Out-of-equilibrium molecular systems hold great promise as dynamic, reconfigurable matter that executes complex tasks autonomously. However, translating molecular scale dynamics into spatiotemporally controlled phenomena emerging at mesoscopic scale remains a challenge—especially if one aims at a design where the system itself maintains gradients that are required to establish spatial differentiation. Here, we demonstrate how surface tension gradients, facilitated by a linear amphiphile molecule, generate Marangoni flows that coordinate the positioning of amphiphile source and drain droplets floating at air-water interfaces. Importantly, at the same time, this amphiphile leads, via buckling instabilities in lamellar systems of said amphiphile, to the assembly of millimeter long filaments that grow from the source droplets and get absorbed at the drain droplets. Thereby, the Marangoni flows and filament organization together sustain the autonomous positioning of interconnected droplet-filament networks at the mesoscale. Our concepts provide potential for the development of non-equilibrium matter with spatiotemporal programmability.

Self-organization of meso- and macroscale structures is a highly active research field that exploits a wide variety of physicochemical phenomena, including surface tension, Marangoni flow, and (elasto)capillary effects. The release of surface-active compounds generates Marangoni flows that cause repulsion, whereas capillary forces attract floating particles via the Cheerios effect. Typically, the interactions resulting from these effects are nonselective because the gradients involved are uniform. In this work, we unravel the mechanisms involved in the self-organization of amphiphile filaments that connect and attract droplets floating at the air–water interface, and we demonstrate their potential for directional gradient formation and thereby selective interaction. We simulate Marangoni flow patterns resulting from the release and depletion of amphiphile molecules by source and drain droplets, respectively, and we predict that these flow patterns direct the growth of filaments from the source droplets toward specific drain droplets, based on their amphiphile depletion rate. The interaction between such droplets is then investigated experimentally by charting the flow patterns in their surroundings, while the role of filaments in source–drain attraction is studied using microscopy. Based on these observations, we attribute attraction of drain droplets and even solid objects toward the source to elastocapillary effects. Finally, the insights from our simulations and experiments are combined to construct a droplet-based system in which the composition of drain droplets regulates their ability to attract filaments and as a consequence be attracted toward the source. 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.

We highlight four different concepts that can be used as a design principe to establish self-organization using chemical reactions as a driving force to sustain gradients: reaction–diffusion, reaction–convection, Marangoni flow and diffusiophoresis.

Dissipative chemical systems hold the potential to enable life‐like behavior in synthetic matter, such as self‐organization, motility, and dynamic switching between different states. Here, out‐of‐equilibrium self‐organization is demonstrated by interconnected source and drain droplets at an air‐water interface, which display dynamic behavior due to a hydrolysis reaction that generates a concentration gradient around the drain droplets. This concentration gradient interferes with the adhesion of self‐assembled amphiphile filaments that grow from a source droplet. The chemical gradient sustains a unique orbiting of the drain droplet, which is proposed to be driven by the selective adhesion of the filaments to the front of the moving droplet, while filaments approaching from behind are destabilized upon contact with the hydrolysis product in the trail of the droplet. Potential applications are foreseen in the transfer of chemical signals amongst communicating droplets in rearranging networks, and the implementation of chemical reactions to drive complex positioning routines in life‐like systems.

<p>Out-of-equilibrium molecular systems hold great promise as dynamic, reconfigurable matter that executes complex tasks autonomously. However, translating molecular scale dynamics into spatiotemporally controlled phenomena at mesoscopic length scales remains a challenge. In living cells, reliable positioning processes such as the centering of the centrosome involve forces that result from dissipative self-assembly. We demonstrate how spatiotemporal positioning emerges in synthetic systems where self-assembly is coupled to molecular fluxes originating from concentration gradients. At the core of our system are millimeter long self-assembled filaments and Marangoni flows induced by non-uniform amphiphile distributions. We demonstrate how repulsive and attractive forces that are generated as filaments organize between source and drain droplets sustain autonomous positioning of dynamic assemblies at the mesoscale. Our concepts provide a new paradigm for the development of non-equilibrium matter with spatiotemporal programmability. </p>

Dissipative chemical systems operate outside of equilibrium, and hold potential to enable life-like behavior in synthetic matter, such as self-organization, motility, and dynamic switching between different states. Here, out-of-equilibrium self-organization is demonstrated at an air-water interface, enabled by amphiphile filaments that self-assemble from source droplets and tether to pivalic anhydride-based drain droplets, which are surrounded by a pivalic acid gradient due to their hydrolysis. The coupling of chemical gradients, self-assembly and Marangoni flow due to release and depletion of amphiphiles at the air-water interface generates a unique orbiting of drain droplets around the source droplet. This orbiting is proposed to be driven by the selective adhesion of filaments to the front of the moving drain, while filaments approaching the drain from behind are destabilized upon contact with the asymmetrical gradient of pivalic acid. The motion sustains itself to complete multiple rotations, ending when the depletion of amphiphiles at the drain, which drives the Marangoni flow towards the drain, becomes too weak to attract new filaments. Potential applications are foreseen in rearranging networks for dynamic transfer of chemical signals amongst interconnected droplets, and the implementation of dissipative chemical reactions in self-organizing systems as a strategy towards life-like behavior is highlighted.

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.

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.