"Fabrication and applications of microfluidic systems with embedded thick film sensors"
Fabrication and applications of microfluidic systems with embedded thick film sensors
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This thesis summarizes the author's work in the field of microfluidics and the embedding of thick film sensors in such systems. Although microfluidics has been established over 30 years ago, and is still a very active field of research, only a small number of examples found their application outside of academics. This is sometimes attributed to unnecessary complicated designs of microfluidic devices. The content of this thesis aims to study the fabrication of simple microfluidic systems, utilizing easy to use processes. Sensors are embedded into the devices, to add further functionality. In this regard, thick film processes are considered to be in accordance with the simplicity goal. The applications of the systems presented in this thesis will not revolutionize the field, and will most certainly not solve the current issues in microfluidics, but may be thought-provoking to some open minds.
The structure of the thesis is split into four major parts. Part I introduces all necessary aspects that are brought up in the research papers later, with the aim to keep the thesis as self-contained as possible. The first chapter is dedicated to the fundamentals of fluid dynamics, and touches topics like the governing equations, some material properties of fluids and flow characteristics prevalent in microfluidics. The following two chapters discuss different aspects of microfluidic systems. A distinction between flow based and droplet based microfluidic systems is made. The flow based microfluidics chapter is concerned with continuous flows inside closed microfluidic channels. Here, the emphasis is on different techniques for chip fabrication. Also, a few analytical results for flows inside microchannels are illustrated. Droplet based microfluidics is interpreted to cover all systems where somehow droplets are involved. This chapter mainly introduces wetting properties of sessile droplets and their evaporation characteristics. The last chapter of the Introduction is devoted to thick film processes and properties of compound materials, with a special focus on effective medium approximations.
In Part II of the thesis, five scientific publications are presented, that are related to the topics discussed in the Introduction. The first three papers can be attributed to the field of flow based to flow based microfluidics. The first article introduces a simple fabrication method for polymeric microfluidic chips, that utilizes gallium as a protective material during a solvent bonding process, in order to keep the intricacies of the design intact. The second paper discusses the applicability of a dielectric coating made from poly(methyl methacrylate) (PMMA) and barium titanate (BaTiO\textsubscript The work presented in the third publication utilizes the technologies that were developed in the first two papers, and puts them to test in form of a microfluidic co-flow viscometer. This device allows the measurement of a sample fluid's viscosity by capacitive means, instead of commonly used optical methods. The fourth and fifth publications are related to droplet based microfluidic systems. Here, a simple and highly parallelizable approach for exact dosing is presented. The underlying principle relies on evaporating droplets resting on a superhydrophobic hole plate. Only after the droplets get small enough due to evaporation, they become able to pass through the hole. This evaporation is further sped up by embedding of microheaters into the hole plate, to increase its temperature. Additionally, sensors are also embedded to monitor the temperature.
Part III concludes the thesis with a critical view on the presented microfluidic systems. Additional derivations to the topics discussed in the introductory part are given in Part IV. These should keep the thesis self-contained, but are intentionally not included in the Introduction, so they do not distract from the key points discussed there.