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 (C12E3) 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 C12E3 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.
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IJS, KILJ, NUK, PNG, UL, UM
Living organisms employ chemical self-organization to build structures, and inspire new strategies to design synthetic systems that spontaneously take a particular form,
via
a combination of ...integrated chemical reactions, assembly pathways and physicochemical processes. However, spatial programmability that is required to direct such self-organization is a challenge to control. Thermodynamic equilibrium typically brings about a homogeneous solution, or equilibrium structures such as supramolecular complexes and crystals. This perspective addresses out-of-equilibrium gradients that can be driven by coupling chemical reaction, diffusion and hydrodynamics, and provide spatial differentiation in the self-organization of molecular, ionic or colloidal building blocks in solution. These physicochemical gradients are required to (1) direct the organization from the starting conditions (
e.g.
a homogeneous solution), and (2) sustain the organization, to prevent it from decaying towards thermodynamic equilibrium. We highlight four different concepts that can be used as a design principle to establish such self-organization, using chemical reactions as a driving force to sustain the gradient and, ultimately, program the characteristics of the gradient: (1) reaction-diffusion coupling; (2) reactionconvection; (3) the Marangoni effect and (4) diffusiophoresis. Furthermore, we outline their potential as attractive pathways to translate chemical reactions and molecular/colloidal assembly into organization of patterns in solution, (dynamic) self-assembled architectures and collectively moving swarms at the micro-, meso- and macroscale, exemplified by recent demonstrations in the literature.
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: reactiondiffusion, reaction-convection, Marangoni flow and diffusiophoresis.
The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together ...to produce a contraction. In spite of this progress, harnessing cooperative molecular motion remains a challenge in this field. Here, we show how the light-induced action of artificial molecular switches modifies not only the shape but also, simultaneously, the stiffness of soft materials. The heterogeneous design of these materials features inclusions of free liquid crystal in a liquid crystal polymer network. When the magnitude of the intrinsic interfacial tension is modified by the action of the switches, photo-stiffening is observed, in analogy with the mechanical response of activated muscle fibers, and in contrast to melting mechanisms reported so far. Mechanoadaptive materials that are capable of active tuning of rigidity will likely contribute to a bottom-up approach towards human-friendly and soft robotics.
Developing shape-shifting materials requires combining the flexibility needed by shape-shifting properties, with the toughness that is demanded to maintain their mechanical performance. Typically, in ...liquid crystal networks, the amplitude of the shape transformation can be hindered by large cross-linking densities. Here, we argue that a promising strategy to address this limitation consists in integrating liquid crystal networks into an anisotropic and porous material that acts as an orienting scaffold. This strategy shows similarities with the principles of stimuli-responsive deformation in plants, where inflexible elements with specific orientations are integrated into a stimuli-responsive matrix. By aligning liquid crystals in a porous polypropylene orienting scaffold, we demonstrate liquid crystal networks that respond to humidity with a shape change, yet they display high elastic modulus and toughness. Various chiral shapes can be generated in single and double layers of these films, and the complexity of their actuation modes is enhanced, including twisting, curling or winding. We anticipate that these hybrid composites and the strategy they embody can find application to other stimuli-responsive anisotropic soft materials.
Out‐of‐equilibrium chemical systems, comprising reaction networks and molecular self‐assembly pathways, rely on the delivery of reagents. Rather than via external flow, diffusion or convection, we ...aim at self‐sustained reagent delivery. Therefore, we explore how the coupling of Marangoni flow with chemical reactions can generate self‐sustained flows, driven by said chemical reactions, and – in turn – sustained by the delivery of reagents for this reaction. We combine a photoacid generator with a pH‐responsive surfactant, such that local UV exposure decreases the pH, increases the surface tension, and triggers the emergence of a Marangoni flow. We study the impact of reagent concentrations and identify threshold conditions at which flow can emerge. Surprisingly, we unraveled an antagonistic influence of the reagents on key features of the flow such as velocity and duration, and rationalize these findings via a kinetic model. Our study displays the potential of reaction‐driven flow to establish autonomous control in fuel delivery of out‐of‐equilibrium systems.
Marangoni flows driven by chemical reactions can sustain themselves via delivery of reagents at the reaction site. Here, it is demonstrated how a flow generated through reactions between a photoacid generator and a surfactant can be maintained for more than 20 minutes at the air‐water interface. The antagonistic influence of the reagents on the system was explored experimentally and theoretically, and its potential as feedback mechanism was shown.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract Quorum sensing enables unicellular organisms to probe their population density and perform behavior that exclusively occurs above a critical density. Quorum sensing is established in ...emulsion droplet swarms that float at a water surface and cluster above a critical density. The design involves competition between 1) a surface tension gradient that is generated upon release of a surfactant from the oil droplets, and thereby drives their mutual repulsion, and 2) the release of a surfactant precursor from the droplets, that forms a strong imine surfactant which suppresses the surface tension gradient and thereby causes droplet clustering upon capillary (Cheerios) attraction. The production of the imine‐surfactant depends on the population density of the droplets releasing the precursor so that the clustering only occurs above a critical population density. The pH‐dependence of the imine‐surfactant formation is exploited to trigger quorum sensing upon a base stimulus: dynamic droplet swarms are generated that cluster and spread upon spatiotemporally varying acid and base conditions. Next, the clustering of two droplet subpopulations is coupled to a chemical reaction that generates a fluorescent signal. It is foreseen that quorum sensing enables control mechanisms in droplet‐based systems that display collective responses in contexts of, e.g., sensing, optics, or dynamically controlled droplet‐reactors.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The front cover artwork is provided by Anne‐Déborah Nguindjel and Peter A. Korevaar, Radboud University. The image represents the concept of self‐sustained Marangoni flows. These flows bring the fuel ...to the reaction center, to keep the flow “alive” in its out‐of‐equilibrium state.. Read the full text of the Article at 10.1002/syst.202100021.
“The coupling between a set of chemical reactions and a physical phenomenon like the Marangoni effect can lead to an emerging out‐of‐equilibrium system. The surface tension of the air–water interface was manipulated by using photochemistry to create a Marangoni flow that keeps itself ‘alive’ by delivering reagents to the reaction site…” This and more about the story behind the front cover can be found in the Article at 10.1002/syst.202100021.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Front Cover represents an artist impression of self‐sustained Marangoni flows. Conversion of the surfactants results in surface tension gradients that drive the Marangoni flow (blue waves) ...towards the site of UV illumination (yellow) – such that in turn new reagents are delivered which keep the flow “alive”. The spheres in the background represent the oleic acid droplets that are accumulated at the site of illumination. The work points out how gradients can be sustained by combining chemical reactions to a physicochemical mechanism that translates a chemical gradient into a flux of reagents – which in turn drive the chemical reaction. Cover design by Anne‐Déborah C. Nguindjel. More information can be found in the Article by Anne‐Déborah C. Nguindjel and Peter A. Korevaar.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Developing shape-shifting materials requires combining the flexibility needed by shape-shifting properties, with the toughness that is demanded to maintain their mechanical performance. Typically, in ...liquid crystal networks, the amplitude of the shape transformation can be hindered by large cross-linking densities. Here, we argue that a promising strategy to address this limitation consists in integrating liquid crystal networks into an anisotropic and porous material that acts as an orienting scaffold. This strategy shows similarities with the principles of stimuli-responsive deformation in plants, where inflexible elements with specific orientations are integrated into a stimuli-responsive matrix. By aligning liquid crystals in a porous polypropylene orienting scaffold, we demonstrate liquid crystal networks that respond to humidity with a shape change, yet they display high elastic modulus and toughness. Various chiral shapes can be generated in single and double layers of these films, and the complexity of their actuation modes is enhanced, including twisting, curling or winding. We anticipate that these hybrid composites and the strategy they embody can find application to other stimuli-responsive anisotropic soft materials.
Integrating liquid crystal networks in an anisotropic porous scaffold allows enhancing their mechanical performance and promotes their alignment. By combining this approach with humidity-responsive liquid crystals and gradients in cross-linking density, we demonstrate soft yet tough actuators that curl, wind or twist in response to humidity.
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