Interfacial Dynamics in the Spontaneous Motion of an Aqueous Droplet

Nonequilibrium structure formation in electrohydrodynamic emulsions

Application of an electric field across the interface of two fluids with low, but non-zero, conductivities gives rise to a sustained electrohydrodynamic (EHD) fluid flow. In the presence of neighboring drops, drops interact via the EHD flows of their neighbors, as well as through a dielectrophoretic (DEP) force, a consequence of drops encountering disturbance electric fields around their neighbors. We explore the collective dynamics of emulsions with drops undergoing EHD and DEP interactions. The interplay between EHD and DEP results in a rich set of emergent behaviors. We simulate the collective behavior of large numbers of drops; in two dimensions, where drops are confined to a plane; and three dimensions. In monodisperse emulsions, drops in two dimensions cluster or crystallize depending on the relative strengths of EHD and DEP, and form spaced clusters when EHD and DEP balance. In three dimensions, chain formation observed under DEP alone is suppressed by EHD, and lost entirely when EHD dominates. When a second population of drops are introduced, such that the electrical conductivity, permittivity, or viscosity are different from the first population of drops, the interaction between the drops becomes non-reciprocal, an apparent violation of Newton's Third Law. The breadth of consequences due to these non-reciprocal interactions are vast: we show selected cases in two dimensions, where drops cluster into active dimers, trimers, and larger clusters that continue to translate and rotate over long timescales; and three dimensions, where drops form stratified chains, or combine into a single dynamic sheet.

Simple model for self-propulsion of microdroplets in surfactant solution

We propose a simple active hydrodynamic model for the self-propulsion of a liquid droplet suspended in micellar solutions. The self-propulsion of the droplet occurs by spontaneous breaking of isotropic symmetry and is studied using both analytical and numerical methods. The emergence of self-propulsion arises from the slow dissolution of the inner fluid into the outer micellar solution as filled micelles. We propose that the surface generation of filled micelles is the dominant reason for the self-propulsion of the droplet. The flow instability is due to the Marangoni stress generated by the nonuniform distribution of the surfactant molecules on the droplet interface. In our model, the driving parameter of the instability is the excess surfactant concentration above the critical micellar concentration, which directly correlates with the experimental observations. We consider various low-order modes of flow instability and show that the first mode becomes unstable through a supercritical bifurcation and is the only mode contributing to the swimming of the droplet. The flow fields around the droplet for these modes and their combined effects are also discussed.

Hydrodynamics of a confined active Belousov-Zhabotinsky droplet

Self-sustained locomotion of synthetic droplet swimmers has been of great interest due to their ability to mimic the behavior of biological swimmers. Here we harness the Belousov-Zhabotinsky (BZ) reaction to induce Marangoni stresses on the fluid-droplet interface and elucidate the spontaneous locomotion of active BZ droplets in a confined two-dimensional channel. Our approach employs the lattice Boltzmann method to simulate a coupled system of multiphase hydrodynamics and BZ-reaction kinetics. Our investigation reveals the mechanism underlying the propulsion of active BZ droplets, in terms of convective and diffusive fluxes and deformation of the droplets. Furthermore, we demonstrate that by manipulating the degree of confinement, strength, and nature of coupling between surface tension and active species' concentration, the motion of the BZ droplet can be directed. In addition, we are able to capture two different kinds of droplet behaviors, namely, sustained and stationary, and establish conditions for the sustained long-time motion. We envisage that our findings can be used not only to understand the mechanics of biological swimmers but also to design reaction-driven self-propelled systems for a variety of biomimetic applications.

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.

Chemically Tuning Attractive and Repulsive Interactions between Solubilizing Oil Droplets

Micellar solubilization is a transport process occurring in surfactant-stabilized emulsions that can lead to Marangoni flow and droplet motility. Active droplets exhibit self-propulsion and pairwise repulsion due to solubilization processes and/or solubilization products raising the droplet's interfacial tension. Here, we report emulsions with the opposite behavior, wherein solubilization decreases the interfacial tension and causes droplets to attract. We characterize the influence of oil chemical structure, nonionic surfactant structure, and surfactant concentration on the interfacial tensions and Marangoni flows of solubilizing oil-in-water drops. Three regimes corresponding to droplet "attraction", "repulsion" or "inactivity" are identified. We believe these studies contribute to a fundamental understanding of solubilization processes in emulsions and provide guidance as to how chemical parameters can influence the dynamics and chemotactic interactions between active droplets.

Motile behaviour of droplets in lipid systems

Motility is the capacity for living organisms to move autonomously and with purpose, and is essential to life. The transition from abiotic chemistry into motile cellular compartments has yet to be understood, but motile behaviour likely followed chemical evolution because primeval cell survival depended on scouting for resources effectively. Minimalistic motile systems provide an experimental framework to delineate the emergence mechanisms of such an evolutionary asset. In this Review, we discuss frontier developments in controlling the movement of droplets in lipid systems, in particular, chemotactic behaviours driven by fluctuations in interfacial tension, because of its simple mechanism and prebiotic relevance. Although most efforts have focused on designing oil droplet motility in lipid-rich aqueous solutions, we highlight that water droplets can also move in lipid-enriched oils. First, we describe how droplets evolve chemotactic motility in lipid systems. Next, we review how these oil droplets can adapt their movement to illumination conditions. Finally, we discuss examples where chemical reactivity brings complexity to motility. This work contributes to systems chemistry, where chemical reactions combined with physicochemical phenomena can yield new functions, such that a limited set of molecules can promote complex movement at larger functional scales by following the rules of molecular chemistry.

Reversible morphology-resolved chemotactic actuation and motion of Janus emulsion droplets

We report, for the first time, a chemotactic motion of emulsion droplets that can be controllably and reversibly altered. Our approach is based on using biphasic Janus emulsion droplets, where each phase responds differently to chemically induced interfacial tension gradients. By permanently breaking the symmetry of the droplets’ geometry and composition, externally evoked gradients in surfactant concentration or effectiveness induce anisotropic Marangoni-type fluid flows adjacent to each of the two different exposed interfaces. Regulation of the competitive fluid convections then enables a controllable alteration of the speed and the direction of the droplets’ chemotactic motion. Our findings provide insight into how compositional anisotropy can affect the chemotactic behavior of purely liquid-based microswimmers. This has implications for the design of smart and adaptive soft microrobots that can autonomously regulate their response to changes in their chemical environment by chemotactically moving towards or away from a certain target, such as a bacterium.

Artificial microswimmers can emulate the autonomous regulation of chemotactic motility of living organisms. Frank et al. realize a chemotactic locomotion of emulsion droplets, composed of two phase-separated fluids, that can be reversibly directed up or down a chemical concentration gradient.

Self-Propulsion of a Light-Powered Microscopic Crystalline Flapper in Water

A key goal in developing molecular microrobots that mimic real-world animal dynamic behavior is to understand better the self-continuous progressive motion resulting from collective molecular transformation. This study reports, for the first time, the experimental realization of directional swimming of a microcrystal that exhibits self-continuous reciprocating motion in a 2D water tank. Although the reciprocal flip motion of the crystals is like that of a fish wagging its tail fin, many of the crystals swam in the opposite direction to which a fish would swim. Here the directionality generation mechanism and physical features of the swimming behavior is explored by constructing a mathematical model for the crystalline flapper. The results show that a tiny crystal with a less-deformable part in its flip fin exhibits a pull-type stroke swimming, while a crystal with a fin that uniformly deforms exhibits push-type kicking motion.

Spontaneous Mode Switching of Self-Propelled Droplet Motion Induced by a Clock Reaction in the Belousov-Zhabotinsky Medium

Interfacial chemical dynamics on a droplet generate various self-propelled motions. For example, ballistic and random motions arise depending on the physicochemical conditions inside the droplet and its environment. In this study, we focus on the relationship between oxidant concentrations in an aqueous droplet and its mode of self-propelled motion in an oil phase including surfactant. We demonstrated that the chemical conditions inside self-propelled aqueous droplets were changed systematically, indicating that random motion appeared at higher concentrations of oxidants, which were H₂SO₄ and BrO₃^-, and ballistic motion at lower concentrations. In addition, spontaneous mode switching from ballistic to random motion was successfully demonstrated by adding malonic acid, wherein the initially observed reduced state of the aqueous solution suddenly changed to the oxidized state. Although we only observed one-time transition and have not yet succeeded to realize alternation between ballistic (reduced state) and random motion (oxidized state), such spontaneous transitions are fundamental steps in realizing artificial cells and understanding the fundamental mechanisms of life-like behavior, such as bacterial chemotaxis originating from periodical run-and-tumble motion.

Spontaneous Motion and Rotation of Acid Droplets on the Surface of a Liquid Metal

Self-propulsion of droplets is of great significance in many fields. The spontaneous horizontal motion and self-jumping of droplets have been well realized in various ways. However, there is still a lack of an effective method to enable a droplet to rotate spontaneously and steadily. In this paper, by employing an acid droplet and a liquid metal, the spontaneous and steady rotation of droplets is achieved. For an acid droplet, it may spontaneously move when it is deposited on the surface of the liquid metal. By adjusting experimental parameters to the proper range, the self-rotation of droplet happens. This phenomenon originates from the fluctuation of the droplet boundary and the collective movement of bubbles that are generated by the chemical reactions between the acid droplet and liquid metal. This rotation has a simpler implementation method and more steady rotation state. Its angular velocity is much higher than that driven by other mechanisms. Moreover, the movements of acid droplets on the liquid metal are classified according to experimental conditions. The internal flow fields, the movements and distribution of bubbles, and the fluctuation of the droplet boundary are also explored and discussed. The theoretical model describing the rotational droplet is given. Our work may deepen the understanding of the physical system transition affected by chemical reactions and provide a new way for the design of potential applications, e.g., micro- and nanodevices.

Engineering motile aqueous phase-separated droplets via liposome stabilisation

There are increasing efforts to engineer functional compartments that mimic cellular behaviours from the bottom-up. One behaviour that is receiving particular attention is motility, due to its biotechnological potential and ubiquity in living systems. Many existing platforms make use of the Marangoni effect to achieve motion in water/oil (w/o) droplet systems. However, most of these systems are unsuitable for biological applications due to biocompatibility issues caused by the presence of oil phases. Here we report a biocompatible all aqueous (w/w) PEG/dextran Pickering-like emulsion system consisting of liposome-stabilised cell-sized droplets, where the stability can be easily tuned by adjusting liposome composition and concentration. We demonstrate that the compartments are capable of negative chemotaxis: these droplets can respond to a PEG/dextran polymer gradient through directional motion down to the gradient. The biocompatibility, motility and partitioning abilities of this droplet system offers new directions to pursue research in motion-related biological processes.

Spontaneous Formation of Double Emulsions at Particle-Laden Interfaces

HYPOTHESIS

Traditionally, double emulsions are produced in the presence of both oil-soluble and water-soluble surfactants in sequential droplet formation settings or unique fluidic designs. Micelles, assemblies of surfactants in liquid mediums, can generate single emulsion droplets without requiring input energy. We hypothesize that the synergy between nanoparticles in one phase, and micelles in the other phase can spontaneously generate double emulsions. Nanoparticles can become surface-activated by adsorbing surfactants and form the second type of emulsions from the initially emulsified phase by micelles.

EXPERIMENTS

We design a thermodynamically-driven emulsification platform where double emulsions are spontaneously formed as soon an aqueous nanoparticle dispersion is placed in contact with an oleic micellar solution. Confocal and cryogenic-scanning electron microscopies are utilized to characterize structure and intensity of emulsions at various concentrations of silica nanoparticle and Span micelles. The rate of particle surface activation and emulsification and the amount of water intake are quantified using dynamic light scattering, dynamic interfacial tension, and density measurements.

FINDINGS

Nanoscale water droplets nucleate in the oil in form of swollen micelles. Over time, nanoparticles form a water-shell encapsulating the swollen-micelle rich oil phase. The gradual surfaceactivation of nanoparticles is key in self-double emulsification and controlling the emulsion intensity. We build on this new discovery and design a novel system for double emulsification. Incorporating nanoparticles into spontaneous emulsification systems opens novel routes for designing emulsion-based materials.

Programmable Design of Self-Organized Patterns through a Precipitation Reaction

Nature uses self-organized spatiotemporal patterns to construct systems with robustness and flexibility. Furthermore, understanding the principles underlying self-organization in nature enables programmable design of artificial patterns driven by chemical energy. The related mechanisms are however not clearly understood because most of these patterns are formed in reaction-diffusion (RD) systems consisting of intricate interaction between diffusion and reaction. Therefore, comprehensive understanding of the pattern formation may provide critical knowledge for developing novel strategies in both natural science and chemical engineering. Liesegang patterns (LPs) are one of the typical programmable patterns. This study demonstrates that appropriate tuning of gel concentration distribution is a key programming factor for controlling LP periodicities. The gel distribution was realized in bi- or multilayered gels constructed by stacking agarose gels of different concentrations. Thus, exceptional LP periodicities were achieved locally in bilayered gels. Furthermore, RD simulations revealed that the nucleation process modulated by the gel distribution determines the LP periodicity in bilayered gels. Finally, based on this concept, desired LP periodicities were successfully realized by programming gel distributions in multilayered gels. Thus, deep insights into the fundamental role of nucleation in designing LPs can lead to the practical applications of LPs and the understanding of self-organization in nature.

Effect of a product on spontaneous droplet motion driven by a chemical reaction of surfactant

We focus on the self-propelled motion of an oil droplet within an aqueous phase or an aqueous droplet within an oil phase, which originates from an interfacial chemical reaction of surfactant. The droplet motion has been explained by mathematical models, which require the assumption that the chemical reaction increases the interfacial tension. However, several experimental reports have demonstrated self-propelled motion with the chemical reaction decreasing the interfacial tension. Our motivation is to construct an improved mathematical model, which explains these experimental observations. In this process, we consider the concentrations of the reactant and product on the interface and of the reactant in the bulk. Our numerical calculations indicate that the droplet potentially moves in the cases of both an increase and a decrease in the interfacial tension. In addition, the reaction rate and size dependencies of the droplet speed observed in experiments were well reproduced using our model. These results indicate the potential of our model as a universal one for droplet motion.