Structure and dynamics of colloidal hard spheres in real-space

Abstract

This thesis deals with various aspects of the structure and dynamics of colloidal hard spheres. A general introduction on colloids as experimental realization of hard spheres is presented in Chapter 1. The basic principles of confocal microscopy, the main technique used in this thesis, as well as its advantages over conventional microscopy are discussed in Chapter 2.

Chapter 3 describes the synthesis and characterization of monodisperse crosslinked polymethyl methacrylate (PMMA) latex particles consisting of a fluorescent core and a large nonfluorescent shell. Since only the fluorescent cores are visible, quantitative confocal microscopy studies in three dimensions (3D) and on a single-particle level are feasible. In addition, we demonstrate that the properties of the core and the shell(s) can be controlled independently, which allows the preparation of different composite PMMA particles.

The behavior of crosslinked PMMA particles in the good solvents tetrahydrofurane, chloroform and toluene is explored in Chapter 4 using light scattering and confocal microscopy. We find that the particles almost instantaneously swell, up to more than 7 times their volume in a poor solvent like hexane. Furthermore, it is likely that the particles are charged in tetrahydrofurane, whereas signs of attractions are found in toluene.

In Chapter 5, we use a fast (spinning disc) confocal microscope to acquire 3D snapshots of an equilibrium hard sphere fluid at various densities. The available volume to insert another sphere and the surface area of that volume are determined. Applying exact relations between geometry and thermodynamics, we directly obtain the pressure, the chemical and the free energy density from microscopy images only.

Chapter 6 deals with the 3D analysis of the crystal-fluid interface of colloidal hard spheres. We vary the growth rate of the crystal by adjusting the mass density of the solvent with respect to the mass density of the particles and determine the number density profiles and in-plane bond-order profiles normal to the interfacial plane. We find that the interfacial width increases from about 8 particle diameters close to mass density matching, to 15 diameters for the largest mass density difference studied.

In Chapter 7 we show that both structure and dynamics are significantly affected by the presence of a wall. Whereas in bulk (i.e. far away from a wall) the system forms a glass, hexagonal order is clearly observed at a wall. Moreover, we show that the system at the wall exhibits a reentrant melting transition upon increasing volume fraction. The reentrant melting transition is accompanied by an increase in the mean squared displacement. Surprisingly, the ordered phase at a wall has a hexatic character rather than crystalline.

The correlation between local structure and mobility is addressed in Chapter 8 by introducing the topological lifetime, being the average time that a particle spends having the same coordination number. Subsequently, we show that defective particles exhibit shorter lifetimes than sixfold coordinated particles, which directly implies that the mobility generally increases near defects.

In Chapter 9 we compare the structures of 'monodisperse polyhedral colloids' and equivalent spherical particles. We find that the polyhedral shape significantly increases the degree of polycrystallinity. Furthermore, within single domains of the polyhedral colloids the inherent shape-induced geometrical frustration leads to a hexatic-like structure of the polyhedral colloids.