New light on soft matter Spectroscopic and microscopic studies of complex fluids, active matter and supercooled liquids

This thesis investigates a variety of open questions in soft matter systems including the viscosity on different length scales, the coffee ring effect and the mystery of supercooled water. The experimental techniques comprise light-based methods, such as UV-vis absorption spectroscopy, fluorescence spectroscopy, fluorescence (lifetime) microscopy and infrared spectroscopy.
To study the viscosity on different length scales, we use viscosity-sensitive molecular rotors to probe the local environments of polymer solutions and oil-in-water emulsions. Our results suggest that molecular rotors measure the mesh size of polymer solutions and even allow to deduce the molecular weight. In oil-in-water emulsions we use molecular rotors as a proxy for lipophilic drugs, showing how their transport is affected by confinement in their nanoscale environment.
For the studies related to the coffee ring effect we employed a variety of microscopic techniques to demonstrate how the deposits left behind drying emulsion drops and drops containing living algae cells can be influenced by the properties of the substrates, and particle activity, respectively.
Eventually, we use infrared spectroscopy coupled with multivariate data analysis to study supercooled aqueous solutions. We provide evidence for the existence of an isotope in the supercooled regime of water, shifting the onset of a liquid-liquid transition by approximately 5 kelvin. This value is similar to the one observed for the Widom line of pure water and in the transition between HDA and LDA ices, strongly supporting the second critical point scenario.