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A02-Research Projects

Paper | Original Paper

2017

Masaya Kato, Xiao-Fei Zhang, and *Hiroki Saito,
Moving obstacle potential in a spin-orbit-coupled Bose-Einstein condensate,
Physical Review A 96, 033613/1-8 (2017).

[Summary] We investigate the dynamics around an obstacle potential moving in the plane-wave state of a pseudospin-1/2 Bose-Einstein condensate with Rashba spin-orbit coupling. We numerically investigate the dynamics of the system and find that it depends not only on the velocity of the obstacle but also significantly on the direction of obstacle motion, both of which are verified by the Bogoliubov analysis. The excitation diagram with respect to the velocity and direction is obtained. The dependence of the critical velocity on the strength of the spin-orbit coupling and the size of the obstacle is also investigated.

Yasuyuki Kobayashi*, Hiroyuki Kitahata, and Masaharu Nagayama,
Sustained dynamics of a weakly excitable system with nonlocal interactions,
Physical Review E 96, 022213/1-8 (2017).

[Summary] We investigate a two-dimensional spatially extended system that has a weak sense of excitability, where an excitation wave has a uniform profile and propagates only within a finite range. Using a cellular automaton model of such a weakly excitable system, we show that three kinds of sustained dynamics emerge when nonlocal spatial interactions are provided, where a chain of local wave propagation and nonlocal activation forms an elementary oscillatory cycle. Transition between different oscillation regimes can be understood as different ways of interactions among these cycles. Analytical expressions are given for the oscillation probability near the onset of oscillations.

Hiroki Saito,
Solving the Bose-Hubbard model with machine learning,
Journal of the Physical Society of Japan 86, 093001/1-4 (2017).

[Summary] Motivated by the recent successful application of artificial neural networks to quantum many-body problems [G. Carleo and M. Troyer, Science 355, 602 (2017)], a method to calculate the ground state of the Bose–Hubbard model using a feedforward neural network is proposed. The results are in good agreement with those obtained by exact diagonalization and the Gutzwiller approximation. The method of neural-network quantum states is promising for solving quantum many-body problems of ultracold atoms in optical lattices.

*Yuki Koyano, Marian Gryciuk, Paulina Skrobanska, Maciej Malecki, Yutaka Sumino, Hiroyuki Kitahata, and Jerzy Gorecki,
Relationship between the size of a camphor-driven rotor and its angular velocity,
Physical Review E 96, 021609/1-8 (2017).

[Summary] We consider a rotor made of two camphor disks glued below the ends of a plastic stripe. The disks are floating on a water surface and the plastic stripe does not touch the surface. The system can rotate around a vertical axis located at the center of the stripe. The disks dissipate camphor molecules. The driving momentum comes from the nonuniformity of surface tension resulting from inhomogeneous surface concentration of camphor molecules around the disks. We investigate the stationary angular velocity as a function of rotor radius ℓ. For large ℓ the angular velocity decreases for increasing ℓ. At a specific value of ℓ the angular velocity reaches its maximum and, for short ℓ it rapidly decreases. Such behavior is confirmed by a simple numerical model. The model also predicts that there is a critical rotor size below which it does not rotate. Within the introduced model we analyze the type of this bifurcation.

*Jerzy Gorecki, Hiroyuki Kitahata, Nobuhiko J. Suematsu, Yuki Koyano, Paulina Skrobanska, Marian Gryciuk, Maciej Malecki, Takahiro Tanabe, Hiroya Yamamoto, and Satoshi Nakata,
Unidirectional motion of a camphor disk on water forced by interactions between surface camphor concentration and dynamically changing boundaries,
Physical Chemistry Chemical Physics 19, 18767-18772 (2017).

[Summary] We study the motion of a camphor disk on the water surface in a system with flexible boundaries. The boundaries can be dynamically modified by non-uniform surface tension resulting from the nonhomogeneous surface concentration of the camphor molecules dissipated by the disk. We investigate the geometry of the boundaries that forces unidirectional motion of the disk. The studied system can be regarded as a signal diode if the presence or absence of a camphor disk at a specific point is interpreted as the binary TRUE and FALSE ariables. The diode can be incorporated into more complex devices, like a ring that imposes unidirectional rotation of camphor disks.

*Kei Nishi, Shogo Suzuki, Katsuhiko Kayahara, Masakazu Kuze, Hiroyuki Kitahata, *Satoshi Nakata, and *Yasumasa Nishiura,
Achilles’ heel of a traveling pulse subject to a local external stimulus,
Physical Review E 95, 062209/1-8 (2016).

[Summary] The response of a traveling pulse to a local external stimulus is considered numerically for a modified threecomponent Oregonator, which is a model system for the photosensitive Belousov-Zhabotinsky (BZ) reaction. The traveling pulse is traced and constantly stimulated, with the distance between the pulse and the stimulus being kept constant.We are interested in theminimal strength of the spatially localized stimulus in order to eliminate the pulse. The use of a stimulus of small width allows us to detect the point in the pulse most sensitive to the external stimulus, referred to as the “Achilles’ heel” of the traveling pulse, at which minimal strength of stimulus causes a collapse of the pulse. Our findings are demonstrated experimentally as well with the photosensitive BZ reaction.

Naoko Ueno, Taisuke Banno, Arisa Asami, Yuki Kazayama, Yuya Morimoto, Toshihisa Osaki, Shoji Takeuchi, Hiroyuki Kitahata, and *Taro Toyota,
Self-propelled motion of monodisperse underwater oil droplets formed by a microfluidic device,
Langmuir 33, 5593-5597 (2017).

[Summary] We evaluated the speed profile of self-propelled underwater oil droplets comprising a hydrophobic aldehyde derivative in terms of their diameter and the surrounding surfactant concentration using a microfluidic device. We found that the speed of the oil droplets is dependent on not only the surfactant concentration but also the droplet size in a certain range of the surfactant concentration. This tendency is interpreted in terms of combination of the oil and surfactant affording spontaneous emulsification in addition to the Marangoni effect.

Masaya Kato, Xiao-Fei Zhang, and *Hiroki Saito,
Vortex pairs in a spin-orbit coupled Bose-Einstein condensate,
Physical Review A 95, 043605/1-7 (2017).

[Summary] Static and dynamic properties of vortices in a two-component Bose-Einstein condensate with Rashba spin-orbit coupling are investigated. The mass current around a vortex core in the plane-wave phase is found to be deformed by the spin-orbit coupling, and this makes the dynamics of the vortex pairs quite different from those in a scalar Bose-Einstein condensate. The velocity of a vortex-antivortex pair is much smaller than that without spin-orbit coupling, and there exist stationary states. Two vortices with the same circulation move away from each other or unite to form a stationary state.

Xiao-Fei Zhang, Masaya Kato, Wei Han, Shou-Gang Zhang, and Hiroki Saito,
Spin-orbit-coupled Bose-Einstein condensates held under a toroidal trap,
Physical Review A 95, 033620 (2017).

[Summary] We study a quasispin-1/2 Bose-Einstein condensate with synthetically generated spin-orbit coupling in a toroidal trap and show that the system has a rich variety of ground states. As the central hole region increases, i.e., the potential changes from harmoniclike to ringlike, the condensate exhibits a variety of structures, such as a modified stripe, an alternately arranged stripe, and countercircling states. In the limit of a quasi-one-dimensional ring, the quantum many-body ground state is obtained, which is found to be the fragmented condensate.

*Hiroyuki Kitahata, Hiroya Yamamoto, Misato Hata, Yumihiko S. Ikura, *Satoshi Nakata,
Relaxation dynamics of the Marangoni convection roll structure induced by camphor concentration gradient,
Colloids and Surfaces A 520, 436-441 (2017).

[Summary] When a camphor disk is placed close to a water surface, Marangoni convection occurs due to the surfacetension gradient originating from the spatial distribution of camphor molecules at the water surface. Weput plastic floats on the water surface to investigate the surface Marangoni flow, and observed that theplastic floats moved away from the camphor disk due to Marangoni convection. When the camphor diskwas pulled up away from the water surface, the Marangoni convection weakened and finally disappeared.At that time, we observed that the floats approached the position just below the camphor disk. We discussthe mechanism of such float motion as related to the change in the structure of Marangoni convectionand the change in the water level.

2016

Yoriaki Nishioka, Fumiaki Kobayashi, Nobutaka Sakurai, *Yuji Sasaki, and Hiroshi Orihara,
Microscopic characterisation of self-assembled colloidal particles in electrohydrodynamic convection of a low-birefringence nematic liquid crystal,
Liquid Crystals 43(4), 427-435 (2016).

[Summary] Electrokinetics of small particles immersed in anisotropic fluids is attracting attention in recent years. Here we focus on microscopic appearance of single as well as self-assembled particles moving in the electrohydrodynamic convection (EHC) of a nematic liquid crystal with low birefringence. Characterisation of the birefringent properties is made by polarised light microscopy under different illumination conditions. Because of the small optical anisotropy, the director distortion around the particles clearly exhibits distinctive colours on both sides depending on the height in the cell. The observation can be explained as the change in the net phase retardation of the light. It is also found that a caterpillar-like motion is possible by tuning temperature, although the horizontal size of the EHC rolls is relatively narrow.

Dai Akita, Itsuki Kunita, Mark D Fricker, Shigeru Kuroda, Katsuhiko Sato and *Toshiyuki Nakagaki,
Experimental models for Murray’s law,
Journal of Physics D: Applied Physics 50, 024001 (2016).

[Summary] Transport networks are ubiquitous in multicellular organisms and include leaf veins, fungal mycelia and bloodvessels. While transport of materials and signals through the network plays a crucial role in maintaining theliving system, the transport capacity of the network can best be understood in terms of hydrodynamics. Wereport here that plasmodium from the large, single-celled amoeboid Physarum was able to construct ahydrodynamically optimized veinnetwork when evacuating biomass from confined arenas of various shapes througha narrow exit. Increasingly thick veins developed towards the exit, and the network spanned the arena viarepetitive bifurcations to give a branching tree. The Hausdorff distance from all parts of the plasmodium tothe vein network was kept low, whilst the hydrodynamic conductivity from distal parts of the network to theexit was equivalent, irrespective of the arena shape. This combination of spatial patterning and differentialvein thickening served to evacuate biomass at an equivalent rate across the entire arena. The scalingrelationship at the vein branches was determined experimentally to be 2.53.3.29, consistent with predictionsfrom Murray’s law. Furthermore, we show that mathematical models for self-organised, adaptive transport inPhysarum simulate the experimental network organisation well if the scaling coefficient of thecurrent-reinforcement rule is set to 3. In simulations, this resulted in rapid development of an optimalnetwork that minimised the combined volume and frictional energy in comparison with other scalingcoefficients. This would predict that the boundary shear forces within each vein are constant throughout thenetwork, and would be consistent with a feedback mechanism based on a sensing a threshold shear at the veinwall.

Yuji Sasaki, *V.S.R. Jampani, Chiharu Tanaka, Nobutaka Sakurai, Shin Sakane, Khoa V. Le, *Fumito Araoka, and *Hiroshi Orihara,
Large-scale self-organization of reconfigurable topological defect networks in nematic liquid crystals,
Nature Communications 7, 13238 (2016).

[Summary] Topological defects in nematic liquid crystals are ubiquitous. The defects are important in understanding the fundamental properties of the systems, as well as in practical applications, such as colloidal self-assembly, optical vortex generation and templates for molecular self-assembly. Usually, spatially and temporally stable defects require geometrical frustration imposed by surfaces; otherwise, the system relaxes because of the high cost of the elastic energy. So far, multiple defects are kept in bulk nematic liquid crystals by top-down lithographic techniques. In this work, we stabilize a large number of umbilical defects by doping with an ionic impurity. This method does not require pre-patterned surfaces. We demonstrate that molecular reorientation controlled by an AC voltage induces periodic density modulation of ions accumulated at an electrically insulating polymer interface, resulting in self-organization of a two-dimensional square array of umbilical defects that is reconfigurable and tunable.

*Xiao-Fei Zhang, Wei Han, Hai-Feng Jiang, Wu-Ming Liu, Hiroki Saito, and Shou-Gang Zhang,
Topological defect formation in rotating binary dipolar Bose-Einstein condensate,
Annals of Physics 375, 368-377 (2016).

[Summary] We investigate the topological defects and spin structures of a rotating binary Bose–Einstein condensate, which consists of both dipolar and scalar bosonic atoms confined in spin-dependent optical lattices, for an arbitrary orientation of the dipoles with respect to their plane of motion. Our results show that the tunable dipolar interaction, especially the orientation of the dipoles, can be used to control the direction of stripe phase and its related half-vortex sheets. In addition, it can also be used to obtain a regular arrangement of various topological spin textures, such as meron, circular and cross disgyration spin structures. We point out that such topological defects and regular arrangement of spin structures arise primarily from the long-range and anisotropic nature of dipolar interaction and its competition with the spin-dependent optical lattices and rotation.

Masaya Kato, Xiao-Fei Zhang, Daichi Sasaki, and Hiroki Saito,
Twisted spin vortices in a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction,
Physical Review A 94, 043633/1-6 (2016).

[Summary] We consider a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction confined in a cigar-shaped trap. Due to the combined effects of spin-orbit coupling, dipole-dipole interaction, and trap geometry, the system exhibits a rich variety of ground-state spin structures, including twisted spin vortices. The ground-state phase diagram is determined with respect to the strengths of the spin-orbit coupling and dipole-dipole interaction.

Yuki Koyano, Tatsunari Sakurai, and *Hiroyuki Kitahata,
Oscillatory motion of a camphor grain in a one-dimensional finite region,
Physical Review E 94, 042215/1-8 (2016).

[Summary] The motion of a self-propelled particle is affected by its surroundings, such as boundaries or external fields. In this paper, we investigated the bifurcation of the motion of a camphor grain, as a simple actual self-propelled system, confined in a one-dimensional finite region. A camphor grain exhibits oscillatory motion or remains at rest around the center position in a one-dimensional finite water channel, depending on the length of the water channel and the resistance coefficient. A mathematical model including the boundary effect is analytically reduced to an ordinary differential equation. Linear stability analysis reveals that the Hopf bifurcation occurs, reflecting the symmetry of the system.

Hiroki Saito and Rina Kanamoto,
Self-rotation and synchronization in exciton-polariton condensates,
Physical Review B 94, 165306 (2016).

[Summary] Self-rotation occurs in an exciton-polariton condensate in a two-dimensional semiconductor microcavity pumped by a nonresonant Gaussian laser beam. A wave packet of the condensate spontaneously rotates around the center of the pumped region at a constant frequency breaking the rotation symmetry of the system. When two self-rotating condensates are created with an appropriate distance, synchronization occurs between the dynamics of the self-rotating condensates.

Akihiko Nakajima, Motohiko Ishida, Taihei Fujimori, Yuichi Wakamoto, *Satoshi Sawai,
The Microfluidic lighthouse: an omnidirectional gradient generator,
Lab on Chip 16(22), 4382-4394 (2016).

[Summary] Studies of chemotactic cell migration rely heavily on various assay systems designed to evaluate the ability of cells to move in response to attractant molecules. In particular, the development of microfluidics-based devices in recent years has made it possible to spatially distribute attractant molecules in graded profiles that are sufficiently stable and precise to test theoretical predictions regarding the accuracy and efficiency of chemotaxis and the underlying mechanism of stimulus perception. However, because the gradient is fixed in a direction orthogonal to the laminar flow and thus the chamber geometry, conventional devices are limited for the study of cell re-orientation to gradients that move or change directions. Here, we describe the development of a simple radially symmetric microfluidics device that can deliver laminar flow in 360°. A stimulant introduced either from the central inlet or by photo uncaging is focused into the laminar flow in a direction determined by the relative rate of regulated flow from multiple side channels. Schemes for flow regulation and an extended duplexed device were designed to generate and move gradients in desired orientations and speed, and then tested to steer cell migration of Dictyostelium and neutrophil-like HL60 cells. The device provided a high degree of freedom in the positioning and orientation of attractant gradients, and thus may serve as a versatile platform for studying cell migration, re-orientation, and steering.

*Takayuki Narumi, Yosuke Mikami, Tomoyuki Nagaya, Hirotaka Okabe, Kazuhiro Hara, and Yoshiki Hidaka,
Relaxation with long-period oscillation in defect turbulence of planar nematic liquid crystals,
Physical Review E 94, 042701/1-6 (2016).

[Summary] Through experiments, we studied defect turbulence, a type of spatiotemporal chaos in planar systems of nematic liquid crystals, to clarify the chaotic advection of weak turbulence. In planar systems of large aspect ratio, structural relaxation, which is characterized by the dynamic structure factor, exhibits a long-period oscillation that is described well by a combination of a simple exponential relaxation and underdamped oscillation. The simple relaxation arises as a result of the roll modulation while the damped oscillation is manifest in the repetitive gliding of defect pairs in a local area. Each relaxation is derived analytically by the projection operator method that separates turbulent transport into a macroscopic contribution and fluctuations. The analysis proposes that the two relaxations are not correlated. The nonthermal fluctuations of defect turbulence are consequently separated into two independent Markov processes. Our approach sheds light on diversity and universality from a unified viewpoint for weak turbulence.

*Wei Han, Xiao-Fei Zhang, Shu-Wei Song, Hiroki Saito, Wei Zhang, Wu-Ming Liu, and Shou-Gang Zhang,
Double-quantum spin vortices in SU(3) spin-orbit coupled Bose gases,
Physical Review A 94, 033611/1-9 (2016).

[Summary] We show that double-quantum spin vortices, which are characterized by doubly quantized circulating spin currents and unmagnetized filled cores, can exist in the ground states of SU(3) spin-orbit-coupled Bose gases. It is found that the SU(3) spin-orbit coupling and spin-exchange interaction play important roles in determining the ground-state phase diagram. In the case of effective ferromagnetic spin interaction, the SU(3) spin-orbit coupling induces a threefold degeneracy to the magnetized ground state, while in the antiferromagnetic spin interaction case, the SU(3) spin-orbit coupling breaks the ordinary phase rule of spinor Bose gases and allows the spontaneous emergence of double-quantum spin vortices. This exotic topological defect is in stark contrast to the singly quantized spin vortices observed in existing experiments and can be readily observed by the current magnetization-sensitive phase-contrast imaging technique.

Elisa Herawati, Daisuke Taniguchi, Hatsuho Kanoh, Kazuhiro Tateishi, Shuji Ishihara, and Sachiko Tsukita,
Multiciliated cell basal bodies align in stereotypical patterns coordinated by the apical cytoskeleto,
The Journal of Cell Biology 214, 571–586 (2016).

[Summary] Multiciliated cells (MCCs) promote fluid flow through coordinated ciliary beating, which requires properly organized basal bodies (BBs). Airway MCCs have large numbers of BBs, which are uniformly oriented and, as we show here, align linearly. The mechanism for BB alignment is unexplored. To study this mechanism, we developed a long-term and high-resolution live-imaging system and used it to observe green fluorescent protein–centrin2–labeled BBs in cultured mouse tracheal MCCs. During MCC differentiation, the BB array adopted four stereotypical patterns, from a clustering “floret” pattern to the linear “alignment.” This alignment process was correlated with BB orientations, revealed by double immunostaining for BBs and their asymmetrically associated basal feet (BF). The BB alignment was disrupted by disturbing apical microtubules with nocodazole and by a BF-depleting Odf2 mutation. We constructed a theoretical model, which indicated that the apical cytoskeleton, acting like a viscoelastic fluid, provides a self-organizing mechanism in tracheal MCCs to align BBs linearly for mucociliary transport.

Yuki Koyano, Hiroyuki Kitahata, *Alexander S. Mikhailov,
Hydrodynamic collective effects of active proteins in biological membranes,
Physical Review E 94, 022416/1-11 (2016).

[Summary] Lipid bilayers forming biological membranes are known to behave as viscous two-dimensional fluids on submicrometer scales; usually they contain a large number of active protein inclusions. Recently, it was shown that such active proteins should induce nonthermal fluctuating lipid flows leading to diffusion enhancement and chemotaxislike drift for passive inclusions in biomembranes. Here, a detailed analytical and numerical investigation of such effects is performed. The attention is focused on the situations when proteins are concentrated within lipid rafts. We demonstrate that passive particles tend to become attracted by active rafts and are accumulated inside them.

Koutaro Nakagome, Katsuhiko Sato, Seine A. Shintani, *Shin’ichi Ishiwata,
Model simulation of the SPOC wave in a bundle of striated myofibrils,
Biophysics and Physicobiology 13, 217-226 (2016).

[Summary] SPOC (spontaneous oscillatory contraction) is a phenomenon observed in striated muscle under intermediateactivation conditions. Recently, we constructed a theoretical model of SPOC for a sarcomere, a unit sarcomeremodel, which explains the behavior of SPOC at each sarcomere level. We also constructed a single myofibrilmodel, which visco-elastically connects the unit model in series, and explains the behaviors of SPOC at themyofibril level. In the present study, to understand the SPOC properties in a bundle of myofibrils, weextended the single myofibril model to a two-dimensional (2D) model and a three-dimensional (3D) model, inwhich myofibrils were elastically connected side-by-side through cross-linkers between the Z-lines andM-lines. These 2D and 3D myofibril models could reproduce various patterns of SPOC waves experimentallyobserved in a 2D sheet and a 3D bundle of myofibrils only by choosing different values of elastic constants ofthe cross-linkers and the external spring. The results of these 2D and 3D myofibril models provide insightinto the SPOC properties of the higher-ordered assembly of myofibrils.

Taisuke Banno, Arisa Asami, Naoko Ueno, Hiroyuki Kitahata, Yuki Koyano, Kouichi Asakura, *Taro Toyota,
Deformable self-propelled micro-object comprising underwater oil droplets,
Scientific Reports 6, 31292/1-9 (2016).

[Summary] The self-propelled motion with deformation of micrometer-sized soft matter in water has potential application not only for underwater carriers or probes in very narrow spaces but also for understanding cell locomotion in terms of non-equilibrium physics. As far as we know, there have been no reports about micrometer-sized self-propelled soft matter mimicking amoeboid motion underwater. Here, we report an artificial molecular system of underwater oil droplets exhibiting self-propelled motion with deformation as an initial experimental model. We describe the heterogeneity in a deformable self-propelled oil droplet system in aqueous and oil phases and at their interface based on the behavior and interaction of surfactant and oil molecules. The current results have great importance for scientific frontiers such as developing deformable micro-swimmers and exploring the emergence of self-locomotion of oil droplet-type protocells.

*Satoshi Nakata, Hiroya Yamamoto, Yuki Koyano, Osamu Yamanaka, Yutaka Sumino, Nobuhiko J. Suematsu, Hiroyuki Kitahata, Paulina Skrobanska, and Jerzy Gorecki,
Selection of the Rotation Direction for a Camphor Disk Resulting from Chiral Asymmetry of a Water Chamber,
Journal of Physical Chemistry B 120, 9166-9172 (2016).

[Summary] Self-motion of a camphor disk rotating inside a water chamber composed of two half-disks was investigated. The half-disks were joined along their diameter segments, and the distance between their midpoints (ds) was considered as the control parameter. Various types of camphor disk motions were observed depending on ds. When ds = 0, the chamber had a circular shape, so it was symmetric. A camphor disk showed either a clockwise (CW) or ounterclockwise (CCW) rotation with the direction determined by its initial state. The symmetry of the chamber was broken for ds > 0. For moderate distances between the midpoints, a unidirectional orbital motion of the disk was observed. The preferred rotation direction was determined by the shape of the chamber, and it did not depend on the initial rotation direction. For yet larger ds, the unidirectional circular motion was no longer observed and the trajectory became irregular. A mathematical model coupling the camphor disk motion with the dynamics of the developed camphor molecular layer on water was constructed, and the numerical results were compared with the experimental results. The selection of motion type can be explained by considering the influence of camphor concentration on the disk trajectory through the surface tension gradient.

Takayuki Torisawa, Daisuke Taniguchi, Shuji Ishihara, Kazuhiro Oiwa,
Spontaneous Formation of a Globally Connected Contractile Network in a Microtubule-Motor System,
Biophysical Journal 111, 373–385 (2016).

[Summary] Microtubule (MT) networks play key roles in cell division, intracellular transport, and cell motility. These functions of MT networks occur through interactions between MTs and various associated proteins, notably motor proteins that bundle and slide MTs. Our objective in this study was to address the question of how motors determine the nature of MT networks. We conducted in vitro assays using homotetrameric kinesin Eg5, a motor protein involved in the formation and maintenance of the mitotic spindle. The mixing of Eg5 and MTs produced a range of spatiotemporal dynamics depending on the motor/filament ratio. Low motor/filament ratios produced globally connected static MT networks with sparsely distributed contractile active nodes (motor-accumulating points with radially extending MTs). Increasing the motor/filament ratio facilitated the linking of contractile active nodes and led to a global contraction of the network. When the motor/filament ratio was further increased, densely distributed active nodes formed local clusters and segmented the network into pieces with their strong contractile forces. Altering the properties of the motor through the use of chimeric Eg5, which has kinesin-1 heads, resulted in the generation of many isolated asters. These results suggest that the spatial distribution of contractile active nodes determines the dynamics of MT-motor networks. We then developed a coarse-grained model of MT-motor networks and identified two essential features for reproducing the experimentally observed patterns: an accumulation of motors that form the active nodes necessary to generate contractile forces, and a nonlinear dependency of contractile force on motor densities. Our model also enabled us to characterize the mechanical properties of the contractile network. Our study provides insight into how local motor-MT interactions generate the spatiotemporal dynamics of macroscopic network structures.

*Yujiro Eto, Masahiro Takahashi, Masaya Kunimi, Hiroki Saito, and Takuya Hirano,
Nonequilibrium dynamics induced by miscible-immiscible transition in binary Bose-Einstein condensates,
New Journal of Physics 17, 0703029/1-6 (2016).

[Summary] We have observed and characterized the nonequilibrium spatial dynamics of a two-component 87Rb Bose–Einstein condensate (BEC) that is controllable switched back and forth between the miscible and immiscible phases of the phase separation transition by changing the internal states of the 87Rb atoms. The subsequent evolution exhibits large scale oscillations of the spatial structure that involve component mixing and separation. We show that the larger total energy of the miscible system results in a higher oscillation frequency. This investigation introduces a new technique to control the miscibility and the spatial degrees of freedom in atomic BECs.

Fumihito Fukujin, Akihiko Nakajima, Nao Shimada, *Satoshi Sawai,
Self-organization of chemoattractant waves in Dictyostelium depends on F-actin and cell–substrate adhesion,
Journal of The Royal Society Interface 13 (2016).

[Summary] In the social amoeba Dictyostelium discoideum, travelling waves of extracellular cyclic adenosine monophosphate (cAMP) self-organize in cell populations and direct aggregation of individual cells to form multicellular fruiting bodies. In contrast to the large body of studies that addressed how movement of cells is determined by spatial and temporal cues encoded in the dynamic cAMP gradients, how cell mechanics affect the formation of a self-generated chemoattractant field has received less attention. Here, we show, by live cell imaging analysis, that the periodicity of the synchronized cAMP waves increases in cells treated with the actin inhibitor latrunculin. Detail analysis of the extracellular cAMP-induced transients of cytosolic cAMP (cAMP relay response) in well-isolated cells demonstrated that their amplitude and duration were markedly reduced in latrunculin-treated cells. Similarly, in cells strongly adhered to a poly-l-lysine-coated surface, the response was suppressed, and the periodicity of the population-level oscillations was markedly lengthened. Our results suggest that cortical F-actin is dispensable for the basic low amplitude relay response but essential for its full amplification and that this enhanced response is necessary to establish high-frequency signalling centres. The observed F-actin dependence may prevent aggregation centres from establishing in microenvironments that are incompatible with cell migration.

*Tomoyuki Nagaya, Yuki Satou, Yoshitomo Goto, Yoshiki Hidaka, and Hiroshi Orihara,
Viscosity of Liquid Crystal Mixtures in the Presence of Electroconvection,
Journal of the Physical Society of Japan 85, 074002/1-4 (2016).

[Summary] We have experimentally investigated the viscosity of nematic liquid crystal mixtures of p-methoxybenzylidene-pA-n- butylaniline (MBBA) and p-ethoxybenzylidene-pA-cyanoaniline (EBCA) in the presence of electroconvection under an ac electric field with 60Hz. Although the viscosity of the mixtures with negative dielectric anisotropy shows a characteristic decrease in the high-voltage regime, that with positive dielectric anisotropy shows a monotonic increase as the applied voltage is increased. The experimental results suggest that the decrease in viscosity observed only for the mixtures with negative dielectric anisotropy is attributed to the negative contribution of electric stress caused by the anisotropic director distribution of the turbulent state.

Hironobu Nogucci and Shuji Ishihara,
Collective dynamics of active filament complexes,
Physical Review E 93, 052406/1-10 (2016).

[Summary] Networks of biofilaments are essential for the formation of cellular structures that support various biologicalfunctions. For the most part, previous studies have investigated the collective dynamics of rodlike biofilaments;however, the shapes of the actual subcellular components are often more elaborate. In this study, weconsidered an active object composed of two active filaments, which represents the progression from rodlikebiofilaments to complex-shaped biofilaments. Specifically, we numerically assessed the collective behaviorsof these active objects in two dimensions and observed several types of dynamics, depending on the densityand the angle of the two filaments as shape parameters of the object. Among the observed collectivedynamics, a moving density band that we named a “moving smectic” is introduced here for the first time.By analyzing the trajectories of individual objects and the interactions among them, this study demonstrated howinteractions among active biofilaments with complex shapes could produce collective dynamics in a nontrivialmanner.

Yui Matsuda, Nobuhiko J. Suematsu, Hiroyuki Kitahata, Yumihiko S.Ikura, and *Satoshi Nakata,
Acceleration or deeleration of self-motion by the Marangoni effect,
Chemical Physics Letters 654, 92-96 (2016).

[Summary] We investigated the water-depth dependence of the self-motion of a camphor disk and camphor boat. With increasing water depth, the speed of motion of the camphor disk increased, but that of the camphor boat decreased in an annular one-dimensional system. We discussed the difference in the water-depth dependence of the speed of the camphor objects in relation to Marangoni flow. We concluded that Marangoni flow, which became stronger with increasing the water depth, positively and negatively affected the speed of the disk and boat, respectively.

Hiroki Saito,
Path-integral Monte Carlo study on a droplet of a dipolar Bose-Einstein condensate stabilized by quantum fluctuation,
Journal of the Physical Society of Japan 85, 053001 (2016).

[Summary] Motivated by recent experiments [H. Kadau et al., Nature (London) 530, 194 (2016); I. Ferrier-Barbut et al., arXiv:1601.03318] and theoretical prediction (F. Wächtler and L. Santos, arXiv:1601.04501), the ground state of a dysprosium Bose–Einstein condensate with strong dipole–dipole interaction is studied by the path-integral Monte Carlo method. It is shown that quantum fluctuation can stabilize the condensate against dipolar collapse.

Ken H. Nagai, Kunihito Tachibana, Yuta Tobe, Masaki Kazama, Hiroyuki Kitahata, Seiro Omata, *Masaharu Nagayama,
Mathematical model for self-propelled droplets driven by interfacial tension,
Journal of Chemical Physics 144, 114707/1-8 (2016).

[Summary] We propose a model for the spontaneous motion of a droplet induced by inhomogeneity in interfacial tension. The model is derived from a variation of the Lagrangian of the system and we use a time-discretized Morse flow scheme to perform its numerical simulations. Our model can naturally simulate the dynamics of a single droplet, as well as that of multiple droplets, where the volume of each droplet is conserved. We reproduced the ballistic motion and fission of a droplet, and the collision of two droplets was also examined numerically.

*Yujiro Eto, Masahiro Takahashi, Keita Nabeta, Ryotaro Okada, Masaya Kunimi, Hiroki Saito, and Takuya Hirano,
Bouncing motion and penetration dynamics in multicomponent Bose-Einstein condensates,
Physical Review A 93, 033615/1-6 (2016).

[Summary] We investigate the dynamic properties of bouncing and penetration in colliding binary and ternary Bose-Einstein condensates comprised of different Zeeman or hyperfine states of 87Rb. Through the application of magnetic field gradient pulses, two- or three-component condensates in an optical trap are spatially separated and then made to collide. The subsequent evolutions are classified into two categories: repeated bouncing motion and mutual penetration after damped bounces. We experimentally observed mutual penetration for immiscible condensates, bouncing between miscible condensates, and domain formation for miscible condensates. From numerical simulations of the Gross-Pitaevskii equation, we find that the penetration time can be tuned by slightly changing the atomic interaction strengths.

Tomoya Kaneda and Hiroki Saito,
Collision dynamics of Skyrmions in a two-component Bose-Einsteincondensate,
Physical Review A 93, 033611/1-6 (2016).

[Summary] The dynamics of skyrmions in a two-component Bose-Einstein condensate is numerically investigated in the mean-field theory. When two skyrmions collide with each other, they are first united and then scattered into various states. For head-on collisions, skyrmions with unit winding number are scattered. The collision dynamics with an impact parameter are shown to depend on the relative phase. These dynamic processes are characterized by integer winding numbers.

*Yutaka Sumino, Norifumi L. Yamada, Michihiro Nagao, Takuya Honda, Hiroyuki Kitahata, Yuri B. Melnichenko, and Hideki Seto,
Mechanism of Spontaneous Blebbing Motion of an Oil−Water Interface: Elastic Stress Generated by a Lamellar−Lamellar Transition,
Langmuir 32, 2891-2899 (2016).

[Summary] A quaternary system composed of surfactant, cosurfactant, oil, and water showing spontaneous motion of the oil–water interface under far-from-equilibrium condition is studied in order to understand nanometer-scale structures and their roles in spontaneous motion. The interfacial motion is characterized by the repetitive extension and retraction of spherical protrusions at the interface, i.e, blebbing motion. During the blebbing motion, elastic aggregates are accumulated, which were characterized as surfactant lamellar structures with mean repeat distances d of 25 to 40 nm. Still unclear is the relationship between the structure formation and the dynamics of the interfacial motion. In the present study, we find that a new lamellar structure with d larger than 80 nm is formed at the blebbing oil–water interface, while the resultant elastic aggregates, which are the one reported before, have a lamellar structure with smaller d (25 to 40 nm). Such transition of lamellar structures from the larger d to smaller d is induced by a penetration of surfactants from an aqueous phase into the aggregates. We propose a model in which elastic stress generated by the transition drives the blebbing motion at the interface. The present results explain the link between nanometer-scale transition of lamellar structure and millimeter-scale dynamics at an oil–water interface.

Masanobu Horie, Tatsunari Sakurai, and *Hiroyuki Kitahata,
Experimental and theoretical approach for the clustering of globally coupled density oscillators based on phase response,
Physical Review E 93, 012212/1-9 (2016).

[Summary] We investigated the phase-response curve of a coupled system of density oscillatorswith an analytical approach. The behaviors of two-, three-, and four-coupled systems seen in the experiments were reproduced by the model considering the phase-response curve. Especially in a four-coupled system, the clustering state and its incidence rate as functions of the coupling strength are well reproduced with this approach.Moreover, we confirmed that the shape of the phase-response curve we obtained analytically was close to that observed in the experiment where a perturbation is added to a single-density oscillator. We expect that this approach to obtaining the phase-response curve is general in the sense that it could be applied to coupled systems of other oscillators such as electrical-circuit oscillators, metronomes, and so on.

Haruka Sugiura, Manami Ito, Tomoya Okuaki, Yoshihito Mori, Hiroyuki Kitahata, and *Masahiro Takinoue,
Pulse-density modulation control of chemical oscillation far from equilibrium in a droplet open-reactor system,
Nature Communications 7, 10212/1-9 (2016).

[Summary] The design, construction and control of artificial self-organized systems modelled on dynamical behaviours of living systems are important issues in biologically inspired engineering. Such systems are usually based on complex reaction dynamics far from equilibrium; therefore, the control of non-equilibrium conditions is required. Here we report a droplet open-reactor system, based on droplet fusion and fission, that achieves dynamical control over chemical fluxes into/out of the reactor for chemical reactions far from equilibrium. We mathematically reveal that the control mechanism is formulated as pulse-density modulation control of the fusion–fission timing. We produce the droplet open-reactor system using microfluidic technologies and then perform external control and autonomous feedback control over autocatalytic chemical oscillation reactions far from equilibrium. We believe that this system will be valuable for the dynamical control over self-organized phenomena far from equilibrium in chemical and biomedical studies.

Kui-Tian Xi and Hiroki Saito,
Droplet formation in a Bose-Einstein condensate with strong dipole-dipole interaction,
Physical Review A 93, 011604(R) (2016).

[Summary] Motivated by the recent experiment [H. Kadau et al., arXiv:1508.05007], we study roton instability and droplet formation in a Bose-Einstein condensate of Dy164 atoms with strong magnetic dipole-dipole interaction. We numerically solve the cubic-quintic Gross-Pitaevskii equation with dipole-dipole interaction, and show that the three-body interaction plays a significant role in the formation of droplet patterns. We numerically demonstrate the formation of droplet patterns and crystalline structures, decay of droplets, and hysteresis behavior, which are in good agreement with the experiment. Our numerical simulations provide the first prediction on the values of the three-body interaction in a Dy164 Bose-Einstein condensate. We also predict that the droplets remain stable during the time-of-flight expansion. From our results, further experiments investigating the three-body interaction in dipolar quantum gases are required.

2015

Boris Guirao, Stéphane Rigaud, Floris Bosveld, Anaïs Bailles, Jesus Lopez-Gay, Shuji Ishihara, Kaoru Sugimura, *François Graner, *Yohanns Bellaïche,
Unified quantitative characterization of epithelial tissue development,
eLIFE , 08519 (2015).

[Summary] Understanding the mechanisms regulating development requires a quantitative characterization of cell divisions, rearrangements, cell size and shape changes, and apoptoses. We developed a multiscale formalism that relates the characterizations of each cell process to tissue growth and morphogenesis. Having validated the formalism on computer simulations, we quantifed separately all morphogenetic events in the Drosophila wing and dorsal thorax pupal epithelia to obtain comprehensive statistical maps linking cell and tissue scale dynamics. While globally cell shape changes, rearrangements and divisions all signifcantly participate in tissue morphogenesis, locally, their relative participations display major variations in space and time. By blocking division we analyzed the impact of division on rearrangements, cell shape changes and tissue morphogenesis. Finally, by combining the formalism with mechanical stress measurement, we evidenced unexpected interplays between patterns of tissue elongation, cell division and stress. Our formalism provides a novel and rigorous approach to uncover mechanisms governing tissue development.

*Hiroyuki Kitahata, Rui Tanaka, Yuki Koyano, Satoshi Matsumoto, Katsuhiro Nishinari, Tadashi Watanabe,Koji Hasegawa, Tetsuya Kanagawa, Akiko Kaneko, and Yutaka Abe,
Oscillation of a rotating levitated droplet: Analysis with a mechanical model,
Physical Review E 92, 062904/1-8 (2015).

[Summary] A droplet of millimeter-to-centimeter scale can exhibit electrostatic levitation, and such levitated droplets can be used for the measurement of the surface tension of the liquids by observing the characteristic frequency of oscillatory deformation. In the present study, a simple mechanical model is proposed by considering a single mode of oscillation in the ellipsoidal deformation of a levitated rotating droplet. By measuring the oscillation frequency with respect to the rotational speed and oscillation amplitude, it is expected that the accuracy of the surface tensionmeasurement could be improved. Using the proposed model, the dependences of the characteristic frequency of oscillatory deformation and the averaged aspect ratio are calculated with respect to the rotational angular velocity of a rotating droplet. These dependences are found to be consistent with the experimental observations.

Masanobu Tanaka, Marcel Hörning, *Hiroyuki Kitahata, and Kenichi Yoshikawa,
Elimination of a spiral wave pinned at an obstacle by a train of plane waves: Effect of diffusion between obstacles and surrounding media,
Chaos 25, 103127/1-9 (2015).

[Summary] In excitable media such as cardiac tissue and Belousov-Zhabotinsky reaction medium, spiral waves tend to anchor (pin) to local heterogeneities. In general, such pinned waves are difficult to eliminate and may progress to spatio-temporal chaos. Heterogeneities can be classified as either the absence or presence of diffusive interaction with the surrounding medium. In this study, we investigated the difference in the unpinning of spiral waves from obstacles with and without diffusive interaction, and found a profound difference. The pacing period required for unpinning at fixed obstacle size is larger in case of diffusive obstacles. Further, we deduced a generic theoretical framework that can predict the minimal unpinning period. Our results explain the difference in pacing periods betweenfor the obstacles with and without diffusive interaction, and the difference is interpreted in terms of the local decrease of spiral wave velocity close to the obstacle boundary caused in the case of diffusive interaction.

*Hiroki Saito,
Can we swim in superfluids?: Numerical demonstration of self-propulsion in a Bose-Einstein condensate,
Journal of the Physical Society of Japan 84, 114001/1-6 (2015).

[Summary] We numerically investigated whether a deformable object can propel itself in a superfluid. Articulated bodies and multicomponent condensates are examined as swimmers. An articulated two-body swimmer cannot obtain locomotion without emitting excitations. More flexible swimmers can do so without the need to excite waves.

*Shigeru Kuroda, Seiji Takagi, Toshiyuki Nakagaki and Tetsuo Ueda,
Allometry in Physarum plasmodium during free locomotion: size versus shape, speed and rhythm,
Journal of Experimental Biology 218, 3729-3738 (2015).

[Summary] Physarum plasmodium is a giant unicellular organism whose lengthcan vary by more than three orders of magnitude. Using plasmodiaranging in size from 100 μm to 10 cm, we investigated the sizedependency of their thickness distributions and locomotion speedsduring free locomotion. (1) In the longitudinal direction, the organismis thickest close to the front, and decreases exponentially in thicknesstowards the rear. The slenderness ratio varies with body sizeaccording to a power law, such that large plasmodia are long andflat, whereas small plasmodia are short and thick. (2) The meanlocomotion speed is proportional to the mean maximum thickness ofthe frontal part. By conducting a dimensional analysis, possiblephysical models are discussed. (3) The intrinsic period of thethickness oscillation, which is related to shuttle streaming (period1–2 min), increases logarithmically with body size. (4) Variouscharacteristics exhibit size-independent, long-period (20±10 min)oscillations, including speed, shape and intrinsic thicknessoscillation period. These variations are closely coupled to formationof the entire cell shape, including undulation of thickness along thelongitudinal axis and timing of branching of the frontal tip. Based onthese experimental results and those reported previously, wepropose a simple mathematical model for cell locomotion.

*Yoshiki Hidaka, Megumi Hashiguchi, Noriko Oikawa, Shoichi Kai,
Lagrangian chaos and particle diffusion in electroconvection of planar nematic liquid crystals,
Physical Review E 92, 032909/1-6 (2015).

[Summary] Two types of spatiotemporal chaos in the electroconvection of nematic liquid crystals, such as defect turbulence and spatiotemporal intermittency, have been statistically investigated according to the Lagrangian picture. Here fluctuations are traced using the motion of a single particle driven by chaotic convection. In the defect turbulence (fluctuating normal rolls), a particle is mainly trapped in a roll but sometimes jumps to a neighboring roll. Its activation energy is then obtained from the jumping (hopping) rate. This research clarifies that diffusion in the defect turbulence regime in electroconvection can be regarded as a kind of hopping process. The spatiotemporal intermittency appears as a coexistent state of ordered grid domains and turbulent domains. The motion of a single particle shows weak and strong diffusion, respectively, in the ordered and turbulent domains. The diffusion characteristics intermittently change from one to another with certain durations as the domains change. This research has found that the distribution function of the duration that a particle remains in an ordered area has a power-law decay for which the index is different from that obtained by the Eulerian measurement.

Shingo Miyazaki, Tatsunari Sakurai, and *Hiroyuki Kitahata,
Coupling between a chemical wave and motion in a Belousov-Zhabotinsky droplet,
Current Physical Chemistry 5, 82-90 (2015).

[Summary] As a simple physico-chemical system that exhibits droplet motion induced by the pattern formation inside it, we investigate the motion of a droplet of Belousov-Zhabotinsky (BZ) reaction medium depending on the size and initial point of the chemical wave. We also observe the profile of Marangoni flow induced in the BZ droplet. In ourprevious paper, we reported the BZ-droplet motion and proposed a mechanism based on low-Reynolds-number hydrodynamics. Here, we discuss the validity of the uggested mechanism based on experimental results.

*Yasuaki Kobayashi, Hiroyuki Kitahata, and Masaharu Nagayama,
Model for calcium-mediated reduction of structural fluctuations in epidermis,
Physical Review E 92, 022709/1-7 (2015).

[Summary] We propose a reaction-advection-diffusion model of epidermis consisting of two variables, the degree of differentiation and the calcium ion concentration, where calcium ions enhance differentiation. By analytically and numerically investigating this system, we show that a calcium localization layer formed beneath the stratum corneum helps reduce spatiotemporal fluctuations of the structure of the stratum corneum. In particular, spatially or temporally small-scale fluctuations in the lower structure are suppressed and do not affect the upper structure, due to acceleration of differentiation by calcium ions. Analytical expressions for the reduction rate of fluctuation amplitudes are shown.

Misato Iino, *Yoshiki Hidaka, Fahrudin Nugroho, Rinto Anugraha, Hirotaka Okabe, Kazuhiro Hara,
Responses of spatiotemporal chaos to oscillating forces,
Physical Review E 92, 012916/1-5 (2015).

[Summary] The responses of soft-mode turbulence, a kind of spatiotemporal chaos seen in electroconvection of a nematic liquid crystal, to alternating-current magnetic fields is investigated to uncover the dynamical properties of spatiotemporal chaos. The dynamical responses can be measured by an order parameter, Mp(t), which indicates ordering in the convective roll pattern induced by the magnetic field. Determined by properties of the liquid crystal in a magnetic field, Mp(t) oscillates in accordance with the square of the magnetic field. The relaxation time of the system was obtained by fitting the frequency dependence of the complex susceptibility for the pattern obtained from the oscillation of Mp(t) to the Debye-type relaxation spectra. However, for the high-frequency regime, the susceptibility deviates from the spectra because slow and large fluctuations of Mp(t) contribute to the oscillation. The properties of this type of fluctuation were investigated by introducing a dynamic ordering parameter defined as the period average of Mp(t).

*Yujiro Eto, Masaya Kunimi,Hidekatsu Tokita,Hiroki Saito, and Takuya Hirano,
Suppression of relative flow by multiple domains in two component Bose-Einstein condensates,
Physical Review A 92, 013611/1-5 (2015).

[Summary] We investigate flow properties of immiscible Bose-Einstein condensates composed of two different Zeeman spin states of 〈sup〉87〈/sup〉Rb. Two spatially overlapping condensates in the optical trap are prepared by application of a resonant radio-frequency pulse, and then the magnetic field gradient is applied in order to produce the atomic flow. We find that the spontaneous multiple-domain formation arising from the immiscible nature drastically changes the fluidity. The homogeneously overlapping condensates readily separate under the magnetic field gradient, and they form a stable configuration composed of the two layers. In contrast, the relative flow between two condensates is largely suppressed in the case where the magnetic field gradient is applied after spontaneous domain formation.

Yuki Koyano, Natsuhiko Yoshinaga, and *Hiroyuki Kitahata,
General criteria for determining rotation or oscillation in a two-dimensional axisymmetric system,
Journal of Chemical Physics 143, 014117/1-6 (2015).

[Summary] A self-propelled particle in a two-dimensional axisymmetric system, such as a particle in a central force field or confined in a circular region, may show rotational or oscillatory motion. These motions do not require asymmetry of the particle or the boundary, but arise through spontaneous symmetry breaking. We propose a generic model for a self-propelled particle in a two-dimensional axisymmetric system. A weakly nonlinear analysis establishes criteria for determining rotational or oscillatory motion.

Tomohiro Sasaki, Nobuhiko J. Suematsu, Tatsunari Sakurai, and *Hiroyuki Kitahata,
Spontaneous recurrence of deposition and dissolution of a solid layer on a solution surface,
Journal of Physical Chemistry B 119, 9970-9974 (2015).

[Summary] We investigated the spontaneous recurrence of deposition and dissolution of camphor layer on the surface of camphor methanol solution. This recurrence is a novelrhythmic process concerned with solid−liquid phase transition. To elucidate the underlying mechanism, we measured the solution temperature at different times, and found that the temperature increased and decreased repetitively, correlating with the camphor layer’s deposition and dissolution. These experimental results show that the solution temperature plays an important role in recurrence of deposition and dissolution.

Jaka Fajar Fatriansyah and *Hiroshi Orihara,
Electric-field-induced flow-aligning state in a nematic liquid crystal,
Physical Review E 91, 042508/1-7 (2015).

[Summary] The response of shear stress to a weak ac electric field as a probe is measured in a nematic liquid crystal undershear flow and dc electric fields. Two states with different responses are clearly observed when the dc electricfield is changed at a constant shear rate: the flow aligning and non–flow aligning states. The director lies in theshear plane in the flow aligning state and out of the plane in the non–flow aligning state. Through applicationof dc electric field, the non–flow aligning state can be changed to the flow aligning state. In the transition fromthe flow aligning state to the non–flow aligning state, it is found that the response increases and the relaxationtime becomes longer. Here, the experimental results in the flow aligning state are discussed on the basis of theEricksen-Leslie theory.

*Hiroki Saito and Masaya Kunimi,
Energy shift of magnons in a ferromagnetic spinor-dipolar Bose-Einstein condensate,
Physical Review A 91, 041603(R)/1-4 (2015).

[Summary] Motivated by the recent experiment performed by the Berkeley group [G. E. Marti et al., Phys. Rev. Lett. 113, 165301 (2014)], we consider the dynamics of magnons in a spin-1 spinor-dipolar Bose-Einstein condensate, using mean-field theory. We show that the effective mass of a magnon is increased by the magnetic dipole-dipole interaction, as observed in the experiment. The magnon mass is also decreased by changing the direction of the magnetic field. The increase and decrease in the magnon mass manifest themselves in the acceleration of the magnons.

Masaya Kunimi* and Hiroki Saito,
Upper bound of one-magnon excitation and lower bound of effective mass for ferromagnetic spinor Bose and Fermi gases,
Physical Review A 91, 043624/1-6 (2015).

[Summary] Using a variationalmethod, we derive an exact upper bound for one-magnon excitation energy in ferromagneticspinor gases, which limits the quantum corrections to the effective mass of a magnon to be positive. We alsoderive an upper bound for one-magnon excitation energy in lattice systems. The results hold for both Bose andFermi systems in d dimensions as long as the interaction is local and invariant under spin rotation.

*Kenji Kashima, Toshiyuki Ogawa, Tatsunari Sakurai,
Selective pattern formation control: Spatial spectrum consensus and Turing instability approach,
Automatica 56, 25-35 (2015).

[Summary] Autonomous pattern formation phenomena are ubiquitous throughout nature. The goal of this paper is to show the possibility to effectively generate various desired spatial patterns by guiding such phenomena suitably. To this end, we employ a reaction–diffusion system as a mathematical model, and formulate and solve a novel pattern formation control problem. First, we describe the control objective in terms of spatial spectrum consensus, which enables utilize recent advances on networked control system theory. Next, the effectiveness of the proposed control law is evaluated theoretically by exploiting the center manifold theorem, and also numerically by simulation. The Turing instabilities play a crucial role throughout the paper.

*Yuji Sasaki, Hikaru Hoshikawa, Takafumi Seto, Fumiaki Kobayashi, V. S. R. Jampani, Stephan Herminghaus, Christian Bahr, and Hiroshi Orihara,
Direct visualization of spatiotemporal structure of self-assembled colloidal particles in electrohydrodynamic flow of a nematic liquid crystal,
Langmuir 31, 3815-3819 (2015).

[Summary] Characterization of spatiotemporal dynamics is of vital importance to soft matter systems far from equilibrium. Using a confocal laser scanning microscopy, we directly reveal three-dimensional motion of surface-modified particles in the electrohydrodynamic convection of a nematic liquid crystal. Particularly, visualizing a caterpillar-like motion of a self-assembled colloidal chain demonstrates the mechanism of the persistent transport enabled by the elastic, electric, and hydrodynamic contributions. We also precisely show how the particles’ trajectory is spatially modified by simply changing the surface boundary condition.

*Jean-Paul Rieu, Helene Delano-Ayari, Seiji Takagi, Yoshimi Tanaka, Toshiyuki Nakagaki,
Periodic traction in migrating large amoeba of Physarum Polycephalum,
Journal of Royal Society Interface 12, 20150099 (2015).

[Summary] The slime mould Physarum polycephalum is a giant multinucleated cell exhibiting well-known Ca2þ-dependent actomyosin contractions of its vein network driving the so-called cytoplasmic shuttle streaming. Its actomyosinnetwork forms both a filamentous cortical layer and large fibrils. In order to understand the role of each structure in the locomotory activity, we performedbirefringence observations and traction force microscopy on excised fragments of Physarum. After several hours, these microplasmodia adopt three main morphologies: flat motile amoeba, chain types with round contractile heads connected by tubes and motile hybrid types. Each typeexhibits oscillations with a period of about 1.5 min of cell area, traction forces and fibril activity (retardance) when fibrils are present. The amoeboid types show only peripheral forces while the chain types present a neverreported force pattern with contractile rings far from the cell boundary under the spherical heads. Forces are mostly transmitted where the actomyosin cortical layer anchors to the substratum, but fibrils maintain highly invaginated structures and contribute to forces by increasing the length of the anchorage line. Microplasmodia are motile only when there is an asymmetry in the shape and/or the force distribution.

*Satoshi Nakata, Shogo Suzuki, Takato Ezaki, Hiroyuki Kitahata, Kei Nishi, and Yasumasa Nishiura,
Response of a chemical wave to local pulse irradiation in the ruthenium-catalyzed Belousov–Zhabotinsky reaction,
Physical Chemistry Chemistry Physics 17, 9148-9152 (2015).

[Summary] 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.

2014

*Yang Ho Na, Yuki Aburaya, Hiroshi Orihara, Kazuyuki Hiraoka, and Youngbae Han,
Electrically induced deformation in chiral smectic elastomers with different domain structures,
Physical Review E 90, 062507/1-6 (2014).

[Summary] Electrical actuation is investigated in two kinds of chiral smectic liquid-crystal elastomers (LCEs) with different domain structures LCE1 and LCE2: The latter is better than the former in orientational order. Tracking fluorescent beads dispersed on the samples enables us to measure the two-dimensional strain tensors in ferroelectric elastomer films. It turns out that the electric-field-induced strain is polarity dependent and the type of molecular orientation responsible for the strain is specified. In LCE1 the shear strain is dominant, whereas in LCE2 it is comparable to the elongation strain, which is explained by the rotation of the principal axes. The essential differences of the two elastomers are observed in the eigenvalues of the strain tensors. The absolute values for LCE1 are larger than those for LCE2. The difference is discussed on the basis of the domain structures.

*Masaya Kunimi,
Metastable spin textures and Nambu-Goldstone modes of a ferromagnetic spin-1 Bose-Einstein condensate confined in a ring trap,
Physical Review A 90, 063632/1-8 (2014).

[Summary] We investigate the metastability of a ferromagnetic spin-1 Bose-Einstein condensate confined in a quasi-onedimensionalrotating ring trap by solving the spin-1 Gross-Pitaevskii equation. We find analytical solutions thatexhibit spin textures. By performing linear stability analysis, it is shown that the solutions can become metastablestates. We also find that the number of Nambu-Goldstone modes changes at a certain rotation velocity withoutchanging the continuous symmetry of the order parameter.

Yuji Sasaki, Yoshinori Takikawa, V. S. R. Jampani, Hikaru Hoshikawa, Takafumi Seto, Christian Bahr, Stephan Herminghaus, Yoshiki Hidaka, and *Hiroshi Orihara,
Colloidal caterpillars for cargo transportation,
Soft Matter 10, 8813–8820 (2014).

[Summary] Tunable transport of tiny objects in fluid systems is demanding in diverse fields of science such as drug delivery,active matter far from equilibrium, and lab-on-a-chip applications. Here, we report the directed motion ofcolloidal particles and self-assembled colloidal chains in a nematic liquid crystal matrix using electrohydrodynamic convection (EHC) rolls. The asymmetric distortion of the molecular orientation around the particles results – for single particles – in a hopping motion from one EHC roll to the next and – for colloidal chains – in a caterpillar-like motion in the direction perpendicular to the roll axes. We demonstrate the use of colloidal chains as microtraction engines for the transport of various types of microcargo.

Tomoya Kaneda and Hiroki Saito,
Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate,
Physical Review A 90, 053632/1-7 (2014).

[Summary] The dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which magnetic phases are spatially distributed is investigated. When a vortex dipole travels from the ferromagnetic phase to the polar phase, or vice versa, it penetrates the phase boundary and transforms into one of the various spin vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a half-quantum vortex dipole. Topological connections of spin wave functions across the phase boundary are discussed.

Akihiko Nakajima, Shuji Ishihara, Daisuke Imoto, and *Satoshi Sawai,
Rectified directional sensing in long-range cell migration,
Nature Communications 5, 5367/1-14 (2014).

[Summary] How spatial and temporal information are integrated to determine the direction of cell migration remains poorly understood. Here, by precise microfluidics emulation of dynamic chemoattractant waves, we demonstrate that, in Dictyostelium, directional movement as well as activation of small guanosine triphosphatase Ras at the leading edge is suppressed when the chemoattractant concentration is decreasing over time. This ‘rectification’ of directional sensing occurs only at an intermediate range of wave speed and does not require phosphoinositide-3-kinase or F-actin. From modelling analysis, we show that rectification arises naturally in a single-layered incoherent feedforward circuit with zero-order ultrasensitivity. The required stimulus time-window predicts ~5 s transient for directional sensing response close to Ras activation and inhibitor diffusion typical for protein in the cytosol. We suggest that the ability of Dictyostelium cells to move only in the wavefront is closely associated with rectification of adaptive response combined with local activation and global inhibition.

Jaka Fajar Fatriansyah, Yuji Sasaki, and *Hiroshi Orihara,
Nonequilibrium steady-state response of a nematic liquid crystal under simple shear flow and electric fields,
Physical Review E 90, 032504/1-8 (2014).

[Summary] The effect of a dc electric field on the response of a nematic liquid crystal under shear flow has been investigated by measuring the shear stress response to an ac electric field used as a probe. It was found that both the first- and second-order responses do not vanish at high frequencies, but have constant nonzero values. The experimental results are in good agreement with calculations based on the Ericksen-Leslie theory. The role of the Parodi relation (which is derived from the Onsager reciprocal relation) in the stress response is discussed.

*Yujiro Eto, Mark Sadgrove, Sho Hasegawa, Hiroki Saito, and Takuya Hirano,
Control of spin current in a Bose gas by periodic application of π pulses,
Physical Review A 90, 013626/1-6 (2014).

[Summary] We generate spin currents in an 87Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic π pulses (bang-bang control), producing longer spin-coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.

Tsuyoshi Kadokura, Jun Yoshida, and Hiroki Saito,
Hysteresis in quantized vortex shedding,
Physical Review A 90, 013612/1-5 (2014).

[Summary] It is shown using numerical simulations that flow patterns around an obstacle potential moving in a superfluid exhibit hysteresis. In a certain velocity region, there is a bistability between stationary laminar flow and periodic vortex shedding. The bistability exists in two- and three-dimensional systems.

*Yoshiki Hidaka and Noriko Oikawa,
Chaos and Spatiotemporal Chaos in Convective Systems,
FORMA 29, 29-32 (2014).

[Summary] Much of early research on chaos from the viewpoint of physics was performed using spatially confined convective systems. In spatially extended convective systems, on the other hand, spatiotemporal chaos occurs. However, there is no unified definition for the term spatiotemporal chaos as for chaos. To unify definition, a property common to the three kinds of spatiotemporal chaos observed in electroconvection of nematic liquid crystals is presented.

*Satoshi Nakata, Tomoaki Ueda, Tatsuya Miyaji, Yui Matsuda, Yukiteru Katsumoto, Hiroyuki Kitahata, Takafumi Shimoaka, and Takeshi Hasegawa,
Transient reciprocating motion of a self-propelled object controlled by a molecular layer of a N‑stearoyl‑p‑nitroaniline: dependence on the temperature of an aqueous phase,
Journal of Physical Chemistry C 118, 14888-14893 (2014).

[Summary] The mode-bifurcation of a self-propelled system inducedby the property of a N-stearoyl-p-nitroaniline (C18ANA) monolayer developed on an aqueous phase was studied. A camphor disk was placed on a C18ANA monolayer, which indicated a characteristic surface pressure−area (π−A) isotherm. A camphor disk transiently exhibited reciprocating motion at a higher surface density of C18ANA. The amplitude of the reciprocating motion increased with an increase in the temperature of the aqueous phase below 290 K, but reciprocating motionvaried to irregular motion over 290 K. The temperature-dependent reciprocating motion is discussed in terms of the π−A curve for C18ANA depending on the temperature. The interaction between C18ANA molecules was measured by Fourier transform IR spectrometry and Brewster-angle microscopy. As an extension of the study, the trajectory ofreciprocating motion could be determined by writing with a camphor pen on the C18ANA monolayer.

*Nobuhiko J. Suematsu, Tomohiro Sasaki, Satoshi Nakata, and Hiroyuki Kitahata,
Quantitative estimation of the parameters for self-motion driven by difference in surface tension,
Langmuir 30, 8101-8108 (2014).

[Summary] 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 μN, 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.

Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
FFLO Multi-Phase Transition in Two-Band Superconductor,
JPS Conference Proceedings 3, 015022 (2014).

[Summary] We study the phase diagram of Pauli-limiting two-band superconductors in the plane of an external magnetic field and temperature, based on the mean field approximation. In the case of a single-band superconductor, the superconducting phase diagram consists of two phases, Bardeen–Cooper–Schrieffer state in the lower field and Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state in the higher field. In the case of two-band superconductors, there are competing two length scales originated by first and second bands. Due to the competing length scales, the FFLO phase is divided into multiple — actually infinitely many — phases. We discuss the mechanism of the multiple phase transition.

*Itsuki Kunita, Shigeru Kuroda, Kaito Ooki, Toshiyuki Nakagaki,
Attempts to retreat from a dead-ended long capillary by backward swimming in Paramecium,
Frontiers in Microbiology 5, Article 270 (1-8) (2014).

[Summary] We have observed how the ciliate {\it Paramecium} attempts to retreat from the dead-end of a long capillary that is too narrow in which to turn. After many trial-and-error episodes of short-term backward swimming (SBS), which is the conventional avoidance behavior exhibited in free swimming when an obstacle is faced, long-term backward swimming (LBS) that lasted five to ten times longer was developed. LBS may have a beneficial effect for complete withdrawal from the capillary space, although in our experiment it was impossible for the organism to do so due to the capillary length. In order to identify a physically possible mechanism for LBS, we propose model equations for the membrane potential of Hodgkin-Huxley type, which describe the control of ciliary movement. The physiological implications and physical mechanism of the development of LBS are discussed.

*Itsuki Kunita, Shigeru Kuroda, Kaito Ooki, Toshiyuki Nakagaki,
Attempts to retreat from a dead-ended long capillary by backward swimming in Paramecium,
Frontiers in Microbiology 5, 270 (2014).

[Summary] We have observed how the ciliate Paramecium attempts to retreat from the dead-end of a long capillary that is too narrow for turning. After many trial-and-error episodes of short-term backward swimming (SBS), which is the conventional avoidance behavior exhibited in free swimming when an obstacle is faced, long-term backward swimming(LBS) that lasted five to ten times longer was developed. LBS may have ab eneficial effect for complete withdrawal from the capillary space, although in our experiment it was impossible for the organism to do so due to the capillary length. In order to identify a physically possible mechanism for LBS, we propose model equations for the membrane potential of Hodgkin–Huxley type, which describe the control of ciliary movement. The physiological implications and physical mechanism of the development of LBS are discussed.

*Nen Saito, Shuji Ishihara, and Kunihiko Kaneko,
Evolution of Genetic Redundancy : The Relevance of Complexity in Genotype-Phenotype Mapping,
New Journal of Physics 16, 063013/1-14 (2014).

[Summary] Despite its ubiquity among organisms, genetic redundancy is presumed to reduce total population fitness and is therefore unlikely to evolve. This study evaluates an evolutionary model with high-dimensional genotype–phenotype mapping (GPM) by applying a replica method to deal with quenched randomness. From the method, the dependence of fitness on genetic redundancy is analytically calculated. The results demonstrate that genetic redundancy can have higher population fitness under complex GPM, which tends to favor gene duplication in selection processes, further enhancing the potential for evolutionary innovations.

*Hiroki Saito,
Comment on “Ground-state fragmentation phase transition for attractive bosons in anisotropic traps”,
Physical Review A 89, 067601/1-2 (2014).

[Summary] In a recent article [Phys. Rev. A 88, 063641 (2013)], Cizek and Kasevich claimed that quantum fragmentation occurs in the metastable state of a Bose-Einstein condensate with an attractive interaction confined in an elongated harmonic potential. This result was obtained using the two-state Gaussian variational method. However, modified methods show that no fragmentation occurs for the parameters used by Cizek and Kasevich.

*Yujiro Eto, Hiroki Saito, and Takuya Hirano,
Observation of dipole-induced spin texture in an 87Rb spin-2 Bose-Einstein condensate,
Physical Review Letters 112, 185301/1-5 (2014).

[Summary] We report the formation of spin texture resulting from the magnetic dipole-dipole interaction in a spin-2 87Rb Bose-Einstein condensate. The spinor condensate is prepared in the transversely polarized spin state and the time evolution is observed under a magnetic field of 90 mG with a gradient of 3  mG/cm using Stern-Gerlach imaging. The experimental results are compared with numerical simulations of the Gross-Pitaevskii equation, which reveals that the observed spatial modulation of the longitudinal magnetization is due to the spin precession in an effective magnetic field produced by the dipole-dipole interaction. These results show that the dipole-dipole interaction has considerable effects even on spinor condensates of alkali metal atoms.

Satoshi Aya, Yuji Sasaki, Damian Pociecha, Fumito Araoka, Ewa Gorecka, Kenji Ema, Igor Musevic, Hiroshi Orihara, Ken Ishikawa, and *Hideo Takezoe,
Stepwise heat-capacity change at an orientation transition in liquid crystals,
Physical Review E 89, 022512 (2014).

[Summary] During a phase transition in a bulk material, heat is exchanged with matter to balance the changes in the internal energy and the entropy of the system. Here we report on the thermal detection of a surface-mediated anchoring transition, a spontaneous and discontinuous orientation change between planar (P) and homeotropic (H) alignments within a single nematic phase by changing temperature. In this case a stepwise change in the heat flow, similar to a glass transition, is observed by means of high-resolution differential scanning calorimetry. We found that the jump in the specific heat does not depend on the sample volume, although the contribution of molecules in the vicinity of surfaces, which trigger the transition, becomes less with increasing the sample volume. This means that different molecular orientations, H and P, with respect to surfaces have different thermodynamic free energies. We also address why the anchoring transition occurs by means of Grazing-incidence x-ray diffraction measurements, which clearly reveal the formation of quasi-smectic layers parallel to surfaces in the nematic phase.

*Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
Multi-band effects on Fulde-Ferrell-Larkin-Ovchinnikov states of Pauli-limited superconductors,
Physical Review B 89, 064505/1-16 (2014).

[Summary] Multi-band effects on Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of a Pauli-limiting two-band superconductor are studied theoretically, based on self-consistent calculations of the Bogoliubov-de Gennes equation. First, we examine the phase diagrams of two-band systems with a passive band in which the intraband pairing interaction is absent and superconductivity is induced by a Cooper pair tunneling from an active band. It is demonstrated that the temperature of the Lifshitz point at which three second-order transition lines meet is independent of the Cooper pair tunneling strength. The BCS-FFLO critical field becomes lower than the Lifshitz point with increasing the interband tunneling strength, and the resultant phase diagram is qualitatively different from that in a single-band superconductor. We also study the thermodynamics of Pauli-limiting two-band superconductors with comparable intraband pairing interactions. As a consequence of a competing effect between two bands, the FFLO phase is divided into two phases: Q1- and Q2-FFLO phases. The Q1-FFLO is favored in a high field regime and the Q2-FFLO becomes stable in the lower field. In a particular case, the latter is further subdivided into a family of FFLO states with rational modulation lengths, leading to a devil's staircase structure in the field-dependence of physical quantities. The critical field, above which the FFLO is stabilized, is lower than that in a single band superconductor, while the temperature of tricritical Lifshitz point is invariant under the change of two-band parameters.

*Hiroki Saito,
Many-body dynamics of a Bose-Einstein condensate collapsing by quantum tunneling,
Physical Review A 89, 023610/1-6 (2014).

[Summary] The dynamics of a Bose-Einstein condensate of atoms having attractive interactions is studied using quantum many-body simulations. The collapse of the condensate by quantum tunneling is numerically demonstrated, and the tunneling rate is calculated. The correlation properties of the quantum many-body state are investigated.

Keita Iida, Hiroyuki Kitahata, and *Masaharu Nagayama,
Theoretical study on the translation and rotation of an elliptic camphor particle,
Physica D 272, 39-50 (2014).

[Summary] 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 umerical computation. We then introduce a small parameter ε into the definition of the particle shape, which represents an elliptic deformation from a circular shape and, by means of perturbation theory, we analytically alculate the travelling solution to within O(ε). 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 O(ε2), 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.

Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
Fulde-Ferrell-Larkin-Ovchinnikov States in Two-Band Superconductors,
Journal of the Physical Society of Japan 83, 023703/1-5 (2014).

[Summary] We examine the possible phase diagram in an H–T plane for Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states in a two-band Pauli-limiting superconductor. We here demonstrate that, as a result of the competition of two different modulation lengthscales, the FFLO phase is divided into two phases by the first-order transition: the Q1- and Q2-FFLO phases at the higher and lower fields. The Q2-FFLO phase is further divided by successive first order transitions into an infinite family of FFLO subphases with rational modulation vectors, forming a devil’s staircase structure for the field dependences of the modulation vector and paramagnetic moment. The critical magnetic field above which the FFLO is stabilized is lower than that in a single-band superconductor. However, the tricritical Lifshitz point L at TL is invariant under two-band parameter changes.

2013

Yoshinori Takikawa and *Hiroshi Orihara,
Persistence of Brownian motion in shear flow,
Physical Review E 88, 062111/1-5 (2013).

[Summary] The persistence of a Brownian particle in a shear flow is investigated. The persistence probability P(t) , which is the probability that the particle does not return to its initial position up to time t, is known to obey a power law . Since the displacement of a particle along the flow direction due to convection is much larger than that due to Brownian motion, we define an alternative displacement in which the convection effect is removed. We derive theoretically the two-time correlation function and the persistence exponent  of this displacement. The exponent has different values at short and long times. The theoretical results are compared with experiment and a good agreement is found.

Masaru Suzuki, Hiroshi Sueto, Yusaku Hosokawa, Naoyuki Muramoto, Takayuki Narumi, Yoshiki Hidaka, and Shoichi Kai,
Duality of diffusion dynamics in particle motion in soft-mode turbulence,
Physical Review E 88, 42147 (2013).

[Summary] Nonthermal Brownian motion is investigated experimentally by injecting a particle into soft-mode turbulence (SMT), in the electroconvection of a nematic liquid crystal. It is clarified that the particle motion can be classified into two phases: fast motion, where particles move with the local convective flow, and slow motion, where they are carried by global slow pattern dynamics. We propose a simplified model to clarify the mechanism of the short-time and asymptotic behavior of diffusion. In our model, the correlation time is estimated as a function of a control parameter ɛ. The scaling of the SMT pattern correlation time, τ_d ∼ ɛ^−1, is estimated from the particle dynamics, which is consistent with a previous report observed from the Eulerian viewpoint. The origin of the non-Gaussian distribution of the displacement in the short-time regime is also discussed and an analytical curve is introduced that quantitatively agrees with the experimental data. Our results clearly illustrate the characteristics of diffusive motion in SMT, which are considerably different from the conventional Brownian motion.

Ryo Tanaka, Tomonori Nomoto, Taro Toyota, Hiroyuki Kitahata, and *Masanori Fujinami,
Delayed response of interfacial tension in propagating chemical waves of the Belousov-Zhabotinsky reaction without stirring,
The Journal of Physical Chemistry B 117, 13893–13898 (2013).

[Summary] 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.

*Yujiro Eto, Hayato Ikeda, Hirosuke Suzuki, Sho Hasegawa, Yasushi Tomiyama, Sawako Sekine, Mark Sadgrove, and Takuya Hirano,
Spin-echo-based magnetometry with spinor Bose-Einstein condensates,
Physical Review A 88, 031602(R)/1-4 (2013).

[Summary] We demonstrate detection of a weak alternate-current magnetic field by application of the spin-echo technique to F=2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/√Hz is attained for an atom number of 5×10^3 at a spatial resolution of 100 μ㎡. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.

Jaka Fajar Fatriansyah and *Hiroshi Orihara,
Dynamical properties of nematic liquid crystals subjected to shear flow and magnetic fields: Tumbling instability and non-equilibrium fluctuations,
Physival Review E 88, 012510/1-9 (2013).

[Summary] We investigate the dynamical properties of monodomain nematic liquid crystals under shear flow and magnetic fields on the basis of the Ericksen-Leslie theory. Stable and unstable states appear depending on the magnetic field and the shear rate. The trajectory of the unstable state shows tumbling motion. The phase diagram of these states is plotted as a function of the three components of the magnetic field at a constant shear rate. The phase diagram changes depending on the viscous properties of different types of nematic liquid crystals. In this non-equilibrium steady state, we calculate the correlation function of director fluctuations and the response function, and discuss the non-equilibrium fluctuations and the modified fluctuation dissipation relation in connection with non-conservative forces due to shear flow.