We report additive-free physical strategies to regulate ZIF-8 particle size and uniformity using co-solvents. This fluid-shear-mediated strategy regulates nuclei coalescence to dictate the final size, yielding small particles with a quasi-single-crystalline structure under mild shear and larger ones with a polycrystalline nature under intense mixing, fully preserving their adsorption capacity.

Monodisperse spherical colloidal particles confined within emulsion droplets can crystallize into icosahedral clusters. Experimentally, it was observed that a few large colloidal particles added as defects preferentially migrate to the vertices of the icosahedral clusters. To understand this structure formation phenomenon, we simulate the confined self-assembly of hard spheres in the presence of a small number of larger particles. The results demonstrate that large spheres are significantly influenced by concentric shells of small spheres near the crystallization transition. Entropic forces drive the large spheres to the cluster surface, where they settle into free energy minima at the icosahedron vertices. Notably, the addition of twelve large spheres results in the formation of a perfect icosahedral frame. Free energy calculations via umbrella sampling are used to quantify this process and show that both the migration to the cluster surface and the trapping at the vertices with trapping strength of multiple k_{B}T results from free energy minimization. Moreover, our study reveals that the crystallization pathway and dynamics of large spheres are consistent across different systems, suggesting robustness of entropic trapping.

Magic numbers in finite particle systems correspond to specific system sizes that allow configurations with low free energy, often exhibiting closed surface shells to maximize the number of nearest neighbors. Since their discovery in atomic nuclei, magic numbers have been essential for understanding the number-structure-property relationships in finite clusters across different scales. However, as the system size increases, the significance of magic numbers diminishes, and the precise system size at which magic number phenomena disappear remains uncertain. In this study, we investigate colloidal clusters formed through confined self-assembly. Small magic number clusters display icosahedral symmetry with closed surface shells, corresponding to pronounced free energy minima. Our findings reveal that beyond a critical system size, closed surface shells disappear, and free energy minima become less pronounced. Instead, we observe a distinct type of colloidal cluster, termed a football cluster, which retains icosahedral symmetry but features lower-coordinated facets disconnected by terraces. A sphere packing model demonstrates that forming closed surface shells becomes impossible beyond a critical system size, explaining the breakdown of magic numbers in large confined systems.

Bifurcations in kinetic pathways decide the evolution of a system. An example is crystallization, in which the thermodynamically stable polymorph may not form due to kinetic hindrance. Here, we use confined self-assembly to investigate the interplay of thermodynamics and kinetics in the crystallization pathways of finite clusters. We report the observation of decahedral clusters from colloidal particles in emulsion droplets and show that these decahedral clusters can be thermodynamically stable, just like icosahedral clusters. Our hard sphere simulations reveal how the development of the early nucleus shape passes through a bifurcation that decides the cluster symmetry. A geometric argument explains why decahedral clusters are kinetically hindered and why icosahedral clusters can be dominant even if they are not in the thermodynamic ground state.

Metal-organic frameworks (MOFs) are microporous adsorbents for high-throughput gas separation. Such materials exhibit distinct adsorption characteristics owing to the flexibility of the crystal framework in a nanoparticle, which can be different from its bulk crystal. However, for practical applications, such particles need to be compacted into macroscopic pellets, creating mass-transport limitations. In this work, this problem is addressed by forming materials with structural hierarchy, using a supraparticle-based approach. Spherical supraparticles composed of nanosized MOF particles are fabricated by emulsion templating and they are used as the structural component forming a macroscopic material. Zeolitic imidazolate framework-8 (ZIF-8) particles are used as a model system and the gas-adsorption kinetics of the hierarchical material are compared with conventional pellets without structural hierarchy. It is demonstrated that a pellet packed with supraparticles exhibits a 30 times faster adsorption rate compared to an unstructured ZIF-8 powder pellet. These results underline the importance of controlling structural hierarchy to maximize the performance of existing materials. In the hierarchical MOFs, large macropores between the supraparticles, smaller macropores between individual ZIF-8 primary particles, and micropores inherent to the ZIF-8 framework collude to combine large surface area, defined adsorption sites, and efficient mass transport to enhance performance.

Supraparticles are spherical colloidal crystals prepared by confined self-assembly processes. A particularly appealing property of these microscale structures is the structural color arising from interference of light with their building blocks. Here, we assemble supraparticles with high structural order that exhibit coloration from uniform, polyhedral metal-organic framework (MOF) particles. We analyse the structural coloration as a function of the size of these anisotropic building blocks and their internal structure. We attribute the angle-dependent coloration of the MOF supraparticles to the presence of ordered, onion-like layers at the outermost regions. Surprisingly, even though different shapes of the MOF particles have different propensities to form these onion layers, all supraparticle dispersions show well-visible macroscopic coloration, indicating that local ordering is sufficient to generate interference effects.

Single-Core Double Emulsions The fabrication of single-core double emulsions typically requires sophisticated microfluidic devices and meticulous operation protocols. In article number 2107338, Nicolas Vogel and co-workers develop a convenient method to produce such tailored double emulsions with defined droplet structure using only vortex mixing. This fast, low-cost and high-yield method improves the accessibility of droplet-based materials.

Journal Article Crystal Structure and Defect Analysis of Colloidal Supraparticles by Lab-Based X-ray Microscopy Get access Silvan Englisch, Silvan Englisch Institute of Micro- and Nanostructure Research (IMN) and Center for Nanoanalysis and Electron Microscopy (CENEM), IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Corresponding authors: silvan.englisch@fau.de, erdmann.spiecker@fau.de Search for other works by this author on: Oxford Academic Google Scholar Junwei Wang, Junwei Wang Institute of Particle Technology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Lukas J Roemling, Lukas J Roemling Institute of Particle Technology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Johannes Voß, Johannes Voß Institute of Micro- and Nanostructure Research (IMN) and Center for Nanoanalysis and Electron Microscopy (CENEM), IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Janis Wirth, Janis Wirth Institute of Micro- and Nanostructure Research (IMN) and Center for Nanoanalysis and Electron Microscopy (CENEM), IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Chrameh Fru Mbah, Chrameh Fru Mbah Institute for Multiscale Simulation, IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Benjamin Apeleo Zubiri, Benjamin Apeleo Zubiri Institute of Micro- and Nanostructure Research (IMN) and Center for Nanoanalysis and Electron Microscopy (CENEM), IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Michael Engel, Michael Engel Institute for Multiscale Simulation, IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Nicolas Vogel, Nicolas Vogel Institute of Particle Technology, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Erdmann Spiecker Erdmann Spiecker Institute of Micro- and Nanostructure Research (IMN) and Center for Nanoanalysis and Electron Microscopy (CENEM), IZNF, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany Corresponding authors: silvan.englisch@fau.de, erdmann.spiecker@fau.de Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 306–309, https://doi.org/10.1017/S1431927622002008 Published: 01 August 2022

Colloidal supraparticles are finite, spherical assemblies of many primary particles. To take advantage of their emergent functionalities, such supraparticles must retain their structural integrity. Here, we investigate their size-dependent mechanical properties via nanoindentation. We find that the deformation resistance inversely scales with the primary particle diameter, while the work of deformation is dependent on the supraparticle diameter. We adopt the Griffith theory to such particulate systems to provide a predictive scaling to relate the fracture stress to the geometry of supraparticles. The interplay between primary particle material and cohesive interparticle forces dictates the mechanical properties of supraparticles. We find that enhanced stability, associated with ductile fracture, can be achieved if supraparticles are engineered to dissipate more energy via deformation of primary particles than breaking of interparticle bonds. Our work provides a coherent framework to analyze, predict, and design the mechanical properties of colloidal supraparticles.

Double emulsions, such as water-in-oil-in-water droplets, are important material platforms for conducting fundamental research and for technological applications. To date, well-defined double-emulsion droplets consisting of a single water core and a thin oil shell can be exclusively formed with sophisticated microfluidic devices. The fabrication, preparation, and operation of such devices is challenging, which reduces the availability of tailored double emulsions to a limited community of experts. Here, a simple method is introduced to produce single-core double emulsions with high yield in large quantities, using a vortex mixer. Utilizing the density difference between the dispersed droplet and the continuous phase, this two-step emulsification method can achieve very small core droplet diameters below 10 μm and ultrathin shells with thicknesses below 1 μm. A detailed picture of the formation mechanism is provided and it is demonstrated that the process can be extended to produce multishell and multicore emulsions. Finally, its application is demonstrated to produce structurally colored colloidal supraparticles with unprecedented uniformity and yield. The method allows the creation of tailored double emulsions with minimal time, cost, effort, and expertise, and may widen its application to nonspecialized scientific communities.

Colloidal supraparticles are micron-scale spherical assemblies of uniform primary particles, which exhibit emergent properties of a colloidal crystal, yet exist as a dispersible powder. A prerequisite to utilize these emergent functionalities is that the supraparticles maintain their mechanical integrity upon the mechanical impacts that are likely to occur during processing. Understanding how the internal structure relates to the resultant mechanical properties of a supraparticle is therefore of general interest. Here, we take the example of supraparticles templated from water/fluorinated oil emulsions in droplet-based microfluidics and explore the effect of surfactants on their mechanical properties. Stable emulsions can be generated by nonionic block copolymers consisting of a hydrophilic and fluorophilic block and anionic fluorosurfactants widely available under the brand name Krytox. The supraparticles formed in the presence of both types of surfactants appear structurally similar, but differ greatly in their mechanical properties. While the nonionic surfactant induces superior mechanical stability and ductile fracture behavior, the anionic Krytox surfactant leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin light spectroscopy that is very sensitive to the interparticle contacts for subnanometer-thick adsorbed layers atop of the nanoparticle. While the anionic Krytox does not significantly affect the interparticle bonds, the amphiphilic nonionic surfactant drastically strengthens these bonds to the point that individual particle vibrations are not resolved in the experimental spectrum. Our results demonstrate that seemingly subtle changes in the physicochemical properties of supraparticles can drastically impact the resultant mechanical properties.

Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low‐cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepared by a confined self‐assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face‐centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addition, the rotational motion and dynamics of such micrometer‐scale clusters suspended in a liquid can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real‐time information about the crystallization pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.

The structure of finite self-assembling systems depends sensitively on the number of constituent building blocks. Recently, it was demonstrated that hard sphere-like colloidal particles show a magic number effect when confined in spherical emulsion droplets. Geometric construction rules permit a few dozen magic numbers that correspond to a discrete series of completely filled concentric icosahedral shells. Here, we investigate the free energy landscape of these colloidal clusters as a function of the number of their constituent building blocks for system sizes up to several thousand particles. We find that minima in the free energy landscape, arising from the presence of filled, concentric shells, are significantly broadened. In contrast to their atomic analogues, colloidal clusters in spherical confinement can flexibly accommodate excess colloids by ordering icosahedrally in the cluster center while changing the structure near the cluster surface. In-between these magic number regions, the building blocks cannot arrange into filled shells. Instead, we observe that defects accumulate in a single wedge and therefore only affect a few tetrahedral grains of the cluster. We predict the existence of this wedge by simulation and confirm its presence in experiment using electron tomography. The introduction of the wedge minimizes the free energy penalty by confining defects to small regions within the cluster. In addition, the remaining ordered tetrahedral grains can relax internal strain by breaking icosahedral symmetry. Our findings demonstrate how multiple defect mechanisms collude to form the complex free energy landscape of hard sphere-like colloidal clusters.

Journal Article Scale-Bridging 3D-Analysis of Colloidal Clusters Using 360° Electron Tomography and X-Ray Nano-CT Get access Silvan Englisch, Silvan Englisch Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander University Erlangen-Nürnberg, Germany Corresponding authors: silvan.englisch@fau.de, erdmann.spiecker@fau.de Search for other works by this author on: Oxford Academic Google Scholar Janis Wirth, Janis Wirth Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander University Erlangen-Nürnberg, Germany Search for other works by this author on: Oxford Academic Google Scholar Thomas Przybilla, Thomas Przybilla Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander University Erlangen-Nürnberg, Germany Search for other works by this author on: Oxford Academic Google Scholar Benjamin Apeleo Zubiri, Benjamin Apeleo Zubiri Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander University Erlangen-Nürnberg, Germany Search for other works by this author on: Oxford Academic Google Scholar Junwei Wang, Junwei Wang Institute of Particle Technology at the Department for Chemical and Biological Engineering at the Friedrich-Alexander University Erlangen-Nürnberg, Germany Search for other works by this author on: Oxford Academic Google Scholar Nicolas Vogel, Nicolas Vogel Institute of Particle Technology at the Department for Chemical and Biological Engineering at the Friedrich-Alexander University Erlangen-Nürnberg, Germany Search for other works by this author on: Oxford Academic Google Scholar Erdmann Spiecker Erdmann Spiecker Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander University Erlangen-Nürnberg, Germany Corresponding authors: silvan.englisch@fau.de, erdmann.spiecker@fau.de Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 392–393, https://doi.org/10.1017/S1431927619002691 Published: 01 August 2019

We investigate the formation of spherical supra particles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixture of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement determines the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temperatures. We control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supra particles from both silica- and calcium-containing CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, we found that the presence of calcium ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. We minimized these stability issues via the addition of a steric stabilizing agent and by mixing bioactive and silica colloidal particles. We investigated the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.

Clusters in systems as diverse as metal atoms, virus proteins, noble gases, and nucleons have properties that depend sensitively on the number of constituent particles. Certain numbers are termed 'magic' because they grant the system with closed shells and exceptional stability. To this point, magic number clusters have been exclusively found with attractive interactions as present between atoms. Here we show that magic number clusters exist in a confined soft matter system with negligible interactions. Colloidal particles in an emulsion droplet spontaneously organize into a series of clusters with precisely defined shell structures. Crucially, free energy calculations demonstrate that colloidal clusters with magic numbers possess higher thermodynamic stability than those off magic numbers. A complex kinetic pathway is responsible for the efficiency of this system in finding its minimum free energy configuration. Targeting similar magic number states is a strategy towards unique configurations in finite self-organizing systems across the scales.