  Capillary forces guide the motion of biomolecular condensates, water-borne
insects, and breakfast cereal. These surface-mediated interactions can be
harnessed to build units into materials with exotic properties deriving from
mesoscale structure. Droplets are promising building blocks for these
materials, finding applications in tissue engineering, adaptive optics, and
structural colour. However, the instability of water droplets at many
liquid-liquid interfaces hampers the use of capillarity for the assembly of
droplet-based materials. Here, we use nanoparticle surfactants to form
solid-like oil-water interfaces at which aqueous droplets sit for extended
periods. We find that microlitre-sized droplets at these interfaces attract
each other over millimetric scales. We rationalize this interaction with a
modified theory of capillarity. Applying printing methods allows us to finely
control initial droplet positions, from which they self-assemble into cellular
materials. Finally, by functionalising the interface with gold nanoparticles,
we use plasmon-assisted optofluidics to manipulate these droplet-based
materials with temperature gradients.
