Droplet-based assembly of magnetic superballs

The self-assembly of materials driven by the inherent directionality of the constituent particles is of both practical and fundamental interest because it enables the fabrication of complex and hierarchical structures with tailored functionalities. By employing evaporation assisted self-assembly, we form opal-like structures with micro-sized magnetic superball particles. We study the structure formation of different superball shapes during evaporation of a dispersion droplet with in-situ small angle x-ray scattering with microradian resolution in the absence and presence of an external magnetic field. In the absence of a magnetic field, strong shape-dependent structures form as the water evaporates from the system. Applying a magnetic field to the droplet has a unique effect on the system; strong magnetic fields inhibit the growth of well-ordered assemblies due to the formation of out-of-equilibrium dipolar structures while lower magnetic fields allow particles to rearrange and orient without inhibition. In this work, we show how the superball assembly inside a droplet can be controlled by the magnetic field strength and the superball shape. The tunability of these parameters not only enables the controllable formation of macroscopic colloidal assemblies but also opens up possibilities for the development of functional materials with tailored properties on a macro-scale.