北畑 裕之

The photo-sensitive Belousov-Zhabotinsky (BZ) reaction system was investigated to understand the response of wave propagation to local pulse stimulation in an excitable field. When the chemical wave was irradiated with a bright pulse or a dark pulse, the speed of wave propagation decreased or increased. The timing of pulse irradiation that significantly affected the speed of chemical wave propagation was different with the bright and dark pulses. That is, there is a sensitive point in the chemical wave. The experimental results were qualitatively reproduced by a numerical calculation based on a three-variable Oregonator model that was modified for the photosensitive BZ reaction. These results suggest that the chemical wave is sensitive to the timing of pulse irradiation due to the rates of production of an activator and an inhibitor in the photochemical reaction.

The development of self-propelled motors that mimic biological motors is an important challenge for the transport of either themselves or some material in a small space, since biological systems exhibit high autonomy and various types of responses, such as taxis and swarming. In this perspective, we review non-living systems that behave like living matter. We especially focus on nonlinearity to enhance autonomy and the response of the system, since characteristic nonlinear phenomena, such as oscillation, synchronization, pattern formation, bifurcation, and hysteresis, are coupled to self-motion of which driving force is the difference in the interfacial tension. Mathematical modelling based on reaction-diffusion equations and equations of motion as well as physicochemical analysis from the point of view of the molecular structure are also important for the design of non-living motors that mimic living motors.

Addition of Au nanoparticles to a solution of chemical oscillators caused a concentration-dependent change in the oscillation dynamics. The oscillator used in the present system is BrO3--SO32-/HSO3- reaction in a continuous stirred-tank reactor. Increase in the Au nanoparticle concentration induced a decrease and increase in the periods in the high- and low-pH states, respectively, whereas total period was almost remained. We compared these experimental results with those obtained from the simulations involving protonation equilibrium, proton production, and proton consumption reactions. The comparison revealed that the change in the protonation equilibrium constant and the acceleration of the proton production reaction can explain the experimentally observed change in the oscillation dynamics. Our finding provides a novel insight into the mechanism of control of the chemical oscillation dynamics. Chemical oscillators are inherently stable toward chemical and physical fluctuations and lack flexible controllability owing to their characteristic features such as limit cycle and bifurcation; therefore, the addition of the metal nanoparticles to the chemical oscillators can offer a novel approach for the flexible and on-demand control of the chemical oscillations. (C) 2014 Elsevier B.V. All rights reserved.

Quantitative information on the parameters associated with self-propelled objects would enhance the potential of this research field; for example, finding a realistic way to develop a functional self-propelled object and quantitative understanding of the mechanism of self-motion. We therefore estimated five main parameters, including the driving force, of a camphor boat as a simple self-propelled object that spontaneously moves on water due to difference in surface tension. The experimental results and mathematical model indicated that the camphor boat generated a driving force of 4.2 mu N, which corresponds to a difference in surface tension of 1.1 mN m(-1). The methods used in this study are not restricted to evaluate the parameters of self-motion of a camphor boat, but can be applied to other self-propelled objects driven by difference in surface tension. Thus, our investigation provides a novel method to quantitatively estimate the parameters for self-propelled objects driven by the interfacial tension difference.

The mode-bifurcation of a self-propelled system induced by the property of a N-stearoyl-p-nitroaniline (C(18)ANA) monolayer developed on an aqueous phase was studied. A camphor disk was placed on a C(18)ANA monolayer, which indicated a characteristic surface pressure-area (pi-A) isotherm. A camphor disk transiently exhibited reciprocating motion at a higher surface density of C(18)ANA. The amplitude of the reciprocating motion increased with an increase in the temperature of the aqueous phase below 290 K, but reciprocating motion varied to irregular motion over 290 K. The temperature-dependent reciprocating motion is discussed in terms of the pi-A curve for C(18)ANA depending on the temperature. The interaction between C(18)ANA molecules was measured by Fourier transform IR spectrometry and Brewster-angle microscopy. As an extension of the study, the trajectory of reciprocating motion could be determined by writing with a camphor pen on the C(18)ANA monolayer.

The spontaneous motion of an elliptic camphor particle floating on water is studied theoretically and experimentally. Considering a mathematical model for the motion of an elliptic camphor particle in a two-dimensional space, we first investigate the asymptotic solutions with numerical computation. We then introduce a small parameters into the definition of the particle shape, which represents an elliptic deformation from a circular shape and, by means of perturbation theory, we analytically calculate the travelling solution to within O(epsilon). The results show that short-axis-directed travelling solutions primarily bifurcate from stationary solutions and that long-axis-directed ones are secondary which means that elliptic camphor particles are easier to move in the short-axis direction. Furthermore, we show that rotating solutions bifurcate from stationary solutions and that the bifurcation point changes with 0(82), which suggests that elliptic camphor disks easily exhibit translational motion, rather than rotational, within the small deformation. Finally, our theoretical suggestions are confirmed by an experiment. (C) 2014 Elsevier B.V. All rights reserved.

Recent studies have shown that the behavior of calcium in the epidermis is closely related to the conditions of the skin, especially the differentiation of the epidermal keratinocytes and the permeability barrier function, and therefore a correct understanding of the calcium dynamics is important in explaining epidermal homeostasis. Here we report on experimental observations of in vitro calcium waves in keratinocytes induced by mechanical stimulation, and present a mathematical model that can describe the experimentally observed wave behavior that includes finite-range wave propagation and a ring-shaped pattern. A mechanism of the ring formation hypothesized by our model may be related to similar calcium propagation patterns observed during the wound healing process in the epidermis. We discuss a possible extension of our model that may serve as a tool for investigating the mechanisms of various skin diseases.

Intact epidermal barrier function is crucial for survival and is associated with the presence of gradients of both calcium ion concentration and electric potential. Although many molecules, including ion channels and pumps, are known to contribute to maintenance of these gradients, the mechanisms involved in epidermal calcium ion dynamics have not been clarified. We have established that a variety of neurotransmitters and their receptors, originally found in the brain, are expressed in keratinocytes and are also associated with barrier homeostasis. Moreover, keratinocytes and neurons show some similarities of electrochemical behaviour. As mathematical modelling and computer simulation have been employed to understand electrochemical phenomena in brain science, we considered that a similar approach might be applicable to describe the dynamics of epidermal electrochemical phenomena associated with barrier homeostasis. Such methodology would also be potentially useful to address a number of difficult problems in clinical dermatology, such as ageing and itching. Although this work is at a very early stage, in this essay, we discuss the background to our approach and we present some preliminary results of simulation of barrier recovery.

Time-resolved measurements of the interfacial tension of propagating chemical waves of the Belousov-Zhabotinsky reaction based on the iron complex catalysts were carried out without stirring by monitoring the frequency of capillary waves with the quasi-elastic laser scattering method. A delayed response of the interfacial tension with respect to absorption was found with the delay being ligand-dependent when the reaction was conducted at a liquid/liquid interface. This behavior is attributed to differences in adsorption activity of the hydrophobic metal catalyst. The delay time and the increase in interfacial tension were also reproduced by a model considering the rate constants of equilibrium adsorption.

The concept of self-propelled objects is important for the understanding of biological mobility, as well as for the development of autonomous devices in medicine and engineering. In this study, a simple self-propelled object, driven by a difference in surface tension, was found to exhibit intermittent self-motion (alternately in motion and at rest) in an annular water channel, with resting positions and features of motion in subsequent cycles remaining almost the same as those previously visited; that is, memories of the resting positions and features of motion were observed. The occurrence of the memory phenomenon was found to depend on the relationship between the resting time and the period for one lap of the annular channel. The mechanism of memory is discussed in terms of the distribution of surface-active molecules and local surface tension at the resting positions.

The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrim-ethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio similar to 30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation. (C) 2013 Elsevier Inc. All rights reserved.

The propagation of a chemical wave on an inhibitory field, which was wedged between two excitable fields, was investigated for the photosensitive Belousov-Zhabotinsky (BZ) reaction. With an increase in the width of the inhibitory field between the excitable fields (W), the chemical wave divided into two waves at W = W-alpha. The divided chemical waves then coalesced at W = W-beta with a decrease in W. W-alpha was larger than W-beta, i.e., hysteresis on the width of the inhibitory field was observed between the division and coalescence of the chemical wave. The experimental results were qualitatively reproduced by a numerical calculation based on a three-variable Oregonator model modified for the photosensitive BZ reaction. These results suggest that the chemical wave may be preserved on the inhibitory field due to an activator supplied from a chemical wave on the excitable field.

The objective of this study was to observe whether a rotating magnetic field (RMF) could change the anomalous chemical wave propagation induced by a moderate-intensity gradient static magnetic field (SMF) in an unstirred BelousovZhabotinsky (BZ) reaction. The application of the SMF (maximum magnetic flux density=0.22T, maximum magnetic flux density gradient=25.5T/m, and peak magnetic force product (flux densityxgradient)=4T2/m) accelerated the propagation velocity in a two-dimensional pattern. Characteristic anomalous patterns of the wavefront shape were generated and the patterns were dependent on the SMF distribution. The deformation and increase in the propagation velocity were diminished by the application of an RMF at a rotation rate of 1rpm for a few minutes. Numerical simulation by means of the time-averaged value of the magnetic flux density gradient or the MF gradient force over one rotation partially supported the experimental observations. These considerations suggest that RMF exposure modulates the chemical wave propagation and that the degree of modulation could be, at least in part, dependent on the time-averaged MF distribution over one rotation. Bioelectromagnetics 34:220230, 2013. (c) 2012 Wiley Periodicals, Inc.

Gravitational instability occurs at the interface of two solutions when a higher-density solution (HDS) is placed on the surface of a lower-density solution (LDS). As the HDS sinks, a cell pattern forms on the surface. We investigate the size distribution of the cells in this pattern. We show that the cumulative size distribution obeys a power law with a power index that is independent of time as long as it is possible to neglect the interactions among the cells. To understand the power law mechanism, a simple model excluding the interactions is proposed, and we demonstrate that this simple model provides the power law measured in experiments. Our results indicate that independent cell generation and growth are key factors to understand the feature of the cell pattern. DOI: 10.1103/PhysRevE.87.012903

The coupling between deformation and motion in a self-propelled system has attracted broader interest. In the present study, we consider an elliptic camphor particle for investigating the effect of particle shape on spontaneous motion. It is concluded that the symmetric spatial distribution of camphor molecules at the water surface becomes unstable first in the direction of a short axis, which induces the camphor disk motion in this direction. Experimental results also support the theoretical analysis. From the present results, we suggest that when an elliptic particle supplies surface-active molecules to the water surface, the particle can exhibit translational motion only in the short-axis direction. DOI: 10.1103/PhysRevE.87.010901

Spontaneous rotation of a droplet induced by the Marangoni flow is analyzed in a two-dimensional system. The droplet with the small particle which supplies a surfactant at the interface is considered. We calculated flow field around the droplet using the Stokes equation and found that advective nonlinearity breaks symmetry for rotation. Theoretical calculation indicates that the droplet spontaneously rotates when the radius of the droplet is an appropriate size. The theoretical results were validated through comparison with the experiments. DOI: 10.1103/PhysRevE.87.013009

The spontaneous droplet motion of a Belousov-Zhabotinsky (BZ) reaction medium was previously reported; however, the attempts to control such motion have not been successful. In the present study, we first succeeded in controlling the direction of a droplet motion by using a photosensitive BZ reaction catalyzed by ruthenium complex under illumination with a spatial gradient. We were also able to stop the droplet motion using high-intensity light illumination. These results will help in the understanding of motion coupling with pattern formation.

The effect of temperature on the growth of the long axis of gold nanorods is studied. The length of the nanorods decreases gradually with an increase in the growth temperature. By considering the chemical equilibrium between hexadecyltrimethylammonium bromide (HTAB) micelles and Au ions, we suggest that the secondary nucleation of seeds in a growth solution affects the long-axis length. In addition, small-angle X-ray scattering suggests that the equilibrium self-assembled structure of surfactant molecules does not affect the long-axis length in the present system.

In our previous work, we reported that a droplet of a Belousov-Zhabotinsky (BZ) reaction medium moves in one direction and then in the opposite direction, when a chemical wave propagates spherically inside the droplet. In the present study, we report our new results on the continuous motion of the BZ droplet, i.e., on the rotational and repeated back-and-forth motion when a spiral wave and a scroll ring propagated inside the droplet, respectively.

We theoretically derive the amplitude equations for a self-propelled droplet driven by Marangoni flow. As advective flow driven by surface tension gradient is enhanced, the stationary state becomes unstable and the droplet starts to move. The velocity of the droplet is determined from a cubic nonlinear term in the amplitude equations. The obtained critical point and the characteristic velocity are well supported by numerical simulations. © 2012 American Physical Society.

We theoretically derive the amplitude equations for a self-propelled droplet driven by Marangoni flow. As advective flow driven by surface tension gradient is enhanced, the stationary state becomes unstable and the droplet starts to move. The velocity of the droplet is determined from a cubic nonlinear term in the amplitude equations. The obtained critical point and the characteristic velocity are well supported by numerical simulations.

An oscillatory system called a plastic bottle oscillator is studied, in which the downflow of water and upflow of air alternate periodically in an upside-down plastic bottle containing water. It is demonstrated that a coupled two-bottle system exhibits in- and antiphase synchronization according to the nature of coupling. A simple ordinary differential equation is deduced to interpret the characteristics of a single oscillator. This model is also extended to coupled oscillators, and the model reproduces the essential features of the experimental observations.

The motion of an oil water interface that mimics biological motility was investigated in a Hele-Shaw-like cell where elastic surfactant aggregates were formed at the oil water interface. With the interfacial motion, millimeter-scale pillar structures composed of the aggregates were formed. The pillars grew downward in the aqueous phase, and the separations between pillars were roughly equal. Small-angle X-ray scattering using a microbeam X-ray revealed that these aggregates had nanometer-scale lamellar structures whose orientation correlated well with their location in the pillar structure. It is suggested that these hierarchical spatial structures are tailored by the spontaneous interfacial motion.

We investigated the Marangoni flow around a camphor disk on water with the addition of sodium dodecyl sulfate (SDS). The flow velocity decreased with an increase in the concentration of SDS in the aqueous phase, and flow was hardly observed around the critical micelle concentration (cmc), because SDS reduced the driving force of Marangoni flow. However, the flow velocity increased with a further increase in the concentration of SDS. Thus, the Marangoni flow is maximally inhibited around the cmc of SDS. In this paper, we concluded that the regeneration of Marangoni flow originates from an increase in the dissolution rate of camphor into the SDS aqueous solution.

We report the fabrication of topographically guided phase-separated structures of a binary polymer mixture of polystyrene (PS) and poly(2-vinylpyridine) (PVP) spin coated onto sinusoidal microgrooves. Depending on the polymer concentration, dewetted PS islands or uniform PS ribbons were formed in the microgrooves, which were supported by an underlayer of PVP. Uniform PS ribbons were not obtained by sequential coating of the polymers. Tuning the polymer concentration with an appropriate solvent and substrate topography makes this method of spin coating onto microwrinkles a simple, low-cost method for fabricating various topography-guided polymer morphologies.

The growth process of high-aspect-ratio gold nanorods in gelled surfactant solution was studied. As for the application of gold nanorods, the surface plasmon is quite useful, whose absorption depends on their aspect ratio. Hence it is important to synthesize gold nanorods with favorable aspect ratio in high yield. For shorter nanorods (aspect ratio <similar to 10), the synthesis and the growth mechanism have been studied well. For the longer nanorods (aspect ratio >similar to 30), however, the growth mechanism has not yet been understood well, although it has been known that the high-aspect-ratio gold nanorods could be synthesized in high yield in gelled surfactant solution. In this paper, we studied the relationship between the growth process of high-aspect-ratio gold nanorods and the gelation of surfactant growth-solution. Small angle X-ray scattering (SAXS) revealed the microscopic feature of gelation as the structural transition of self-assembly of surfactant molecules from micellar to lamellar. These results will be helpful for better understanding on the growth mechanism of high-aspect-ratio gold nanorods.

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. © 2011 American Physical Society.

The excitation of the photosensitive Belousov-Zhabotinsky (BZ) reaction induced by light stimulation was systematically investigated. A stepwise increase in the light intensity induced the excitation, whereas a stepwise decrease did not induce the excitation. The threshold values for the excitation were found to be a function of the initial and final light intensities, time variation in light intensity, and the concentration of NaBrO3. The experimental results were qualitatively reproduced by a theoretical calculation based on a three-variable Oregonator model modified for the photosensitive BZ reaction. These results suggest that although the steady light irradiation is known to inhibit oscillation and chemical waves in the BZ system under almost all conditions, the stepwise increase in the light irradiation leads to the rapid production of an activator, resulting in the photoexcitation. © 2011 American Chemical Society.

The excitation of the photosensitive Belousov-Zhabotinsky (BZ) reaction induced by light stimulation was systematically investigated. A stepwise increase in the light intensity induced the excitation, whereas a stepwise decrease did not induce the excitation. The threshold values for the excitation were found to be a function of the initial and final light intensities, time variation in light intensity, and the concentration of NaBrO(3). The experimental results were qualitatively reproduced by a theoretical calculation based on a three-variable Oregonator model modified for the photosensitive BZ reaction. These results suggest that although the steady light irradiation is known to inhibit oscillation and chemical waves in the BZ system under almost all conditions, the stepwise increase in the light irradiation leads to the rapid production of an activator, resulting in the photoexcitation.

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.

Gold nanorods have been actively studied for new nanotechnological materials and industrial applications. It is well known that gold nanorods grow spontaneously in surfactant solutions, and a number of procedures for their preparation have been reported; however, the factors that determine the morphology have not been well understood. In this study, we observed the time series of the growth process of gold nanorods in gelled surfactant solutions by completely stopping the growth reaction. This growth process was compared to that in solution without gelation. The comparison indicates that the self-assembly of surfactant molecules affected the resulting shape, especially the short-axis length, of the nanorods. Small-angle neutron scattering (SANS) experiments revealed that the gelled solutions form lamellar structures, whereas nongelled systems form spherical micelles. On the bases of these results, we present a model showing that the short-axis length of gold nanorods is affected by a decrease in the spontaneous curvature of the outer surfactant layer and/or an increase in the bending modulus of the surfactant membrane neighboring the gold surface. (C) 2010 Elsevier Inc. All rights reserved.

Dynamical blebbing in an oil-water system is reported together with a quantitative analysis of interfacial deformation. An oil droplet containing a fatty acid that floats on an aqueous phase containing a cationic surfactant shows blebbing-type deformation at the oil-water interface. Such deformation is caused by an out-of-equilibrium concentration distribution across the interface. The generation and breakage of aggregates at the interface is associated with the time-dependent instability of the interface that accompanies the blebbing. In a quantitative analysis, the size of the bleb depended on the size of the oil droplet: the size of the bleb was inversely proportional to the square root of the radius of the oil droplet approximately. Furthermore, the experimental results showed that an oil droplet undergoes translational motion, i.e., active Brownian motion, under a suitable size of the oil droplet. A simple mathematical model based on the elasticity of the aggregates is proposed, and this agrees well with the experimental results.

The synchronized motion of the water surfaces in contact with two fixed camphor disks was investigated. When the distance between the two camphor disks was greater than 8 mm, the shapes of the water surfaces at the bottoms of the disks oscillated independently. In contrast, synchronized oscillation was observed when the distance was shorter than 7 mm. Depending on the distance, the nature of the Marangoni convection and the difference in the shape of the meniscus changed. The convection was numerically simulated based on the Navier-Stokes equation. The mechanism of synchronization is discussed in relation to the rolling structure of the Marangoni convection. (C) 2010 Elsevier Inc. All rights reserved.

To investigate the nature of the phase wave between two connected oscillators, the photosensitive BelousovZhabotinsky (BZ) reaction was examined for two connected circular reaction fields, which were drawn by using computer software and then projected on a filter paper soaked with BZ solution by using a liquid-crystal projector. The difference in the time at which illumination was terminated between the two circles (Δt0) was changed to control the time at which the phase wave was induced. When Δt0 was small (03 s), the phase wave normally propagated on the two circles in one direction. In contrast, when Δt0 was large (610 s), the velocity of the wave decreased near the intersection of the two circles. These different features are discussed in relation to the excitability of the circles and Δt0. The experimental results were qualitatively reproduced by a numerical calculation based on the modified three-variable Oregonator model that included photosensitivity. © 2010 American Chemical Society.

To investigate the nature of the phase wave between two connected oscillators, the photosensitive Belousov-Zhabotinsky (BZ) reaction was examined for two connected circular reaction fields, which were drawn by using computer software and then projected on a filter paper soaked with BZ solution by using a liquid-crystal projector. The difference in the time at which illumination was terminated between the two circles (Delta t(0)) was changed to control the time at which the phase wave was induced. When Delta t(0) was small (0-3 s), the phase wave normally propagated on the two circles in one direction. In contrast, when Delta t(0) was large (6-10 s), the velocity of the wave decreased near the intersection of the two circles. These different features are discussed in relation to the excitability of the circles and Delta t(0). The experimental results were qualitatively reproduced by a numerical calculation based on the modified three-variable Oregonator model that included photosensitivity.

We demonstrated mode-switching of self-motion coupled with diffusion of molecules at a solid/liquid interface. A camphor boat moved spontaneously on water and the mode of self-motion depended on the setup of the boat. When a camphor disk was connected to the center of a larger plastic plate, intermittent motion (alternating between rest and rapid motion) was observed. When the position of the camphor disk was changed from the center to one of its edges, the period of intermittent motion decreased, and intermittent motion changed to continuous motion. The features of this self-motion and mode-switching were qualitatively reproduced by a numerical calculation using a mathematical model that incorporates the distribution of camphor molecules at the solid/liquid interface.

BackgroundWe previously demonstrated that intracellular calcium propagation was induced by stimulation of epidermal keratinocytes in skin slices or in culture with adenosine triphosphate (ATP). The feature of the calcium wave propagation appeared to be different between differentiated cells and proliferating cells, and so the mechanisms involved might be different.PurposeEstablish a new methodology to abstract cellular information from aggregative dynamics.MethodsWe present a mathematical analysis of the calcium wave to evaluate the mechanism of calcium ion propagation.ResultsA well-defined calcium wave was observed in differentiated cells in comparison with undifferentiated cells. Application of either 2APB [an inositol 1,4,5-trisphosphate (IP3) receptor blocker] or U73122 (an IP3 synthesis blocker) reduced the amplitude of the wave in differentiated cells. Mathematical analysis indicated that U73122 decreased the velocity of the wave, while 2APB altered the wave form. Thus, IP3 synthesis might be important for signal transmission and IP3 movement might be important for pattern formation.ConclusionThe method we present here should be useful to analyze the effects of various reagents in in vitro studies.

A change in the mode of self-motion coupled with complex formation was investigated experimentally and theoretically. A 1,10-phenanthroline disk floating on water exhibited either uniform or intermittent motion (in which the disk altered between rapid motion and a resting state) depending on the concentration of Fe2+ in the aqueous phase. Since the driving force for this motion is considered to be governed by the surface concentrations of phenanthroline (phen) and [Fe(phen)(3)](2+), it is important to understand the relationship between the kinetics near the air/water interface and the mode of motion. We propose a mathematical model for the motion of the phenanthroline disk and discuss the validity of this model based on a comparison with the experimental results.

Spontaneous deformation of a tetradecane droplet with palmitic acid on an aqueous phase with stearyltrimethylammonium chloride is reported. Palmitic acid is transported from the oil droplet to the aqueous phase by the concentration difference between the organic and the aqueous phases. The transport of palmitic acid causes the oil droplet interface to undergo various spontaneous deformations. When the oil droplet is placed on an aqueous surface, its diameter shrinks. Several tens of seconds later, the oil droplet suddenly expands and then shrinks in a second. After such a dramatic deformation, the oil droplet undergoes blebbing on its oil-water interface for over I h. We investigated the physicochemical mechanism of these phenomena. We discuss the cause of these deformations in terms of the spatiotemporal variation of the interfacial tension and elucidate that the blebbing deformation is due to the surfactant aggregate generated by cationic and anionic surfactants.

It is well known that spiral waves are often stabilized by anchoring to a local heterogeneity ("pinning") and that such pinned waves are rather difficult to eliminate. In the present report, we show that pinned spiral waves can be eliminated through collision with a wave train arriving from the outer region, as confirmed in experiments on the Belousov-Zhabotinsky (BZ) reaction as well as in cardiomyocyte tissue culture. A numerical simulation using the Oregonator, a mathematical model for the BZ reaction, provides the parameter area for successful unpinning. The scenario of unpinning is discussed in terms of the dispersion relation of the wave train by taking into account the curvature effect of the excitation wave. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3263167]

A chemical wave train propagating in a narrowing excitable channel surrounded by a nonexcitable field is investigated by using a photosensitive Belousov-Zhabotinsky (BZ) reaction. The considered geometry is created as a dark triangle surrounded by an illuminated area where the reaction is suppressed by the light-induced generation of bromide ion. For a low illumination level, a pulse train terminates at a constant position. However, as the light intensity increases, the position at which subsequent pulses disappear changes periodically, so that the period-doubling of penetration depth occurs. Two-dimensional simulations based on a modified Oregonator model for the photosensitive BZ reaction reproduce the essential features of the experimental observation.