Microfluidic Step Emulsification: From Fundamental Understanding to the Fabrication of Functional Materials
Embargoed until 2022-01-28
- Doctoral Thesis
Microfluidics offers a fascinating playground to manipulate liquids. As a manipulating tool, it enables, for example, the production of size-controlled droplets. A large-volume production of monodisperse droplets is essential for industrial applications, as it allows for templating functional materials, such as microparticles, microcapsules, or porous structures. The monodispersity achieved through this technique enables precise control over droplet volumes, arrangement of droplets into periodic structures, and tailored release of microencapsulated substances. These features offer several benefits for pharmaceutical, cosmetic, and nutraceutical industries. Despite these promising prospects, microfluidic platforms are currently limited by difficulties in parallelizing emulsification processes. Parallelization approaches attempted so far require complicated 3D structures, are pressure-dependent and insufficiently robust, or lack a chemically inert embedding material. Among the several alternatives reported in the literature, step emulsification brings great advantages due to its scalability and pressure-independent droplet formation mechanism. This thesis presents a scalable method to fabricate single and double emulsions of many different fluids using step emulsification devices made in glass. These emulsions are used as templates to fabricate functional materials. Step emulsification devices in glass are fabricated by photolithography, wet etching, and thermal bonding processes. The resulting devices can be used to create functional microparticles and microdroplets, such as polymers, hydrogels, and nanoparticle-loaded emulsions. In addition to the synthesis of functional materials, the glass devices also enables the elucidation of the fundamental physical mechanisms underlying the droplet formation process and the effect of the wetting properties of the glass channels on the droplet size and accessible flow rates. With the use of high-speed imaging at up to 200’000 frames per second, we find that the droplet breakup process is described by two regimes. The initial wetting regime determines the droplet size and is dependent on the contact angle between the two fluids and the channel walls. After the droplet fluid fully de-wets the channel walls, a second regime characterized by a Rayleigh-Plateau type instability determines the droplet pinch-off. The droplet breakup is studied by combining microfluidic experiments with numerical, three-dimensional simulations. Based on these numerical and experimental results, a simple theory predicting the dripping-to-jetting transition as a function of the contact angle is presented. Our findings show that a contact angle higher than 120° is required for successful emulsification. Furthermore, we show that higher contact angles enhance the emulsification speed by shifting the dripping-to-jetting transition towards higher flow rates. This offers a powerful tool for engineers to scale up the process and achieve high droplet formation throughput. Besides the fundamental physics behind step emulsification, this thesis presents a new scaled-up process for the fabrication of monodisperse microcapsules. To reach that, two oppositely-functionalized step devices are connected in series to enable tandem emulsification in a two-step sequential process. The single droplets emulsified in the first device are re-injected into a second radially-shaped device. Such a radial arrangement ensures an equal distribution of the single emulsion towards the droplet makers of the second device. Double emulsion templates formed in the second device are consolidated through polymerization of the middle fluid to generate tailored microcapsules. By controlling the geometry of the two devices and the flow rates of the core and the middle fluids, microcapsules with designed size, shell thickness and number of cores can be produced. Double emulsions made by this approach may exhibit spherical or unusual multifaceted geometries depending on the number of cores. The production of 70 µm microcapsules at throughputs of 48 mL/h is eventually demonstrated using a scaled-up device with 800 droplet makers. Additionally to the large-volume production, the scaled-up device also allows for robust emulsification over more than 5 h. The combination of size control, throughput, and robustness offers the possibility to produce large quantities of microcapsules that can be incorporated into a polymeric matrix to create functional composites. To illustrate this possibility, we produced stress-responsive materials, which exhibit a color change upon a mechanical stress. This system could be potentially expanded to self-healing or stress-adaptive materials by utilizing microcapsules with tailored mechano-response. The scalability of microfluidic step emulsification demonstrated in this thesis fulfills the author’s initial interest in developing engineering tools to make monodisperse microparticles and microcapsules industrially feasible. Towards this goal, a fundamental understanding of the science behind step emulsification was shown to be essential to obtain microfluidic devices with robust and scalable operation. Hence, this thesis shows the benefit of combining interdisciplinary knowledge in physics, materials science, and engineering, as a means to address industrially-relevant challenges and demands Show more
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ContributorsExaminer: Studart, André R.
Examiner: de Mello, Andrew
Examiner: Vladisavljevic, Goran T.
Organisational unit03831 - Studart, André R. / Studart, André R.
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