Reconfigurable Droplet–Droplet Communication Mediated by Photochemical Marangoni Flows

Abstract

Droplets are attractive building blocks for dynamic matter that organizes into adaptive structures. Communication among collectively operating droplets opens untapped potential in settings that vary from sensing, optics, protocells, computing, or adaptive matter. Inspired by the transmission of signals among decentralized units in slime mold Physarum polycephalum, we introduce a combination of surfactants, self-assembly, and photochemistry to establish chemical signal transfer among droplets. To connect droplets that float at an air−water interface, surfactant triethylene glycol monododecylether (C 12 E 3 ) is used for its ability to self-assemble into wires called myelins. We show how the trajectory of these myelins can be directed toward selected photoactive droplets upon UV exposure. To this end, we developed a strategy for photocontrolled Marangoni flow, which comprises (1) the liquid crystalline coating formed at the surface of an oleic acid/sodium oleate (OA/NaO) droplet when in contact with water, (2) a photoacid generator that protonates sodium oleate upon UV exposure and therefore disintegrates the coating, and (3) the surface tension gradient that is generated upon depletion of the surfactant from the air−water interface by the uncoated droplet. Therefore, localized UV exposure of selected OA/ NaO droplets results in attraction of the myelins such that they establish reconfigurable connections that self-organize among the C 12 E 3 and OA/NaO droplets. As an example of communication, we demonstrate how the myelins transfer fluorescent dyes, which are selectively delivered in the droplet interior upon photochemical regulation of the liquid crystalline coating.

Myelin Surfactant Assemblies as Dynamic Pathways Guiding the Growth of Electrodeposited Copper Dendrites

“…Next, the release of C 12 E 3 from the droplet and filaments to the air–water interface and its desorption from the interface to the bulk solution sustain the outbound Marangoni flow (Figure b). As shown in Figure c,d, myelins grow from the source droplet–comparable to results obtained with C 12 E 3 -based droplets we reported earlier. ,, Furthermore, after deposition, the CuCl 2 /C 12 E 3 droplets show a gradual color change from dark yellow-green to clear blue, starting from the boundary of the droplet and penetrating the core over a time course of 2 min (Figure e and Movie S1). As copper­(II) solutions in water are typically blue, even at high concentration, this color change is indicative of the hydration of copper­(II) ions upon water intake among the amphiphile bilayers that is inherent to the myelin formation.…”

“…To this end, we employ myelins assembled from the amphiphile tri­(ethylene glycol) monododecyl ether (C 12 E 3 ), loaded with a copper­(II) salt [copper­(II)­chloride, CuCl 2 ]. Earlier, we demonstrated how C 12 E 3 forms a lamellar phase of closely packed bilayers at the boundary of a C 12 E 3 droplet when deposited at an air–water interface. , Swelling of the bilayer phase upon uptake of water drives the growth of multilamellar myelin filaments from the droplet, , which spread over the air–water interface. The concomitant release of individual C 12 E 3 amphiphiles as a surfactant to the air–water interface generates outbound Marangoni flows which extrude myelins from the C 12 E 3 droplet .…”

Myelin surfactant assemblies shaping the electrodeposition of copper dendrites.

“…We test the filament growth by depositing CuCl 2 /C 12 E 3 "source" droplets (1 μL) on water (5.5 mL, 38 mm diameter Petri dish) with traces of C 12 E 3 at the interface (Figure 2a-b). As shown in Figure 2c-d, myelins grow from the source droplet -comparable to results obtained with C 12 E 3 -based droplets we reported earlier 47,50,51 . Furthermore, after deposition, the CuCl 2 /C 12 E 3 droplets show a gradual colour change from dark yellow-green to clear blue, starting from the boundary of the droplet and penetrating to the core over a time course of 2 min (Figure 2e, Movie 1).…”

“…To this end, we employ myelins assembled from the amphiphile triethyleneglycol monododecyl ether (C 12 E 3 ), loaded with Cu 2+ . Earlier, we demonstrated how C 12 E 3 forms a lamellar phase of closely packed bilayers at the boundary of a C 12 E 3 droplet when deposited at an air-water interface 46,47 . Swelling of the bilayer phase upon uptake of water drives the growth of multilamellar myelin filaments from the droplet 48,49 , which spread over the air-water interface.…”