北畑 裕之

Gold nanorods are generated spontaneously in a surfactant solution. We developed an experimental setup where the growth of gold nanorods can be completely stopped at any instant. With this method, a time series of the growth process of gold nanorods was determined by the direct observation of nanorods with transmission electron microscopy. We estimated the growth rate of nanorods from the change in the average long-axis length over time. To understand the experimental results, we developed a mathematical model for the growth of nanorods. The present results should help to clarify the mechanism of the growth of gold nanorods.

We investigate a simple experimental system using candles; stable combustion is seen when a single candle burns, while oscillatory combustion is seen when three candies burn together. If we consider a set of three candles as a component oscillator, two oscillators, that is, two sets of three candles, can Couple with each other, resulting in both in-phase and antiphase synchronization depending on the distance between the two sets. The mathematical model indicates that the oscillatory combustion in a set of three candles is induced by a lack of oxygen around the burning point. Furthermore, we suggest that thermal radiation may be an essential factor of the synchronization.

We study a discrete model described by coupled excitable elements following the monostable FitzHugh-Nagumo equations. Our model has a weakly coupled activator and a strongly coupled inhibitor. For two-coupled excitable elements, we show that the trivial state always exists stably, while nontrivial stable states appear depending on the coupling strengths. In a one-dimensional array, only the elements near the initial condition step remain at nontrivial states. We discuss stationary pattern formation in a one-dimensional array and a two-dimensional lattice using the analytical results of a two-coupled system.

The anomalous chemical wave propagation of an unstirred Belousov-Zhabotinsky (BZ) reaction was observed under exposure to a gradient static magnetic field (SMF). The gradient SMF effect on the BZ reaction was investigated by increasing the thickness of the BZ medium up to 0.9 mm under the conditions of the extremely reduced water evaporation and surface tension caused by air-water interfaces. The respective maximum values of magnetic flux density (B(max)), magnetic flux gradient (G(max)), and the magnetic force product of the magnetic flux density x its gradient (a magnetic force parameter) are 0.206 T, 37 T m(-1), and 4 T(2) m(-1). The experiments demonstrate that the more increased thickness of the BZ medium induces the larger anomalous wave propagation toward the peak magnetic gradient line but not toward the peak magnetic force product line. The anomalies were significantly enhanced by the increased thickness of the BZ medium at the shorter distance from the maximum magnetic gradient point. The possible mechanism of SMF-induced anomalous wave propagation related to the BZ medium thickness is that the micro-magneto-convection-induced flow of the paramagnetic it-on ion complexes at the wavefronts can be accelerated by increases in both the spatial magnetic gradient and the volumetric depth of the diffusion layer.

The spontaneous generation of a periodic hexagonal superstructure on a giant phospholipid sphere (GPS) with a diameter of 20-200 mu m was studied. The GPS was composed of ternary phospholipids consisting of dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), and dioleoylphosphatidyl-inositol-bisphosphate (DOPIP2). GPSs were prepared by natural swelling of a lipid film formed on a glass substrate. A GPS with a homogeneous lipid mixture tends to form a two-layered structure between the surface and inner parts; the surface layer is attributed to a DOPIP2 rich region (we call this layer SL), and the interior is rich in DOPE and DOPC (we call this layer IL). A hexagonal superstructure develops in the SL, and the topology then changes to form multiple-doughnut structures. Finally, myelin-like tubes are generated through symmetry breaking of the doughnutlike structures. The time-dependent change in the surface-area expansion of a GPS is shown to obey the logistic growth model, and this is attributed to the kinetic process of phase segregation between the surface and bulk phase of the GPS.

One-dimensional gold nanorods can grow spontaneously in a surfactant solution. In this study, a mixture of hexadecyltrimethylammonium bromide ( HTAB) and octadecyltrimethylammonium bromide (OTAB) was used as a surfactant solution. When the solution gelled, high-aspect-ratio gold nanorods, with an average long-axis length of similar to 1100 nm and an aspect-ratio of similar to 50, were synthesized with a yield of similar to 90%, whereas no high-aspect-ratio gold nanorods were synthesized when the solution did not gel. This phenomenon indicates that there is a close relation between the gelation of a surfactant solution and the elongation of gold nanorods. (c) 2008 Elsevier B. V. All rights reserved.

When a plastic bottle with water is placed upside-down with a thin pipe at its head. limit-cycle oscillation call be observed. A simple differential equation can reproduce the properties of this oscillation. In the present study, a three-coupled-oscillator system is investigated experimentally and numerically. The behavior of the three coupled oscillators with frustration is interesting from the viewpoint of mathematical physics, and with all increase in size, this system should be helpful in unconventional computing, such Lis in, for example, optimization problems.

Reaction-diffusion equations can be used only when the medium does not move. However, the medium itself usually moves in the real world. We introduce this effect by adding an advection term to the reaction-diffusion equation. This can be considered the natural generalization of the reaction-diffusion system. In this study. mutual coupling between pattern formation in reaction-diffusion systems and convective flow is investigated. Since the scales of the flow velocity and propaggating-wave velocity are comparable, novel phenomena can be observed in both experiments and numerical calculations. In the present article, some experimental results are given and a theoretical discussion and/or numerical modeling are presented from the viewpoint of reaction-diffusion systems with convective effects.

It is found that a micrometer-sized droplet in an aqueous solution of binary polymers [water/polyethylene-glycol (PEG)/dextran] disappears upon irradiation with a focused yttrium-aluminum-garnet laser. The interface of the dextran-rich droplet broadens and disappears, and it reappears upon turning the laser off, whereas the PEG-rich droplet shrinks and disappears. These phenomena are discussed in terms of the free energy by considering the laser-induced dielectric potential.

It is believed that static magnetic fields (SMF) cannot affect the pattern formation of the Belousov-Zhabotinsky (BZ) reaction, which has been frequently studied as a simplified experimental model of a nonequilibrium open system, because SMF produces no induced current and the magnetic force of SMF far below I T is too low to expect the effects on electrons in the BZ reaction. In the present study, we examined whether the velocity of chemical waves in the unstirred BZ reaction can be affected by a moderate-intensity SMF exposure depending on the spatial magnetic gradient. The SMF was generated by a parallel pair of attracting rectangular NdFeB magnets positioned opposite each other. The respective maximum values of magnetic flux density (B(max)), magnetic flux gradient (G(max)), and the magnetic force product of the magnetic flux density its gradient (a magnetic force parameter) were 206 mT, 37 mT/mm, and 3,000 mT(2)/mm. The ferroin-catalyzed BZ medium was exposed to the SMF for Lip to 16 min at 25 degrees C. The experiments demonstrated that the wave velocity was significantly accelerated primarily by the magnetic gradient. The propagation of the fastest wave front indicated a sigmoid increase along the peak magnetic gradient line, but not along the peak magnetic force product line. The underlying mechanisms of the SMF effects on the anomalous wave propagation could be attributed primarily to the increased concentration gradient of the paramagnetic iron ion complexes at the chemical wave fronts induced by the magnetic gradient. Bioelectromagnetics 29:598-604, 2008. (C) 2008 Wiley-Liss, Inc.

We report the dynamical behavior of mu m-sized phase separation for a homogeneous mixture of D2O and 3-methylpyridine (3MP) induced under continuous photon-flux by a focused YAG laser. At a water-rich composition, stable rhythmic oscillation of the emergence-growth escape of a 3MP-rich droplet is generated. In contrast, at a near-critical composition, an emergent droplet exhibits significant nonequilibrium fluctuation. Such dynamic phenomena are interpreted in terms of the effect of a local dielectric field and a dissipative system generated by a focused laser on the binary mixture.

The oscillatory motion of a water surface in contact with a fixed camphor disk was investigated. A periodic change in the contact length of the upheld water was synchronized with the changes in the force working on the disk. The period of the oscillation changed depending on the surface tension and the surface area of the water phase. The mechanism of oscillation is discussed in relation to the kinetics of the camphor molecular layer on the water surface and the change in the driving force working on the contact line due to the surface tension and the Marangoni flow. (C) 2008 Elsevier B.V. All rights reserved.

We present a model to describe the on-off switching of transcriptional activity in a genetic assembly by considering the intrinsic characteristics of a giant genomic DNA molecule which can undergo a discrete structural transition between coiled and compact states. We propose a model in which the transition in the higher-order structure of DNA plays an essential role in regulating stable on-off switching and/or the oscillation of a large number of genes under the fluctuations in a living cell, where such a structural transition is caused by environmental factors. This model explains the rapid and broad transcriptional response in a genetic assembly as well as its robustness against fluctuations.

We report the vectorial transport of a material object to a desired location by chemical waves generated in an excitable medium (Belousov-Zhabotinsky reaction). The transportation route is determined by the excitable waves induced at a certain point on the excitable medium. This study realized direct coupling between a chemical reaction and vectorial motion; such transduction of energy is generally found in biological motility but has been difficult to achieve in artificial model systems. We discuss the future possible application of these findings to the fabrication of a chemical machine that works under the self-management of motion in response to external stimuli.

Interfacial dynamical blebbing of an oil droplet on an aqueous solution is reported. The oil droplet and the aqueous solution contain a fatty acid and a cationic surfactant, respectively. When the oil droplet was placed on an aqueous surface, the oil-water interface formed blebs, spherical extrusions on the oil-water interface and circular ones on the edge of the oil droplet. With increase of the concentration of the surfactant and/or the fatty acid, the generated blebs become smaller. Based on the experimental observations together with theoretical considerations, we conclude that the bleb formation is induced through generation and destruction of a gel-like intermediate phase.

We recorded time series data of calls of Japanese tree frogs (Hyla japonica; Nihon-Ama-Gaeru) and examined the dynamics of the experimentally observed data not only through linear time series analysis such as power spectra but also through nonlinear time series analysis such as reconstruction of orbits with delay coordinates and different kinds of recurrence plots, namely the conventional recurrence plot (RP), the iso-directional recurrence plot (IDRP), and the iso-directional neighbors plot (IDNP). The results show that a single frog called nearly periodically, and a pair of frogs called nearly periodically but alternately in almost anti-phase synchronization with little overlap through mutual interaction. The fundamental frequency of the calls of a single frog during the interactive calling between two frogs was smaller than when the same frog first called alone. We also used the recurrence plots to study nonlinear and nonstationary determinism in the transition of the calling behavior. Moreover, we quantified the determinism of the nonlinear and nonstationary dynamics with indices of the ratio R of the number of points in IDNP to that in RP and the percentage P-D of contiguous points forming diagonal lines in RP by the recurrence quantification analysis (RQA). Finally, we discuss a possibility of mathematical modeling of the calling behavior and a possible biological meaning of the call alternation.

The oscillatory flow of water draining from an upside-down plastic bottle with a thin pipe attached to its head is studied as an example of a dissipative structure generated under far-from-equilibrium conditions. Mode bifurcation was observed in the water/air flow: no flow, oscillatory flow, and counter flow were found when the inner diameter of the thin pipe was changed. The modes are stable against perturbations. A coupled two-bottle system exhibits either in-phase or anti-phase self-synchronization. These characteristic behaviors imply that the essential features of the oscillatory flow in a single bottle system can be described as a limit-cycle oscillation. (c) 2007 American Association of Physics Teachers.

Excitable media show changes in their basic characteristics that reflect temporal changes in the environment. In the photosensitive Belousov-Zhabotinsky (BZ) reaction, excitability is decreased by illumination. We found that a traveling pulse failed to propagate when a certain level of light intensity was switched on abruptly, but the pulse continued propagating when the light intensity reached the same level gradually. We investigated the mechanism of adaptation of pulse propagation to the change in light intensity using two mathematical models, the Oregonator model (a specific model for the photosensitive BZ reaction), and the FitzHugh-Nagumo model (a generic model for excitable media). The appearance of a characteristic such as adaptation is shown to be a general feature for a traveling pulse in excitable media.

The photosensitive Belousov-Zhabotinsky (BZ) reaction was investigated on a double rectangular field composed of two rectangular routes, which was drawn using computer software and then projected using a liquid-crystal projector on a filter paper soaked with BZ solution. When two chemical waves were generated on the rectangular routes as the initial condition, the nature of the collision of the waves could be theoretically classified into four categories depending on the initial phase difference between the two waves and the aspect ratio of the rectangular routes. The experimental results were consistent with the features of the theoretical prediction. These results suggest that the feature of wave propagation characteristically develops depending on the geometry of the excitable fields.

This paper reports a novel crystal growth system of a coordination framework {[Cu-3(CN)(3){hat(CN)(3)(OEt)(3)}]}n (1) (hat-(CN)(3)(OEt)(3)) 2,6,10-tricyano-3,7,11-triethoxy-1,4,5,8,9,12-hexaazatriphenylene). The coordination polymer is crystallized through the reaction of 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12hexaazatriphenylene (hat-(CN)(6)), ethanol, and copper(I) complex, involving the breaking and forming of covalent bonds. The crystal morphologies obtained in the present system contain dumbbells, cogwheels, and superlattices. Moreover, in the growth perpendicular to the c-axis, periodic ramification at regular interval is observed, affording superlattice morphologies. Observation of the growth of dumbbell crystals shows that the growth rates parallel and perpendicular to the crystallographic c-axis are quite different: the former shows a drastic change with the reaction duration, while the latter is almost constant. These results are reproduced as a simple reaction-diffusion system, indicating that chemical reactions on crystal surfaces play an important role in determining the macroscopic crystal morphologies.

The photosensitive Belousov-Zhabotinsky (BZ) reaction was investigated on a circular ring, which was drawn using computer software and then projected on a film soaked with BZ solution using a liquid-crystal projector. Under the initial conditions, a chemical wave propagated with a constant velocity on the black ring under a bright background. When the background was rapidly changed to dark, coexistence of the oscillation on part of the ring and propagation of the chemical wave on the other part was observed. These experimental results are discussed in relation to the nature of the photosensitive BZ reaction and theoretically reproduced based on a reaction-diffusion system using the modified Oregonator model.

Recently, it has been found that a giant DNA molecule exhibits a repetitive change in conformation between an elongated coil and folded compact states under irradiation by a continuous IR laser. We report here a theoretical model to explain such spontaneous rhythmic motion. The generation of the rhythm can be attributed to the competition between optical attraction in the compact state and the instability of the compact state at the laser focus due to heating. This rhythmic conformational change is discussed in relation to the working mechanism of biological molecular motors.

Recently, it has been found that a giant DNA molecule exhibits a repetitive change in conformation between an elongated coil and folded compact states under irradiation by a continuous IR laser. We report here a theoretical model to explain such spontaneous rhythmic motion. The generation of the rhythm can be attributed to the competition between optical attraction in the compact state and the instability of the compact state at the laser focus due to heating. This rhythmic conformational change is discussed in relation to the working mechanism of biological molecular motors. © 2004 The American Physical Society.

Mode bifurcation was investigated when water was poured into a cup. Three modes, i.e., accumulation flow (mode I), scattering flow (mode II), and oscillatory flow (mode III), were regulated by the flow rate and the size of the tube through which the water was poured. When the tube was wide, the flow rate at bifurcation from mode I to mode II was different from that from mode II to mode I, i.e., hysteresis was observed. Mode III was observed only when the flow rate was increasing and the tube was narrow. The water hollow was an important factor in mode bifurcation. The behavior of the system was reproduced by a numerical simulation using differential equations that included the dynamics of two variables, the radius of the water hollow and the pressure exerted on the water hollow. (C) 2003 American Institute of Physics.

Rhythmic bursting on the order of seconds in a cluster of plastic beads under continuous irradiation of a focused neodymium-doped yttrium aluminum garnet (Nd:YAG) laser beam (1064 nm) is reported. The oscillatory instability is induced as a result of competition between trapping and scattering forces, where both forces are induced by the focused laser beam. Above a critical power of the laser beam, mode bifurcation from the stationary state into periodic bursting is observed. Our model employing ordinary differential equations reproduces the essential aspects of the experimental results.

The generation of convective flow by a chemical wave was studied experimentally on a mm-sized droplet of Belousov-Zhabotinsky (BZ) reaction medium. A propagating chemical wave causes a transient increase in interfacial tension, and this local change in interfacial tension induces convection. The observed flow profile was reproduced with a numerical simulation by introducing the transient increase in interfacial tension to a modified Navier-Stokes equation coupled with a chemical kinetic equation; a modified Oregonator. We also observed the periodic motion of a BZ droplet floating on an oil phase. Such periodic motion is attributed to the rhythmic change in interfacial tension. The observed periodic convective motion coupled with a chemical reaction is discussed in relation to chemo-mechanical energy transduction under isothermal conditions. (C) 2002 American Institute of Physics.

Mode-switching was investigated when water was poured into a cup, Three modes, i.e., accumulation flow (Mode I), scattering flow (Mode II), and oscillatory flow (Mode III), were regulated by the flow rate. The bifurcation flow rate at Mode I --> II was different from that at Mode II --> I, i,e., hysteresis in mode-switching was observed. Mode III was observed at Mode I --> II, The bifurcation point changed depending on the radius of the water tube, and the waterhollow was an important factor in mode-switching. The mode-switching was reproduced by a numerical calculation including the double-minimum profile of the free energy and the flow rate. (C) 2002 Elsevier Science B.V. All rights reserved.