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There is a large debate on the destabilization mechanism of emulsions. We present a simple technique using mechanical compression to destabilize oil-in-water emulsions. Upon compression of the emulsion, the continuous aqueous phase is squeezed out, while the dispersed oil phase progressively deforms from circular to honeycomb-like shapes. The films that separate the oil droplets are observed to thin and break at a critical oil/water ratio, leading to coalescence events. Electrostatic interactions and local droplet rearrangements do not determine film rupture. Instead, the destabilization occurs like an avalanche propagating through the system, starting at areas where the film thickness is smallest., 16 pages, 5 figures

[Image: see text] Dimple colloids with well-defined cavities were synthesized by a modified dispersion polymerization. The key step in the procedure is the delayed addition of cross-linkers into the reaction mixture. By systematically studying the effect of the delayed addition time and the concentration of the cross-linker on the resulting particle morphology, we identified the dominating driving force that underlies dimple formation. The delayed addition of cross-linkers results in colloids with a core–shell morphology consisting of a core rich in linear polymers and a cross-linked shell. This morphology was confirmed by selectively etching non-cross-linked material using dimethylformamide. With polymerization proceeding, consumption of monomers present in the swollen particles leads to contraction of the particles, which is larger for the core composed of linear polymers compared to the stiffer cross-linked shell. To accommodate this decrease in volume, the outer cross-linked shell has to buckle, resulting in a well-defined dimple. Furthermore, we extended the procedure to incorporate functional monomers, yielding chemically modifiable dimple particles. Subsequently, we showed that by leveraging the core–shell structure, these dimple particles can be used to prepare dumbbell-shaped colloids with one hollow and one solid lobe. These partially hollow anisotropic particles assemble into strings with well-defined orientations in an alternating current electric field.

Hypothesis Well-defined two-dimensional colloidal crystal monolayers (CCM) have numerous applications, such as photonic crystal, sensors, and masks for colloidal lithography. Therefore, significant effort was devoted to the preparation of preparing CCM. However, the fabrication of CCM that can float in the continuous phase and readily transfer to other substrate remains an elusive challenge. Experiments In this article a facile approach to prepare floating CCM from polymeric colloids as building blocks is reported. The key to obtain floating CCM is the selection of an appropriate solvent to release the formed CCM from the substrate. There are two steps involved in the preparation of floating CCM: formation and peeling off. Findings First, colloids are dispersed in a solvent. Evaporation of this solvent results in the formation of a meniscus structure of the air–liquid interface between the colloids that are on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids together in a dense monolayer. Once a crystallization nucleus is formed, a convective flow containing additional colloids sets in, resulting in the formation of CCM on the substrate. Second, the remaining bulk dispersion is replaced by an extracting solvent that wets the substrate and peels the formed CCM off. The influence of the several solvents, the substrate materials, and the types of colloids on the CCM formation are investigated systematically. The robustness of the approach facilitates the preparation of CCM. Furthermore, the floating feature of the CCM in principle makes transfer of the CCM to other substrates possible, which broadens its applications.

We numerically investigate the adsorption of a variety of Janus particles (dumbbells, elongated dumbbells and spherocylinders) at a fluid-fluid interface by using a numerical method that takes into account the interfacial deformations. We first determine the equilibrium configuration of a single adsorbed particle, and we find that the overall shape of the induced deformation field has a strong hexapolar mode while non-Janus particles of the same shape do not induce any interfacial deformation. We then calculate the capillary interactions between two Janus spherocylinders adsorbed at an interface. The hexapolar deformation field induces capillary attractions for laterally aligned Janus spherocylinders and repulsions for laterally anti-aligned ones. We also experimentally synthesize micrometer-sized charged Janus dumbbells and let them adsorb at a water-decane interface. After several hours we observe the formation of aggregates of dumbbells predominantly induced by interactions that appear to be capillary in nature. Our Janus dumbbells attach laterally and are all aligned, as predicted by our numerical calculations.

We report the first example of reversible encapsulation of micron-sized particles by oppositely charged submicron smaller colloids. The reversibility of this encapsulation process is regulated by pH-responsive poly(acrylic acid) (PAA) present in solution. The competitive adsorption between the small colloids and the poly(acrylic acid) on the surface of the large colloids plays a key role in the encapsulation behavior of the system. pH offers an experimental knob to tune the electrostatic interactions between the two oppositely charged particle species via regulation of the charge density of the poly(acrylic acid). This results in an increased surface coverage of the large colloids by the smaller colloids when decreasing pH. Furthermore, the poly(acrylic acid) also acts as a steric barrier limiting the strength of the attractive forces between the oppositely charged particle species, thereby enabling detachment of the smaller colloids. Finally, based on the pH tunability of the encapsulation behavior and the ability of the small colloids to detach, reversible encapsulation is achieved by cycling pH in the presence of the PAA polyelectrolytes. The role of polyelectrolytes revealed in this work provides a new and facile strategy to control heteroaggregation behavior between oppositely charged colloids, paving the way to prepare sophisticated hierarchical assemblies.

Shape changes resulting from segmental flexibility are ubiquitous in molecular and biological systems, and are expected to affect both the diffusive motion and (biological) function of dispersed objects. The recent development of colloidal structures with freely-jointed bonds have now made a direct experimental investigation of diffusive shape-changing objects possible. Here, we show the effect of segmental flexibility on the simplest possible model system, a freely-jointed cluster of three spherical particles, and validate long-standing theoretical predictions. We find that in addition to the rotational diffusion time, an analogous conformational diffusion time governs the relaxation of the diffusive motion, unique to flexible assemblies, and that their translational diffusivity differs by a small but measurable amount. We also uncovered a Brownian quasiscallop mode, where diffusive motion is coupled to Brownian shape changes. Our findings could have implications for molecular and biological systems where diffusion plays an important role, such as functional site availability in lock-and-key protein interactions., Comment: Ruben W. Verweij and Pepijn G. Moerman contributed equally to this work. 22 pages, 11 figures

The depletion interaction as induced between colloids by the addition of a polymer depletant is one of the few ways in which short-ranged attractions between particles can be controlled. Due to these tunable interactions, colloid-polymer mixtures have contributed to a better understanding of the role of attractions both in equilibrium phenomena such as phase transitions and liquid surfaces as well as in systems out of equilibrium such as gelation and the glass transition. It is known that, by simple geometric effects, surface roughness decreases the strength of the depletion interaction. In this study, we demonstrate both by Monte Carlo simulations and experiments that it is possible to generate enough difference in attraction strength to induce phase separation in smooth particles but not in rough particles. Roughness was induced by coating smooth particles with smaller spherical colloids. We indicate how effective potentials can be obtained through simulations and how the interplay between gravity and the depletion interaction with a flat container wall can be used to obtain a simple measure of the interaction strengths as a function of roughness.

A general and reproducible heterocoagulation method is presented to prepare sub-micron sized zein protein particles, loaded with negatively charged nanoparticles. These composite carrier particles can be obtained in three different morphologies, and each morphology can be prepared using nanoparticles of various size, shape and composition. An important feature of the zein composites is their long-term stability in water even in conditions where free nanoparticles often aggregate within days. Additionally, we modify the composite particles by coating them with a thin layer of silica via condensation of sodium silicate, opening possibilities for highly specific, functionalized carrier particles. Finally, the formation of hollow silica shells containing negatively charged nanoparticles is demonstrated, using the zein composites as a template.

The self-assembly of anisotropic patchy particles with triangular shape was studied by experiments and computer simulations. The colloidal particles were synthesized in a two-step seeded emulsion polymerization process, and consist of a central smooth lobe connected to two rough lobes at an angle of $\sim$90$^{\circ}$, resembling the shape of a "Mickey Mouse" head. Due to the difference in overlap volume, adding an appropriate depletant induces an attractive interaction between the smooth lobes of the colloids only, while the two rough lobes act as steric constraints. The essentially planar geometry of the "Mickey Mouse" particles is a first geometric deviation of dumbbell shaped patchy particles. This new geometry is expected to form one-dimensional tube-like structures rather than spherical, essentially zero-dimensional micelles. At sufficiently strong attractions, we indeed find tube-like structures with the sticky lobes at the core and the non-sticky lobes pointing out as steric constraints that limit the growth to one direction, providing the tubes with a well-defined diameter but variable length both in experiments and simulations. In the simulations, we found that the internal structure of the tubular fragments could either be straight or twisted into so-called Bernal spirals.

Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as $F\ensuremath{\sim}1/{r}^{2}$. Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, $l=16\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivity.

Chemically anisotropic dumbbell-shaped colloids are prepared starting from cross-linked polymer seed particles coated with a chlorinated outer layer. These chlorinated seeds are swollen with monomer. Subsequently, a liquid protrusion is formed on the surface of the seed particle by phase separation between the monomer and the swollen polymer network. Solidification of these liquid lobes by polymerization leads to the desired dumbbell-shaped colloids. The chlorine groups remain confined on the seed lobe of the particles, ensuring chemical anisotropy of the resulting particles. Exploiting the asymmetric distribution of the chemically reactive surface chlorine groups allows for site-specific surface modifications. Here we show that the geometry of the resulting chemically anisotropic dumbbells can be systematically tuned by a number of experimental parameters including the volume of styrene by which the seeds are swollen, the cross-link density of the chlorinated seeds and chemical composition/thickness of the chlorinated coating deposited on the seed particles. Being able to control the particle geometry, and therefore the Janus balance of these chemically anisotropic particles, provides a promising starting point for the synthesis of sophisticated building blocks for future (self-assembly) studies.

The phase behavior of colloids that interact via competing interactions - short-range attraction and long-range repulsion - is studied by computer simulation. In particular, for a fixed strength and range of repulsion, the effect of the strength of an attractive interaction (epsilon) on the phase behavior is investigated at various colloid densities (rho). A thermodynamically stable equilibrium colloidal cluster phase, consisting of compact crystalline clusters, is found below the fluid-solid coexistence line in the epsilon-rho parameter space. The mean cluster size is found to linearly increase with the colloid density. At large epsilon and low densities, and at small epsilon and high densities, a non-equilibrium cluster phase, consisting of elongated Bernal spiral-like clusters, is observed. Although gelation can be induced either by increasing epsilon at constant density or vice versa, the gelation mechanism is different in either route. While in the rho route gelation occurs via a glass transition of compact clusters, gelation in the epsilon route is characterized by percolation of elongated clusters. This study both provides the location of equilibrium and non-equilibrium cluster phases with respect to the fluid-solid coexistence, and reveals the dependencies of the gelation mechanism on the preparation route.

We report on the differences between colloidal systems left to evolve in the earth's gravitational field and the same systems for which a slow continuous rotation averaged out the effects of particle sedimentation on a distance scale small compared to the particle size. Several systems of micron-sized colloidal particles were studied: a hard sphere fluid, colloids interacting via long-range electrostatic repulsions above the freezing volume fraction, an oppositely charged colloidal system close to either gelation and/or crystallization, colloids with a competing short-range depletion attraction and a long-range electrostatic repulsion, colloidal dipolar chains, and colloidal gold platelets under conditions where they formed stacks. Important differences in the structure formation were observed between the experiments where the particles were allowed to sediment and those where sedimentation was averaged out. For instance, in the case of colloids interacting via long-range electrostatic repulsions, an unusual sequence of dilute-Fluid/dilute-Crystal/dense-Fluid/dense-Crystal phases was observed throughout the suspension under the effect of gravity, related to the volume fraction dependence of the colloidal interactions, whereas the system stayed homogeneously crystallized with rotation. For the oppositely charged colloids, a gel-like structure was found to collapse under the influence of gravity with a few crystalline layers grown on top of the sediment, whereas when the colloidal sedimentation was averaged out, the gel completely transformed into crystallites that were oriented randomly throughout the sample. Rotational averaging out gravitational sedimentation is an effective and cheap way to estimate the importance of gravity for colloidal self-assembly processes., Comment: 13 pages, 13 figures

Self-assembly in nature has many facets and occurs on all length scales. This thesis deals with selected examples on the colloidal length scale, including solid-stabilized emulsions, tobacco mosaic virus particles as well as colloidal and colloidal micelles. Part 1 of this thesis deals with the synthesis of anisotropic model colloids, utilizing self-assembly processes. Analogous to atoms that bind to form molecules, colloids with liquid patches are shown to bind with each other forming

Self-assembly in nature has many facets and occurs on all length scales. This thesis deals with selected examples on the colloidal length scale, including solid-stabilized emulsions, tobacco mosaic virus particles as well as colloidal and colloidal micelles. Part 1 of this thesis deals with the synthesis of anisotropic model colloids, utilizing self-assembly processes. Analogous to atoms that bind to form molecules, colloids with liquid patches are shown to bind with each other forming

Assemblies of Polyoxometalates Polyoxometalates, or POMs for short, come in a variety of shapes and sizes. Some of them are among the largest inorganic molecules known today. These molecules can be highly symmetrical and, as the name already implies, consist of (mainly) metal (molybdenum, tungsten, vanadium, iron, etc) and oxygen atoms in the form of metal oxide polyhedrons. Although individual POM molecules are in itself already a very challenging field of research, assemblies of POMs have great challenges of their own. Take for instance the solution behavior of a subclass of POMs known as Keplerates. Keplerates are highly symmetric, hollow, spherically shaped POMs. In solution they spontaneously assemble into large, hollow, spherical superstructures or POM-shells. These POM-shells have an average radius in water of several tens of nanometer and are composed of a monolayer of more than a thousand of individual POMs. Although the underlying principles behind this spontaneous assembly are still not precisely known, much can already be understood about this curious phenomenon. For instance, the equilibrium size of POM-shells of Keplerate type POMs is inversely proportional to the relative dielectric constant of the medium in which they are dispersed. This behavior is in line with predictions made by a stabilization mechanism based on Coulomb repulsion combined with charge regulation. This model also explains a new structural instability of the POM-shells. Besides the colloidal instability, i.e. the formation of aggregates that consist of many single layered shells, these systems also display an instability on a structural scale within the shell-like assemblies when the ionic strength reaches a critical value. Not only the ionic strength has an influence on the properties of POM-shells in solution. It has been claimed that the number of charges on the POMs themselves plays an important roll in their formation and stabilization as well. For instance, for the Keplerate {Mo72Fe30} experiments show that the interaction energy between the POMs in the shell becomes stronger when the charge density on the POMs increases. This interaction energy can also be estimated from analysis based on the charge regulation model in combination with a model for defects on a sphere. The charge regulation model therefore gives much insight into the behavior of the POM-shells. Up till now the POM-shells have been considered thermodynamically stable. Our experimental results however, challenge this view. It is shown that POM-shells change over time and that the preparation route of solutions of POMs affects the resulting species in solution. Moreover, in concentrated samples, a transition from spherical objects to elongated agglomerates was observed. The elongated objects subsequently grow into large, crystalline, needle-like structures. From these observations it is concluded that POMs follow an unusual nucleation route in which the POM-shells are in fact meta-stable intermediates. The term assembly can also be taken in a more general way. For instance polyoxomolybdates can be used to create extended structures. By using elementary molybdenum oxide building blocks and urea in a one-pot directed synthesis, an inorganic-organic hybrid extended structure, consisting of right- and left-handed helical units can be obtained.

Assemblies of Polyoxometalates Polyoxometalates, or POMs for short, come in a variety of shapes and sizes. Some of them are among the largest inorganic molecules known today. These molecules can be highly symmetrical and, as the name already implies, consist of (mainly) metal (molybdenum, tungsten, vanadium, iron, etc) and oxygen atoms in the form of metal oxide polyhedrons. Although individual POM molecules are in itself already a very challenging field of research, assemblies of POMs have great challenges of their own. Take for instance the solution behavior of a subclass of POMs known as Keplerates. Keplerates are highly symmetric, hollow, spherically shaped POMs. In solution they spontaneously assemble into large, hollow, spherical superstructures or POM-shells. These POM-shells have an average radius in water of several tens of nanometer and are composed of a monolayer of more than a thousand of individual POMs. Although the underlying principles behind this spontaneous assembly are still not precisely known, much can already be understood about this curious phenomenon. For instance, the equilibrium size of POM-shells of Keplerate type POMs is inversely proportional to the relative dielectric constant of the medium in which they are dispersed. This behavior is in line with predictions made by a stabilization mechanism based on Coulomb repulsion combined with charge regulation. This model also explains a new structural instability of the POM-shells. Besides the colloidal instability, i.e. the formation of aggregates that consist of many single layered shells, these systems also display an instability on a structural scale within the shell-like assemblies when the ionic strength reaches a critical value. Not only the ionic strength has an influence on the properties of POM-shells in solution. It has been claimed that the number of charges on the POMs themselves plays an important roll in their formation and stabilization as well. For instance, for the Keplerate {Mo72Fe30} experiments show that the interaction energy between the POMs in the shell becomes stronger when the charge density on the POMs increases. This interaction energy can also be estimated from analysis based on the charge regulation model in combination with a model for defects on a sphere. The charge regulation model therefore gives much insight into the behavior of the POM-shells. Up till now the POM-shells have been considered thermodynamically stable. Our experimental results however, challenge this view. It is shown that POM-shells change over time and that the preparation route of solutions of POMs affects the resulting species in solution. Moreover, in concentrated samples, a transition from spherical objects to elongated agglomerates was observed. The elongated objects subsequently grow into large, crystalline, needle-like structures. From these observations it is concluded that POMs follow an unusual nucleation route in which the POM-shells are in fact meta-stable intermediates. The term assembly can also be taken in a more general way. For instance polyoxomolybdates can be used to create extended structures. By using elementary molybdenum oxide building blocks and urea in a one-pot directed synthesis, an inorganic-organic hybrid extended structure, consisting of right- and left-handed helical units can be obtained.

This thesis reports results from a study of particle dynamics in colloidal glasses in the absence and presence of a gravitational field. It also investigates the reentrant glass transition. All the experimental results are obtained after a real space analysis using a confocal scanning laser microscopy as experimental technique that significantly widens the time window over which colloidal systems can be studied. A fluorescent recovery of bleached regions in concentrated suspensions of fluorescent colloidal hard spheres was followed in real space. This method provided data for mean squared particle displacements up to time scales that are three orders of magnitude beyond those available by present experimental techniques like dynamic or static light scattering. It was shown that, above the (hard sphere) glass transition density, particles move over distances on the order of their own diameter on time scales of 106 to 108 Brownian times. Moreover, the mean squared displacement, , showed power-law behavior over seven time (?) decades: ??^ (0.30?0.05). This behavior is different from earlier observations by dynamic light scattering. It was argued that these differences are caused by gravity effects. Further on, the influence of gravity on the long-time behavior of the mean squared displacement in glasses of colloidal hard spheres was studied. For the first time, a significant influence of gravity on the mean squared displacements of the particles was presented. In particular, the systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) showed anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. It was shown that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explained the observation that colloidal hard sphere systems which are a glass on earth rapidly crystallize in space. A quantitative analysis of the structure and dynamics of concentrated suspensions of colloids in which the magnitude of the short range attractive potential was increased by adding non-adsorbing polymers was done. These systems undergo a reentrant glass transition upon increasing polymer concentration. The melting of the glass is accompanied by significant changes in the displacement distribution and its moments. However, no significant variations were detected in the shapes of the displacement distributions. Moreover, structural correlation functions and the magnitude of local density fluctuations did not vary significantly between the glass states and the fluid. The influence of gravity on long-time diffusion in glasses was also studied using a new experimental setup that allowed a real space determination of particle displacements in directions parallel and perpendicular to the gravity field. Dispersions of particles were prepared in different solvent combinations that provide different gravitational lengths. It was shown that the direction of the external gravitational field couples to the particle dynamics and influences the values of mean square particle displacements.

This thesis reports results from a study of particle dynamics in colloidal glasses in the absence and presence of a gravitational field. It also investigates the reentrant glass transition. All the experimental results are obtained after a real space analysis using a confocal scanning laser microscopy as experimental technique that significantly widens the time window over which colloidal systems can be studied. A fluorescent recovery of bleached regions in concentrated suspensions of fluorescent colloidal hard spheres was followed in real space. This method provided data for mean squared particle displacements up to time scales that are three orders of magnitude beyond those available by present experimental techniques like dynamic or static light scattering. It was shown that, above the (hard sphere) glass transition density, particles move over distances on the order of their own diameter on time scales of 106 to 108 Brownian times. Moreover, the mean squared displacement, , showed power-law behavior over seven time (?) decades: ??^ (0.30?0.05). This behavior is different from earlier observations by dynamic light scattering. It was argued that these differences are caused by gravity effects. Further on, the influence of gravity on the long-time behavior of the mean squared displacement in glasses of colloidal hard spheres was studied. For the first time, a significant influence of gravity on the mean squared displacements of the particles was presented. In particular, the systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) showed anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. It was shown that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explained the observation that colloidal hard sphere systems which are a glass on earth rapidly crystallize in space. A quantitative analysis of the structure and dynamics of concentrated suspensions of colloids in which the magnitude of the short range attractive potential was increased by adding non-adsorbing polymers was done. These systems undergo a reentrant glass transition upon increasing polymer concentration. The melting of the glass is accompanied by significant changes in the displacement distribution and its moments. However, no significant variations were detected in the shapes of the displacement distributions. Moreover, structural correlation functions and the magnitude of local density fluctuations did not vary significantly between the glass states and the fluid. The influence of gravity on long-time diffusion in glasses was also studied using a new experimental setup that allowed a real space determination of particle displacements in directions parallel and perpendicular to the gravity field. Dispersions of particles were prepared in different solvent combinations that provide different gravitational lengths. It was shown that the direction of the external gravitational field couples to the particle dynamics and influences the values of mean square particle displacements.

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.

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.