Dancing performance of organic droplets in aqueous surfactant solutions

Quorum Sensing in Emulsion Droplet Swarms Driven by a Surfactant Competition System

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.

Reconfigurable Droplet-Droplet Communication Mediated by Photochemical Marangoni Flows

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.

Sequentially Selective Coalescence of Binary Self-Propelled Droplets upon Collective Motion

Subsequent synthesis and detection using droplets as microreactors have shown promise in the development of novel materials and drugs because microreactors enable small-scale synthesis and detection of covalent/non-covalent intermolecular interactions. Self-organization exhibited by autonomous droplets under non-equilibrium conditions is beneficial for manipulating the sequentiality and selectivity of droplet coalescence because expensive equipment or elaborate techniques are not required with the autonomy of droplets. However, to our knowledge, selective coalescence caused by the collective motion of self-propelled droplets has not been demonstrated in inanimate systems. Here, we report sequentially selective coalescence based on the dynamic collective pattern of self-propelled droplets composed of ethyl salicylate (ES) or butyl salicylate (BS). When ES and BS droplets were placed on an aqueous sodium dodecyl sulfate (SDS) solution, the collective motion of droplets resulted in three stages of selective coalescence on the time development. Initially, coalescence was observed only between different types of self-propelled droplets. Subsequently, the formed droplets selectively coalesced with ES droplets. Finally, mature droplets merged with BS droplets. The sequentially selective coalescence was discussed from the dynamic pattern formation of swarming droplets and the collapse of the SDS monolayer at the o/w interface caused by the difference in Laplace pressure and the interfacial instability at the contact point between droplets. Thus, this study formulates a strategy of sequentially selective coalescence of droplets via the collective motion of non-identical self-propelled droplets, promoting a new type of powerful and efficient automation technology based on an autonomous inanimate manner of spatiotemporal pattern formation under non-equilibrium conditions for the droplet manipulation.

Spatial programming of self-organizing chemical systems using sustained physicochemical gradients from reaction, diffusion and hydrodynamics

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) reaction-convection; (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.

Self-Organization Emerging from Marangoni and Elastocapillary Effects Directed by Amphiphile Filament Connections

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.

Complexity and bifurcations in the motion of a self-propelled rectangle confined in a circular water chamber

We consider the motion of a self-propelled object of rectangular shape inside a circular water chamber. The mathematical model of self-motion includes equations for the orientation and location of the rectangle and reaction-diffusion equation with an effective diffusion coefficient for the time evolution of the surface concentration of active molecules. Numerical simulations of motion were performed for different values of the ratio between the supply rate S and the evaporation rate a of active molecules. Treating S₀ = S/a as a control parameter, we found the critical behavior in variables characterizing the trajectory and identified different types of motion. If the value of S₀ is small, the rectangle rests at the chamber center. For larger S₀, a reciprocal motion during which the rectangle passes through the center is observed. At yet higher supply rates, the star-polygonal motion appears, and the trajectory remains at a distance from the chamber center. In the experiments with a rectangle made of camphor-camphene-polypropylene plastic moving in a Petri dish, we observed the transition from the star-polygonal motion to the reciprocal motion in time. This transition can be understood on the basis of the developed model if we assume that the supply rate decreases in time.

Visual Sensing System to Investigate Self-Propelled Motion and Internal Color of Multiple Aqueous Droplets

This study proposes a visual sensing system to investigate the self-propelled motions of droplets. In the visual sensing of self-propelled droplets, large field-of-view and high-resolution images are both required to investigate the behaviors of multiple droplets as well as chemical reactions in the droplets. Therefore, we developed a view-expansive microscope system using a color camera head to investigate these chemical reactions; in the system, we implemented an image processing algorithm to detect the behaviors of droplets over a large field of view. We conducted motion tracking and color identification experiments on the self-propelled droplets to verify the effectiveness of the proposed system. The experimental results demonstrate that the proposed system is able to detect the location and color of each self-propelled droplet in a large-area image.

Vibrational spectroscopic analysis of critical micelle concentration in sodium decanoate solutions

The presented study is devoted to the investigation of the micellization-induced liquid-liquid fluctuations in sodium decanoate (NaD) aqueous solutions, based on the vibrational spectroscopic study of NaD and the determination of critical micelle concentration (CMC) of this system. At the same time, we focused on monitoring the effect of the addition of decanol to this system and changing its basic parameters from the point of view of CMC. CMC is an important parameter from a practical point of view and a characteristic feature of each micelle-forming compound. Upon analyzing the spectroscopic data we focus our attention mainly on the intensity and band position variations of both the symmetrical and antisymmetrical vibrational modes of CH₂ groups situated in the high-frequency part of the spectrum. The study used normal (non-enhanced) Raman spectroscopy with excitation wavelength 785 nm, surface-enhanced Raman spectroscopy (SERS) on large-scaled gold-coated SERS-active substrates and infrared spectral measurements. The results of spectroscopic measurements were supported by tensiometry and potentiometry.

A surfactant reaction model for the reciprocating motion of a self-propelled droplet

We report herein experimental observations of the reciprocating motion of a self-propelled droplet floating on the surface of an aqueous surfactant solution and a simple reaction model capable of reproducing the observed behavior of the droplet. The reciprocating motion was observed in a quasi-one-dimensional annular channel, so the reciprocation was not caused by reflections at boundaries. To understand the reciprocation, our model assumes a reaction between the surface active substance emitted from the droplet and surfactants dissolved in the aqueous solution. This reaction invokes an inversion of the surface tension gradient and thus the droplet's reciprocation. We show that the model can reproduce experimental results semi-quantitatively using numerical simulations with realistic parameters.

Interfacial Dynamics in the Spontaneous Motion of an Aqueous Droplet

Self-propelled droplets can spontaneously move using chemical energy. In several reports of self-propelled droplets, interfacial chemical reactions occur at the oil/aqueous interface to induce the Marangoni flow. While the dynamics of interfacial tension is essential to the droplet motion, there are few reports that quantitatively discuss the moving mechanism based on interfacial tension measurements. In this study, we focused on the self-propelled motion of an aqueous droplet in the oil phase, where the surfactant monoolein reacts with bromine at the interface, and estimated the physicochemical parameters related to the droplet motion based on the time series of interfacial tension. These results may reveal the general mechanism for the self-propelled motion of aqueous/oil droplets driven by the interfacial chemical reaction.