Anisotropic self-assembly structures from isotropic building blocks

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Spherical particles form structures with hexagonal packing at all scales. The densest possible packing of spheres in three dimensions is formed by stacking hexagonally close packed arrangements of spheres in three dimensions. This intuitive structure translates all scales and is observed from the crystal structure of many atomic crystals to the macroscopic ordering of spherical objects. Self-assembly of spherical building blocks in two dimensions, for example at the surface of a water body, similarly results in hexagonally close-packed structures. In the first funding period, we have demonstrated that spherical colloidal particles can self-assemble into highly unexpected structures, for example anisotropic chains or phases with square symmetry. We have elaborated that these structures form at the air/water interface in the presence of amphiphilic additives, provided the following criteria are fulfilled. First, the amphiphiles need to adsorb to the interface irreversibly; second, they must not phase separate but mix with the colloidal particles; and third, they must be compressible. The essential finding of the first funding period was that these criteria lead to the formation of a two-dimensional, compressible shell around the particles at the interface. Upon compression on a Langmuir trough, these shells induce a repulsive component to the interaction potential of the particles. This repulsive component, in turn, causes the formation of the unconventional phases as minimum energy structures. Such phases were theoretically predicted by Jagla using particles interacting via a square-shoulder repulsion potential two decades ago and were for the first time experimentally realized in this project. Depending on the shape of the potential, theoreticians have predicted a wide range of complex phases, including different quasicrystalline structures. The approach developed in the first funding period provides a general strategy to achieve the required interaction potentials. However, the two-component mixtures prevented an accurate engineering of the interaction potential, required to experimentally access the entire range of complex assemblies predicted theoretically. In the second funding period, we will use the established knowledge to design one-component core-shell particles with controllable interfacial interaction potentials to experimentally realize the broad range of assembly structures predicted from Jagla-type interactions. The advantage of a one-component system is the potential to accurately engineer the interaction potential via the size and compressibility of the shell. With these tailored particle systems, we will deepen our understanding of the interfacial properties of particles in general, demonstrate that we can engineer their interaction potential, and use this possibility to generate self-assembled structures with unprecedented structural versatility, for example for surface nanostructuration using colloidal lithography.