Fluorinated surfactants in droplet-based microfluidics: Influence of their composition on the properties of emulsion drops

Miniaturization has been a driving force in many areas of science and technology, most notably in the electronics industry. Droplet-based microfluidics, a methode to produce emulsion drops in a controlled way and at high-throughputs, enables the miniaturization and automation of biological and chemical experiments. Each emulsion drop is used as a closed reaction vessel, enabling the performance of thousands of experiments per second, at throughputs traditional technologies cannot meet. Droplet-based microfluidics is a driving technology in the advances of genomics, proteomics, single-cell analysis, high-throughput screening, and diagnostics. Crucial for using emulsion drops as reaction vessels is that they do not break and that they are tight, not allowing material to be exchanged between drops. To prevent emulsion drops from coalescing, they can be stabilized with surfactants. They adsorb at the liquid-liquid interface, lower the interfacial tension, and add steric stability. For most drop-based biological assays, aqueous drops are dispersed in fluorinated oils. These drops are stabilized with fluorinated surfactants, composed of two perfluoropolyether blocks linked to a polyethylene glycol (PEG) block. These surfactants impart good stability to emulsion drops but can contribute to an exchange of reagents between them. In this thesis, we studied how the composition of fluorinated surfactants influences the prop- erties of emulsion drops. Therefore we synthesized different fluorinated surfactants and varied the length of the hydrophilic block and the number of hydrophobic blocks a surfactant contains. We show that the mechanical and thermal stability of single emulsion drops strongly depends on the composition of the surfactants. The stability of drops stabilized with triblock copolymers inversely scales with the interfacial tension, where as the stability of drops coated with diblock copolymers scales with the surfactant packing density. To investigate the transport of reagents across oil phases, we employ water-oil-water double emulsions as templates. We show that the leakage, resulting in cross-contamination, is mainly caused by emulsion drops with sizes around 100 nm that spontaneously form at the water-oil interface. These drops result in leakage of larger objects such as plasmids with 11’000 base pairs or 100 nm polystyrene beads. We show that the leakage can be reduced by an order of magnitude if surfactants that only moderately lower the interfacial tension are used to stabilize double emulsions or if the shell thickness of double emul- sions is reduced to values similar to the diameter of the spontaneously forming drops. Lastly, we developed novel types of surfactants containing a catechol group in the hydrophilic block, that can be ionically cross-linked at the interface of a drop, creating a viscoelastic shell. We demon- strate that by cross-linking the surfactant at the drop interface, we create capsules that show high mechanical stability, and are impermeable to encapsulants. Leveraging their stickiness, we demonstrate that these capsules, if densely packed, can be printed into 3D structures.