Confined Self-Assembly

In bulk, monodisperse colloidal particles crystallize into close-packed structures upon reaching a critical packing fraction. However, when confined in a finite space, the confinement geometry, as well as the number of particles, gives rise to complex crystallization behavior and structures. We experimentally investigate the assembly behavior of sub-micron colloidal particles in the spherical emulsion droplets produced by microfluidics. We demonstrated that such confined self-assembly systems follow a magic number effect and prefer closed-shell structure, in analogy to the formation of atomic clusters or atom nuclei. With the right number of particles in the droplet, the entropy-driven process yields diverse icosahedral structures with precision and suprising structural complexity. Otherwise, defect particles accumulate in a defined wedge region instead of distributing homogeneously to the cluster. This wedge reduces the free energy penalty associated with inevitable structural disorder. We work in close collaboration with the groups of Prof. Michael Engel and Prof. Erdman Spiecker to analyse the 3D structure with advanced microscopy methods (Spiecker group) and to study the kinetics and thermodynamics properties of these confined colloidal clusters in simulation (Engel group).

Colloidal clusters are discrete crystalline assembly of colloidal particles. When the sizes of constituent building blocks match the wavelength of visible light, the periodic arrangement of contrasting refractive index medium give rise to vivid structural color. Depending on the crystallinity, symmetry, orientation and size of the colloidal clusters, their optical properties can be designed, including anisotropy, color, intensity and patterns. Structural color is not only visually pleasing but can also be used to elucidate details on the formation pathway and resulting structures of the confined self-assembly process and to monitor movement and rotation of individual supraparticles in the micrometer length scale.

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Particulate products with well-defined properties are of key importance to the chemical, pharma-ceutical and food industry. Supraparticles are particulate entities in the micrometre scale that are produced from defined colloidal nanoparticles as starting material. We are studying the self-assembly of those colloidal building blocks into supraparticles with defined properties within an evaporating dispersion droplet in a spray drying process. During the evaporation of the dispersion medium the concentration of the colloidal nanoparticles will increase continuously, until they come into direct contact with each other, overcome their repulsive interactions and form a solid supraparticle. The formation of such supraparticles can be controlled by the starting material as well as via the external process conditions. This offers various degrees of freedom for the design and resulting properties of supraparticle powders. We investigate the formation of polymeric supraparticles and optimize their product properties like their particle shape, particle size distribution and powder flowability. Furthermore, we study the formation of hierarchically structured composite supraparticles resulting from the choice of polymeric and additive primary particles. This supraparticle approach enables the design of complex composite powder materials with a spatially controlled distribution of the additive material for the application in the fields of chemical, healthcare, or pharmaceutical applications.