Marangoni and elastocapillary effects are well-known as driving forces in the self-organization of floating objects at air-water interfaces. The release of surface active compounds generates Marangoni flows that cause repulsion, whereas capillary forces drive attraction. Typically, these interactions are non-directional and mechanisms to establish directional connections between the self-organizing elements are lacking. In this work, we unravel the mechanisms involved in the self-organization of a linear amphiphile into millimeter-long filaments that form connections between floating droplets. First, we show how the release of the amphiphile tetra(ethylene glycol) monododecyl ether from a floating source droplet onto the air-water interface generates a Marangoni flow. This flow extrudes self-assembled amphiphile filaments which grow from the source droplet, and concomitantly repels floating droplets in the surroundings. A hydrophobic drain droplet that depletes the amphiphiles from the air-water interface directs the Marangoni flow and thereby the growing filaments. We show how these filaments, upon tethering to the drain, potentially facilitate internal Marangoni convection and elastocapillary effects, which attract the drain back towards the source droplet. Furthermore, this concept establishes connections that are selective to the composition of the drain droplets – which influences the rate at which they deplete the amphiphile – such that repulsive and attractive forces can be balanced. Thereby, we provide a novel method through which directional attraction can be established in synthetic self-organizing systems, and advance our understanding of how complexity arises from simple building blocks.
From Two-Dimensional to Three-Dimensional at the Air/Water Interface: The Self-Aggregation of the Acridine Dye in Mixed Monolayers
