chip chemical reactions were successfully demonstrated. Conversion data obtained from on-chip reactions were used in the demonstration of reaction characterization and optimization such as reaction kinetics, solvent screening, and catalyst loading. As the first step toward on-chip combinatorial synthesis, parallel esterification of three different alcohols were demonstrated. Results from this study clearly show that EWOD digital microfluidic platform is a promising candidate for a microscale chemical reaction. Thirdly, we demonstrate in-line, and on-chip workup as a step towards promoting multi-step synthesis on an EWOD device. In this study, we selected an acid-base workup as a model system and successfully shown the consecutive steps, including neutralization reaction, acid-regeneration reaction, evaporation, and recrystallization. Moreover, on-chip recrystallization of benzoic acid and benzophenone manifested the capacity of an EWOD device to handle the solid particles, whether added or formed during the reaction, with neither change in device architecture nor disassembly. The EWOD DMF device, by having robustness, chemical compatibility, and ease of use, is an alternative or a complementary tool to microreactors based on continuous channel flow for organic synthesis, and it can provide a broader range of chemistries and operating conditions.

Keywords

Electrowetting-on-dielectric (EWOD), Digital microfluidics, On-chip organic synthesis, Non-movable fluids, Electromechanical modeling, Engine-and-cargo, On-chip organic chemistry workup

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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