Self-Organized Colloidal Assemblies in Confined Spaces: Formation Mechanism, Internal Structure and Resulting Optical Properties

The spontaneous organization of individual building blocks into ordered structures is extensively used in nature and found at all length scales, from crystallization processes, via composite materials, to living cells constituting complex tissue. Understanding the relationship between building blocks, environmental conditions, and resulting structure is of fundamental importance for controlling materials properties. Here we propose a joint experimental-theoretical investigation of the self-organization of spherical polymer colloidal particles in confined spaces. Such particles can easily be synthesized with high precision and serve as simple, nanoscale building blocks to study structure-property relationships in self-assembly processes. If sufficiently uniform in size, these particles self-assemble in a close-packed face-centered cubic lattice. These so-called colloidal crystals exhibit intense structural colors.Confining elements imposed upon the self-organizing particles can significantly alter the assembly process and may lead to entirely different colloidal crystals. Especially interesting confinements are emulsion droplets that prevent the formation of periodic structures by introducing boundaries and curvature. In preliminary experiments, we observed spherical assemblies with astonishing structural precision and geometry that enable us to investigate the effect of confinement on the resulting structure in unprecedented details. To date, the only model available to describe the assembly in spherical confinement relies on entropy maximization of non-interacting spheres with hard boundaries. While our observed structures follow the general trends predicted by this model, details of the formed structures indicate a richer phase behavior than expected. We anticipate that the kinetics of the assembly as well as the softness and deformability of the interface play a crucial role in the assembly process as well. Within this proposal we aim to establish a coherent model for the mechanism of confined self-assembly, to gain predictive power over the great many structural details arising from the self-organization of colloidal particles under external confinements and to reliably produce such particles in a uniform fashion using droplet-based microfluidics. Towards this aim, we will combine experimental, theoretical, and computational efforts to correlate experimentally observed assembly structures with particle simulations and free energy-minimization computations based on density and entropy as well as surface tension, deformability of the interface and its interaction with the colloidal particles. Finally, we will investigate the resulting optical properties of the assembled superstructures as an example of a functional property that can be tailored via the internal structure of the self-organized spherical colloidal assemblies.