Fluids, Gels and Crystals : Phase behavior of binary thermoresponsive microgel mixtures

Sammanfattning: Thermoresponsive colloidal microgels expel solvent from their interior upon crossing a threshold temperature, resulting in a significantly reduced size. The collapse of microgels can reversibly switch the interparticle interactions from soft repulsive to attractive, and the accompanying size reduction allows for precise control over volume fraction. Mixing two microgels that collapse at different threshold temperatures provides a system where size ratio, volume fraction and specific interparticle interactions can all be controlled externally with temperature. In this thesis, we investigate various transitions exhibited by these highly tunable systems, and analyze the resulting structural properties. Taking advantage of the two-step collapse behavior, it is shown how different gel structures can be formed depending on the temperature profile used: a fast heating profile yields a gel that has a random distribution of the two particles, while a slower heating profile yields a sequentially gelated structure where one particle type decorates a scaffold network of the other type. The sequential network possesses stronger mechanical responses than its homo-gelated counterpart, and its final mechanical properties are determined by the properties of the scaffold at the moment of secondary particle deposition. We furthermore propose a new model based on particles with patchy interactions that is used to simulate such colloidal gels. In addition to the formation of binary gels, we show how binary mixtures can be driven to self-assemble into binary colloidal crystals. Using a newly designed sample cell that allows in-situ salinity manipulation, we observe a binary soft particle crystal-to-crystal transition. Finally, a microgel synthesis method is proposed that allows for enhanced CLSM imaging compared to traditional synthesis methods. We thus demonstrate and investigate a number of systems that all allow extensive control over phase behavior and structure formation using externally controllable parameters.