Structure and Dynamics at Colloidal Boundaries

This thesis is made up of several studies of boundaries occurring in colloidal hard sphere crystals and phase separated colloid-polymer mixtures. These boundaries can be studied on the particle level, in real space and in real time by confocal microscopy. A general introduction on the experimental systems and on confocal microscopy is given in Chapter 1. This thesis consists of four main parts. The first part deals with stacking disorder in hard sphere crystals. The second part deals with geometrically frustrated hard sphere crystals. The third part describes the statistics of Brownian interface fluctuations in phase-separated colloid-polymer mixtures. The final part of this thesis deals with the transport of particles through such an interface. In the first part of this thesis we study stacking disorder in hard sphere crystals. Due to the small difference in free energy between the hexagonal close packed (HCP) and face centered cubic (FCC) crystal configurations, both structures can coexist within these crystals. As a result the crystals are often made up of a random combination of ABA and ABC type stacking sequences of hexagonal layers. Chapters 2 and 3 describe in-plane stacking disorder in hard sphere crystals. Here, we show that stacking can change as well within a hexagonal layer, both via abrupt transitions through line-defects (Chapter 2) and via continuous transitions through lattice deformations (Chapter 3). Therefore the hexagonal layers are made up of multiple islands of FCC or HCP stacking type. In Chapter 3 the relation between lattice vacancies and stacking transitions is investigated as well. Due to the osmotic pressure imbalance, particles are pushed slightly toward the vacancy. Furthermore, in areas where lattice deformations occur due to stacking transitions, the vacancy concentration is much higher, allowing the accommodation of lattice deformations. Chapter 4 describes the relation between grain size and typical stacking island dimension. Using methods to distinguish both the stacking type and the stacking direction, we are able to identify the typical island dimension. The fraction of FCC type particles determines the relative size of FCC and HCP type islands, which we can relate to lateral islands with an A, B or C type lateral position through simulations. The second part of this thesis describes how geometry affects hard sphere crystals and crystallization. Chapter 5 describes the crystallization of hard spheres near large, nearly immovable objects: much larger spheres, which are in a way the simplest conceivable impurities or dopants. Crystal nucleation, the initial formation of sufficiently large crystals may be facilitated if the curvature of the dopant is sufficiently low. The subsequent crystal growth of these small crystals is retarded by the presence of impurities, but due to their high curvature, small impurities slow the crystal growth much more down than larger ones. Chapter 6 deals with the obtained crystal structure after crystallization has completed. We introduce a frustration length which quantifies the extent of lattice distortion depending on the size of the dopants. If the dopants are sufficiently close, we show that grain boundaries may form in between directly, which is a result of the delayed growth process in between the impurities. Whether the frustration lengths persist in the sample or slowly anneal out on time depends on the impurity spacing. Chapter 7 compares lattice frustration by two methods: 1) the insertion of impurities and 2) a polygonal particle shape. Both sources of lattice frustration induce polycrystallinity. The third part of this thesis concerns the liquid-liquid interface of phase separated colloid polymer mixtures. Such interfaces are characterised by large interface fluctuations due to the low interfacial tension. Experiments and theory on the residence time of a certain fluctuation above a certain height and the related waiting time in between such heights are presented in Chapter 8. In the final part of this thesis the transport of rigid spheres through such a fluctuating interface is presented. In Chapter 9 we compare the approach of droplets and spheres to a deformable interface. The similarity of the problems shows that the approach of the sphere already captures much of the physics of the more complicated approach of a droplet. The transport of rigid spheres through interfaces is the subject of Chapters 10 and 11. We distinguish between low Bond numbers (Chapter 10) and high bond numbers (Chapter 11), which respectively represent the limits of dominant interfacial tension and gravity. The low Bond number transport configuration is characterized by a draining film in between the sphere and the interface. In the meantime the interface deforms up to maximally about half the sphere's diameter. When the sphere is sufficiently close, it is wetted by the interface and dragged through it, before it starts sedimenting away again. The high Bond number transport configuration is characterized by a thin film of gas which persists around the sphere, while the interface deforms more than half a sphere's diameter. Depending on the Bond number, two scenarios may be observed. At moderate Bond numbers the sphere leaves a V-shaped interface behind. At even higher Bond numbers, the sphere drags a column of material of the phase it was originally in behind it that eventually breaks up through a Rayleigh-Plateau instability