On the Phase Behaviour of Soft Matter: Understanding Complex Interactions via Quantitative Imaging

The effect of microscopic colloid interactions on the resultant macroscopic phase behaviour is a frequently studied topic in soft matter resarch, and lies at the heart of this thesis. Key structural and dynamic properties of colloidal model systems across liquid-solid transitions are tracked using optical imaging techniques.
The first studied system comprises of thermosensitive microgels. These are soft, crosslinked polymer networks of colloidal size, which have been used as model systems to investigate various phase transitions. They display a rich phase behaviour due to their soft potential and internal core-corona structure. Especially, their thermosensitivity allows us to use temperature as an external control to tune particle size, volume fraction and effective interaction potential in situ. However, a thorough understanding of the effective interactions between microgels is lacking, and constitutes a key research question in this thesis.
We therefore quantitatively compare experimental and numerical pair correlation functions (g(r)s) across the phase diagram, obtained from confocal microscopy and simulations. We find that neutral, swollen microgel interactions are temperature-dependent, but also hinge on whether the core or corona of the microgel is explored.
This approach is repeated for ionic microgels with varying crosslinker density, where the introduction of acrylic acid complicates the resultant swelling behaviour. For this reason, we start by decoupling the core and corona swelling response to various charge regimes via light scattering experiments, and found that dangling polymer strands can extend up to several 100 nm outside of the network. Dangling ends had a pronounced effect on the interactions and phase behaviour of ionic microgels, but their contribution is missing within the current theoretical framework.
Finally, liquid-solid transitions in concentrated protein solutions are investigated. Two well studied globular proteins, lysozyme and γB-crystallin, were used as model systems with completely different interactions. No unambiguous experimental demonstration of the existence of an arrested glassy state had been published so far for either protein. A combination of two passive microrheology techniques now allowed us to confirm the formation of a glass phase at concentrations above a critical arrest concentration, and to obtain quantitative insight into the concentration dependence of the zero shear viscosity prior to arrest.