Photo-Activated Merging Cascades in Droplet Swarms Upon Synergistic Solvation of Surfactants.

Chemically active droplets at fluid interfaces provide versatile building blocks for dynamic matter, yet most systems are limited to behavior encoded within individual droplets or simple pairwise interactions. Here, we report a strategy to generate non-linear, cascading behavior in droplet swarms by combining photo-activation with synergistic solvation of surfactants. At an air/water interface coated with a crystalline layer formed by a decylammonium/carbamate ion pair, mesitylene-based droplets containing a photoacid generator remain immotile until UV irradiation locally destabilizes the layer, which induces droplet motion via capillary effects. While both UV-responsive mesitylene droplets and tri(ethylene glycol) monododecyl ether (C12E3) droplets are individually immobilized within the layer, their fusion produces hybrid droplets that disintegrate the layer through synergistic solvation and exhibit enhanced motility that persists after UV irradiation is stopped. This non-linear activation enables cascades of chasing and merging events that propagate through large droplet fields well beyond the initial space-time window of photo-activation. By introducing repellent boundary structures, these cascades can be spatially confined, allowing control over collective swarm dynamics. Together, our results demonstrate how synergistic solvation allows to program emergent, multistep behavior in active droplet swarms, opening avenues toward droplet-based logics in chemically driven self-organization.