1st AMI-PhD defence: Mathias Reufer

Abstract

This thesis describes model systems of strongly interacting colloids controlled by external stimuli. In condensed matter physics, colloidal particles are used as "model atoms" and their phase behavior is studied to address fundamental questions. In contrast to molecular condensed matter systems, colloids provide the fascinating possibility to access directly scales of length and time comparable to the monomer size and structure. Moreover, the ability of chemists to synthesize particles with desired size, shape and surface chemistry allows to control the interaction strength by external parameters such as temperature and pH or by an electric or magnetic field.

In the first part of this thesis we investigate a system of particles made of thermosensitive microgel particles dispersed in water. The thermosensitive solubility of the polymer allows to tune the particle size and the particle-particle interactions by temperature. We study the temperature dependent density profiles of the microgel particles. The gained insight allows us to develop a novel model that describes the mechanical properties of the quenched solid phase of microgel particles. We measure the viscoelastic properties of a jammed system of microgel particles and test this novel model. Finally, we complete the study of microgel systems by tracking of labeled particles and investigate the structure and dynamics close to the glass transition.

The second part deals with anisotropic magnetic core-shell particles. A detailed characterization of the magnetic properties of these particles addresses the question how the anisotropic particle shape and the silica coating affect the magnetic properties of the particles. Furthermore, we study dispersed particles by scattering methods to reveal particle dimensions, silica shell thickness and porosity. By applying an external magnetic field, the dispersed particle align with the field, and the resulting anisotropic scattering pattern allows us to study the orientation behavior of the particles. Moreover, we study the dynamics of dispersed particles in a magnetic field at different concentrations. When aligned in a field, the dynamics of a system of dispersed particles becomes anisotropic due to the anisotropic particle shape.