Assembly of filament-like supraparticles in confined vortex rings.

Assembling nanoscale building blocks into larger supraparticle constructs has emerged as a promising avenue to create highly functional particle systems for numerous applications. In the vortex ring freezing method, toroidal supraparticles can be fabricated through the consolidation of colloidal particles in a vortical flow field generated by an impinging droplet in a reservoir of coagulant. Interactions between a vortex ring with its surroundings have been studied systematically in the past decades but a similar perspective is lacking in the application of vortex rings to material assembly. Herein, we show that imposing a quasi-two-dimensional vortex confinement, by limiting the vertical dimension of the reservoir, gives rise to a rapid, radially expanding vortex ring that enables the preparation of millimeter-sized supraparticle filaments templated by the vortex tube geometry. We employ superparamagnetic iron oxide nanoparticles (SPIONs) dispersed in chloroform as model building blocks that are introduced into a shallow reservoir with inclined boundaries containing an ethanol-water binary mixture functioning as the coagulant. We show that controlling the droplet SPION concentration, reservoir water content, and the degree of confinement (reservoir volume), can be used to tune the filament assembly process and the ensuing filament morphology. Our results highlight the importance of employing interactions between vortex rings with their surroundings and expand the potential of vortex-based colloidal assembly methods to harness the complex non-equilibrium dynamics of multiphase systems toward the creation of macroscale filamentous nanostructured supraparticle assemblies.