Hypothesis: Nanoparticle surfactants (‘NPS’) assemble at the oil–water interface to form solid-like films. Aqueous droplets placed on these solid-like oil–water interfaces are expected to be stable against coalescence with the underlying water phase. These droplets will deform the solid-like interface at which they are placed, leading to capillary forces and the assembly of large, multi-droplet structures.

Experiments: Aqueous droplets were placed on a solid-like film of cellulose nanocrystals surfactants (‘CNCS’) assembled at an oil–water interface. Droplet dynamics were quantified using single-particle tracking. A custom-made droplet printer was used to control initial droplet positions to guide droplet assembly into large structures. The composition of both the droplets and the NPS assembly was modified to produce heterogeneous droplet structures and light-responsive oil–water interfaces.

Findings: The droplets could be placed at the solid-like oil–water interface for extended periods of time. Microlitre-sized droplets attracted each other over millimetric scales. System dynamics differed from theoretical predictions for pristine interfaces and were captured by a simple model. This inter-droplet capillary attraction facilitated the printing of self-building droplet structures. Embedding gold nanoparticles in the NPS assembly allowed us to generate local temperature gradients by illuminating the system with a laser and manipulate the droplets via plasmon-assisted optofluidics.