  Self-assembly of nanoparticles at fluid-fluid interfaces is a promising route
to fabricate functional materials from the bottom-up. However, directing and
controlling particles into highly tunable and predictable structures -- while
essential -- is a challenge. We present a liquid interface assisted approach to
fabricate nanoparticle structures with tunable properties. To demonstrate its
feasibility, we study magnetic Janus particles adsorbed at the interface of a
spherical droplet placed on a substrate. With an external magnetic field turned
on, a single particle moves to the location where its position vector relative
to the droplet centre is parallel to the direction of the applied field.
Multiple magnetic Janus particles arrange into reconfigurable hexagonal lattice
structures and can be directed to assemble at desirable locations on the
droplet interface by simply varying the magnetic field direction. We develop an
interface energy model to explain our observations, finding excellent
agreement. Finally, we demonstrate that the external magnetic field allows
\revisedtext{one} to tune the particle deposition pattern obtained when the
droplet evaporates. Our results have implications for the fabrication of varied
nanostructures on substrates for use in nanodevices, organic electronics, or
advanced display, printing and coating applications.
