Hierarchical self-assembly of 'hard-soft' Janus particles into colloidal molecules and larger supracolloidal structures.

Here we report the self-assembly of 'hard-soft' micron-sized Janus particles into clusters in aqueous media. The assembly process is induced by the desorption of a polymeric stabiliser from the particles, that is polyvinylpyrrolidone (PVP). Upon contact through collision and coalescence of the soft polymeric lobes, the newly formed clusters adopt a minimized surface area to volume ratio, thereby forming distinct microscopic supracolloidal analogues of simple molecular valance shell electron pair repulsion (VSEPR) space-fill structures. To explain this behaviour, the colloidal stability of our particle suspensions were studied with and without an adsorbed steric surfactant. Simulations of expected cluster morphology, compared with those from cryo-SEM analysis support the mechanism of assembly driven by surface area minimization in the case of soft-soft interactions. Altering the soft lobe size with respect to the hard lobe indicates a moderate effect on number of primary particles per cluster. Additionally, higher order structures of clusters containing a number of primary particles exceeding what is possible for a 'solid' core cluster are observed. As such, we also investigated the formation of suprastructures using a high number of 'hard-soft' Janus particles and verified their effective Pickering stabilization of air bubbles.