Shape-Induced Frustration of Hexagonal Order in Polyhedral Colloids\ud

The effect of a nonspherical particle shape and shape polydispersity on the structure of densely packed hard colloidal particles was studied in real space by confocal microscopy. We show that the first layer at the wall of concentrated size-monodisperse but shape-polydisperse polyhedral colloids exhibits significant deviations from a hexagonal lattice. These deviations are identified as bond-orientational fluctuations which lead to percolating “mismatch lines.” While the shape-induced geometrical frustration of the hexagonal symmetry suppresses translational order, bond-orientational order is clearly retained, indicating a hexaticlike structure of the polyhedral colloid

Structure and dynamics of colloidal hard spheres in real-space

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

Hierarchical self-assembly of polydisperse colloidal bananas into a two-dimensional vortex phase

Self-assembly of microscopic building blocks into highly ordered and functional structures is ubiquitous in nature and found at all length scales. Hierarchical structures formed by colloidal building blocks are typically assembled from monodisperse particles interacting via engineered directional interactions. Here, we show that polydisperse colloidal bananas self-assemble into a complex and hierarchical quasi-two-dimensional structure, called the vortex phase, only due to excluded volume interactions and polydispersity in the particle curvature. Using confocal microscopy, we uncover the remarkable formation mechanism of the vortex phase and characterize its exotic structure and dynamics at the single-particle level. These results demonstrate that hierarchical self-assembly of complex materials can be solely driven by entropy and shape polydispersity of the constituting particles.ISSN:0027-8424ISSN:1091-649

Stabilisation of hollow colloidal TiO2particles by partial coating with evenly distributed lobes

| openaire: EC/H2020/279541/EU//IMCOLMATPhoto-catalytically active crystalline TiO2has attracted special attention due to its relevance for renewable energy and is typically obtained by the calcination of amorphous TiO2. However, stabilising hollow colloidal TiO2particles against aggregation during calcination without compromising their photocatalytic activity poses two conflicting demands: to be stable their surface needs to be coated, while efficient photocatalysis requires an exposed TiO2surface. Here, this incompatibility is resolved by partially coating TiO2shells with evenly distributed 3-trimethoxysilyl propyl methacrylate (TPM) lobes. These lobes act both as steric barriers and surface charge enhancers that efficiently stabilise the TiO2shells against aggregation during calcination. The morphology of the TPM lobes and their coverage, and the associated particle stability during the calcination-induced TiO2crystallization, can be controlled by the pH and the contact angle between TPM and TiO2. The crystal structure and the grain size of the coated TiO2shells are controlled by varying the calcination temperature, which allows tuning their photocatalytic activity. Finally, the durable photocatalytic activity over many usage cycles of the coated TiO2compared to uncoated shells is demonstrated in a simple way by measuring the photo-degradation of a fluorescent dye. Our approach offers a general strategy for stabilising colloidal materials, without compromising access to their active surfaces.Peer reviewe

Shrinkage mechanisms of grain boundary loops in two-dimensional colloidal crystals

We discuss the various mechanisms involved in the spontaneous shrinkage of circular grain boundaries in two-dimensional colloidal crystals. We provide experimental evidence that these grain boundary loops shrink owing to three intermittent mechanisms proposed for atomic materials, namely purely curvature-driven migration, coupled grain boundary migration, and grain boundary sliding. Throughout shrinkage, the product of the radius and misorientation of the grain boundary loop remains higher than a fundamental limit resulting from the specific dislocation structure of grain boundary loops, except for the very last stage where the loop character is lost. Despite its complexity, this process can be effectively described by a single kinetic coefficient, allowing for a simplified description of grain boundary loop kinetics

Single-orientation colloidal crystals from capillary-action-induced shear

We study the crystallization of colloidal dispersions under capillary-action-induced shear as the dispersion is drawn into flat walled capillaries. Using confocal microscopy and small angle x-ray scattering, we find that the shear near the capillary walls influences the crystallization to result in large random hexagonal close-packed (RHCP) crystals with long-range orientational order over tens of thousands of colloidal particles. We investigate the crystallization mechanism and find partial crystallization under shear, initiating with hexagonal planes at the capillary walls, where shear is highest, followed by epitaxial crystal growth from these hexagonal layers after the shear is stopped. We then characterize the three-dimensional crystal structure finding that the shear-induced crystallization leads to larger particle separations parallel to the shear and vorticity directions as compared to the equilibrium RHCP structure. Confocal microscopy reveals that competing shear directions, where the capillary walls meet at a corner, create differently oriented hexagonal planes of particles. The single-orientation RHCP colloidal crystals remain stable after formation and are produced without the need of complex shear cell arrangements

Microscopic dynamics of synchronization in driven colloids.

Synchronization of coupled oscillators has been scrutinized for over three centuries, from Huygens' pendulum clocks to physiological rhythms. One such synchronization phenomenon, dynamic mode locking, occurs when naturally oscillating processes are driven by an externally imposed modulation. Typically only averaged or integrated properties are accessible, leaving underlying mechanisms unseen. Here, we visualize the microscopic dynamics underlying mode locking in a colloidal model system, by using particle trajectories to produce phase portraits. Furthermore, we use this approach to examine the enhancement of mode locking in a flexible chain of magnetically coupled particles, which we ascribe to breathing modes caused by mode-locked density waves. Finally, we demonstrate that an emergent density wave in a static colloidal chain mode locks as a quasi-particle, with microscopic dynamics analogous to those seen for a single particle. Our results indicate that understanding the intricate link between emergent behaviour and microscopic dynamics is key to controlling synchronization