Faceted liquid droplets: when colloids are attracted by topological defects

Particles at disordered droplet interfaces were extensively investigated, aiming both at their fundamental physics and at their applications in particle-stabilized pharmaceuticals and aerosols. Yet, particles residing at the ubiquitous ordered interfaces have never been studied. We study the dynamics of tracer colloids, incorporated into a curved 2nm-thick crystal, forming at T=Ts 26 oC at the interface of liquid oil-in-water emulsion droplets. We demonstrate the particles to be spontaneously dragged to particular surface locations, identified with topological defects within the crystalline structure. At T=Td < Ts, the droplets undergo an unprecedented sphere-to-icosahedron shape transformation, with their bulk remaining liquid. At Td, the attractors self-position onto the vertices of the icosahedra and fix there the colloids' positions. At an even lower temperature, the particles are spontaneously expelled from the droplets. These phenomena allow functional liquid ``atoms'' to be designed, with their ``valency'' fixed by precise temperature-tuned positioning of the interfacial ligands, en route to supra-``atomic'' nano-structures. Our observations also impact upon the understanding of protein positioning on cell membranes, controlling essential biological functions.