Generation of periodic force with oscillating chemical reaction

Pair Interactions of Self-Propelled SiO2-Pt Janus Colloids: Chemically Mediated Encounters

Driven by the necessity to achieve a thorough comprehension of the bottom-up fabrication process of functional materials, this experimental study investigates the pairwise interactions or collisions between chemically active SiO2-Pt Janus colloids. These collisions are categorized based on the Janus colloids' orientations before and after they make physical contact. In addition to the hydrodynamic interactions, the Janus colloids are also known to affect each other's chemical field, resulting in chemophoretic interactions, which depend on the degree of surface anisotropy in reactivity of Janus colloid and the solute-surface interaction at play. Our study reveals that these interactions lead to a noticeable decrease in particle speed and changes in orientation that correlate with the contact duration and yield different collision types. Distinct configurations of contact during collisions were found, whose mechanisms and likelihood are found to be dependent primarily on the chemical interactions. Such estimates of collision and their characterization in dilute suspensions shall have a key impact in determining the arrangement and time scales of dynamical structures and assemblies of denser suspensions and potentially the functional materials of the future.

The excitable nature of polymerizing actin and the Belousov-Zhabotinsky reaction

The intricate regulatory processes behind actin polymerization play a crucial role in cellular biology, including essential mechanisms such as cell migration or cell division. However, the self-organizing principles governing actin polymerization are still poorly understood. In this perspective article, we compare the Belousov-Zhabotinsky (BZ) reaction, a classic and well understood chemical oscillator known for its self-organizing spatiotemporal dynamics, with the excitable dynamics of polymerizing actin. While the BZ reaction originates from the domain of inorganic chemistry, it shares remarkable similarities with actin polymerization, including the characteristic propagating waves, which are influenced by geometry and external fields, and the emergent collective behavior. Starting with a general description of emerging patterns, we elaborate on single droplets or cell-level dynamics, the influence of geometric confinements and conclude with collective interactions. Comparing these two systems sheds light on the universal nature of self-organization principles in both living and inanimate systems.

Electrotaxis behavior of droplets composed of aqueous Belousov-Zhabotinsky solutions suspended in oil phase

Taxis is ubiquitous in biological and physical chemistry systems as a response to various external stimulations. We prepared aqueous droplets containing Belousov-Zhabotinsky (BZ) solutions suspended on an oleic acid oil phase subject to DC electric field and found that these BZ droplets undergo chemically driven translational motion towards the negative electrode under DC electric field. This electrotaxis phenomenon originates from the field-induced inhomogeneous distribution of reactants, in particular Br[Formula: see text] ions, and consequently the biased location of the leading centers towards the positive electrode. We define the 'leading center' (LC) as a specific location within the droplet where the BZ chemical wave (target pattern) is initiated. The chemical wave generated from the LC propagates passing the droplet center of mass and creates a gradient of interfacial tension when reaching the droplet-oil interface on the other side, resulting in a momentum exchange between the droplet and oil phases which drives the droplet motion in the direction of the electric field. A greater electric field strength renders a more substantial electrotaxis effect.

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.

Unidirectional motion of a camphor disk on water forced by interactions between surface camphor concentration and dynamically changing boundaries

Dynamically changing boundaries induce unidirectional motion of a camphor disk on water, which is regarded as a signal diode.

Oscillation of Speed of a Self-Propelled Belousov-Zhabotinsky Droplet

Self-propelled objects can become potential biomimetic micromachines, but a versatile strategy is required to add the desired functions. Introducing a characteristic chemical reaction is a simple answer; however, the problem is how the chemical reaction is coupled to the self-propelled motion. We propose a strategy to select the chemical reaction so that its product or intermediate affects the driving force of a self-propelled object. To demonstrate this strategy, we put an aqueous droplet of nonlinear chemical reaction, the Belousov-Zhabotinsky (BZ) reaction, into an oil phase including a surfactant, where an aqueous droplet was driven by an interfacial reaction of the surfactant and bromine. The results exhibited oscillation of speed, and it was synchronized with the redox oscillation of the BZ reaction in the droplet. Bromine is one of the intermediates of the BZ reaction, and thus the droplet motion well-reflected the characteristics of the BZ reaction.

Self-Organized Traveling Chemo-Hydrodynamic Fingers Triggered by a Chemical Oscillator

Pulsatile chemo-hydrodynamic patterns due to a coupling between an oscillating chemical reaction and buoyancy-driven hydrodynamic flows can develop when two solutions of separate reactants of the Belousov-Zhabotinsky reaction are put in contact in the gravity field and conditions for chemical oscillations are met in the contact zone. In regular oscillatory conditions, localized periodic changes in the concentration of intermediate species induce pulsatile density gradients, which, in turn, generate traveling convective fingers breaking the transverse symmetry. These patterns are the self-organized result of a genuine coupling between chemical and hydrodynamic modes.

Periodic reciprocating motion of a polymer gel on an aqueous phase synchronized with the Belousov-Zhabotinsky reaction

A self-oscillating gel induced by the Belousov-Zhabotinsky (BZ) reaction was investigated on an aqueous phase. When the Ru-catalyst in the gel was rapidly oxidized, the gel was accelerated in a direction opposite to the side of oxidation. The gel then returned to its original position while the Ru-catalyst in the gel was slowly reduced. To clarify the mechanism of this periodic reciprocation of the gel, the contact angle between a sessile bubble and the gel and the time-variation of the adhesive force of the gel on the aqueous phase were measured. The experimental results suggest that the periodic reciprocation of the gel is driven by the periodic change in the contact angle of the gel induced by the BZ reaction.

Segmented waves in a reaction-diffusion-convection system

The interaction of traveling waves, with both Marangoni and buoyancy driven flows, can generate an extraordinary rich array of patterns ranging from stationary structures to chaotic waves. However, the inherent complexity of reaction-diffusion-convection (RDC) systems makes the explanation of the patterning mechanisms very difficult, both numerically and experimentally. In this paper, we describe the appearance of segmented waves in a shallow layer of an excitable Belousov-Zhabotinsky solution. The segmentation process was found to be dependent both on the depth of the solution and on the excitability of the reaction. We caught the essential features of the system through a RDC model, where the chemical waves were coupled both with surface and bulk fluid motions and we found that by varying the excitability of the reaction, and in turn the wavelength of the chemical fronts, it is possible to create a sort of hydrodynamic resonance structures (corridors), which are responsible for the segmentation process.

Spontaneous motion of a droplet coupled with a chemical wave

We propose a framework for the spontaneous motion of a droplet coupled with internal dynamic patterns generated in a reaction-diffusion system. The spatiotemporal order of the chemical reaction gives rise to inhomogeneous surface tension and results in self-propulsion driven by the surrounding flow due to the Marangoni effect. Numerical calculations of internal patterns together with theoretical results of the flow fields at low Reynolds number reproduce well the experimental results obtained using a droplet of the Belousov-Zhabotinsky reaction medium.

Lattice Boltzmann study of convective drop motion driven by nonlinear chemical kinetics

We model a reaction-diffusion-convection system which comprises a liquid drop containing solutes that undergo an Oregonator reaction producing chemical waves. The reactants are taken to have surfactant properties so that the variation in their concentrations induces Marangoni flows at the drop interface which lead to a displacement of the drop. We discuss the mechanism by which the chemical-mechanical coupling leads to drop motion and the way in which the net displacement of the drop depends on the strength of the surfactant action. The equations of motion are solved using a lattice Boltzmann approach.

Steady Marangoni flow traveling with chemical fronts

When autocatalytic chemical fronts propagate in thin layers of solution in contact with air, they can induce capillary flows due to surface tension gradients across the front (Marangoni flows). We investigate here such an interplay between autocatalytic reactions, diffusion, and Marangoni effects with a theoretical model coupling the incompressible Navier-Stokes equations to a conservation equation for the autocatalytic product concentration in the absence of gravity and for isothermal conditions. The boundary condition at the open liquid/air interface takes the surface activity of this product into account and introduces the solutal Marangoni number M representing the intensity of the coupling between hydrodynamics and reaction-diffusion processes. Positive and negative Marangoni numbers correspond, respectively, to the cases where the product decreases or increases surface tension behind the front. We show that, in both cases, such coupled systems reach an asymptotic dynamics characterized by a steady fluid vortex traveling at a constant speed with the front and deforming it, with, however, an asymmetry between the results for positive and negative M. A parametric study shows that increased propagation speed, front deformation, and possible transient oscillating dynamics occur when the absolute value of M is increased.

Nuclear Magnetic Resonance Studies of Convection in the 1,4-Cyclohexanedione−Bromate−Acid Reaction

The manifestation and development of convection during pattern formation in the 1,4-cyclohexanedione-acid-bromate reaction was investigated using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) experiments. An apparatus was devised that enabled convection to be probed inside an NMR spectrometer and prevented hydrodynamic motion arising from extraneous sources, such as poor mixing or temperature gradients imposed by the experimental setup. PGSE experiments were performed concurrently with magnetic resonance imaging (MRI) experiments to show that convection arose spontaneously from inhomogeneities associated with the chemical patterns. Quantitative data on diffusion coefficients and hydrodynamic velocities are reported.

Periodic change of viscosity and density in an oscillating chemical reaction

It was found that the periodic change of the solution viscosity and density was generated in the Belousov-Zhabotinsky (BZ) reaction. This rhythmic phenomenon was observed in both the iron catalyst [[Fe(Phen)(3)](2+)-[Fe(Phen)(3)](3+)] and the cerium catalyst [Ce(III)-Ce(IV)] system, where the solution viscosity and density were synchronized with the redox potential in the in-phase mode. However, the time delay existed between the redox potential and the solution viscosity and density. The behavior of the BZ reaction was also monitored in the presence of the nonionic surfactant. This experiment revealed that, beyond the critical micelle concentration, the phase between the redox potential and the solution viscosity and density was synchronized into the antiphase mode. We suggested that the variation of the catalyst drove the oscillation of the solution viscosity and density in the BZ reaction.

Self-Sustaining Peristaltic Motion on the Surface of a Porous Gel

We report the structural color behavior of a periodic ordered mesoporous gel synchronized with the Belousov-Zhabotinsky (BZ) reaction. The structural colored concentric rings which were spatiotemporally spread out on the porous gel were observed during the BZ reaction. The color tone of the structural color, which is determined by the swelling ratio of the gel, was periodically changed. This is the first evidence that a self-sustaining peristaltic motion occurs on the surface of a gel.

Bromomalonic-acid-induced transition from trigger wave to big wave in the Belousov-Zhabotinsky reaction

The Marangoni effects on chemical waves in the ferroin-catalyzed Belousov-Zhabotinsky reaction were studied. The main purpose of the present study was to understand the mechanism of the big wave, an accelerative chemical wave involving surface-tension-driven fluid motions. Spatiotemporal variations of surface tension caused by a chemical wave were measured using the Wilhelmy method. The transition from conventional trigger waves to big waves, due to a concentration change of bromomalonic acid, was observed. The strong surface activity of the bromomalonic acid which was responsible for the transition was also observed. It led to an acceleration of the big waves through the Marangoni effect.