Seven daily habits of a crystal nucleus: Homogeneous nucleation in colloidal systems

Nucleation is the first step of a phase transition, such as boiling, condensation, melting, or crystallization. It is the process through which the first tiny portion of the new phase forms. This initial microscopic cluster is called the nucleus. The nucleus is the core from which the new phase can grow further. My dissertation focuses on computer simulations of nucleation during crystallization and melting in colloidal systems: how a colloidal fluid transforms into a crystal and vice versa. Colloids are small particles that are nevertheless about 100-10,000 times larger than atoms. Due to their size, the movements of colloids are slower than those of atoms and can be observed more easily with microscopes. This makes colloids particularly suitable for studying nucleation. Using computer simulations, I measure how quickly nucleation occurs and what kinds of crystals are formed through nucleation. I compare these measurements with experimental results and theoretical predictions. Of particular importance is the so-called classical nucleation theory, which plays a central role in my research. A key conclusion of my work is that classical nucleation theory provides accurate predictions for the rate of nucleation and the types of crystals formed. This also offers practical prospects for applying this theory to the design of colloidal materials. In more technical terms, computer simulations are used in this thesis to investigate homogeneous nucleation in atomic, molecular and colloidal systems. The focus is on the crystallization of colloidal systems, i.e. the phase transition in which a colloidal fluid transforms into a colloidal crystal. Additionally, I have studied the reverse phase transition of melting, meaning the transition from a colloidal crystal to a colloidal fluid. Finally, I have studied crystal nucleation of Lennard-Jones particles and water. Generally, I have used classical nucleation theory to make predictions concerning the nucleation rate as well as the nucleation mechanism and have tested these predictions with independent measurements. A common theme is the presence of metastable phases, e.g. during two-step nucleation and crystal polymorph selection.