Surface patterning droplets: A new route to hierarchical self-assembly*

In this study, we present a strategy for controlling the spatial distribution of binary nanoparticle mixtures on a liquid droplet interface. Molecular dynamics simulations reveal that mixtures of dipolar and non-dipolar particles assemble into dynamically frustrated amorphous structures at the interface. A time-dependent magnetic field is used to overcome kinetic barriers, forming configurations resembling the thermodynamic ground state. These features are encapsulated in a dynamic phase diagram with field strength (relative to dipole strength) and frequency (relative to nanoparticle relaxation time) acting as state-controlling variables. The orienting field biases dipolar particles near the equator while non-dipolar particles are localized at the poles which are along the axis of the field. This patterning is reversible, as changing field direction will redirect the poles on the droplet to align with the field. Finally, we discuss droplet ferromagnetic properties determined by micro-magnetic simulations.