Reconfigurable droplet-droplet communication mediated by photo-chemical Marangoni flows.

Communication amongst droplets via reconfigurable connections enables next generations of “smart” matter. In analogy to the self-organization of slime mold Physarum polycephalum, exchange of chemical signals through a network of wires allows decentralized units to collectively emerge into functional structures. Here, we present a combination of surfactants, self-assembly and photochemistry to establish chemical signal transfer amongst droplets. Surfactant C12E3 is used for its ability to spontaneously generate wire-like structures called myelins when deposited at the air/water interface. Next, we demonstrate photo-controlled drain droplets that deplete surfactants from the air-water interface and thereby generate surface tension gradients that drive Marangoni flows, which in turn attract the myelins towards the drain. At the core of our design is a liquid crystalline coating that is formed at the drain droplet surface by oleic acid (OA) and sodium oleate (NaO) when placed on water. This coating inhibits the depletion of surfactant by the droplet. UV exposure activates photo-acid generator 2-nitrobenzaldehyde in the droplet, which protonates NaO, disintegrates the coating, and increases the Marangoni flow. Thereby, localized UV exposure of selected OA/NaO droplets results in attraction of the myelins, such that they establish connections amongst the droplets. Varying the photo-acid generator concentration, NaO content and UV exposure allows for re-configurability of these connections. As an example of communication, we demonstrate how the myelins enable a photo-controlled transfer of fluorescent dyes, which are selectively delivered in the droplet interior upon photo-chemical regulation of the liquid crystalline coating.