arXiv subjects

Hiroyuki Kitahata

Publications and source records attributed to Hiroyuki Kitahata.

At least 37 records · Page 2Linked to original sources

arXiv2022

Anomalous diffusion and transport by a reciprocal convective flow

Under low-Reynolds-number conditions, dynamics of convection and diffusion are usually considered separately because their dominant spatial and temporal scales are different, but cooperative effects of convection and diffusion can cause diffusion enhancement [Koyano et al., Phys. Rev. E, 102, 033109 (2020)]. In this study, such cooperative effects are investigated in detail. Numerical simulations based on the convection-diffusion equation revealed that anisotropic diffusion and net shift as well as diffusion enhancement occur under a reciprocal flow. Such anomalous diffusion and transport are theoretically derived by the analyses of the Langevin dynamics.

physics.flu-dyn↗

arXiv2022

Bifurcation structure of the flame oscillation

A flame exhibits a limit-cycle oscillation, which is called "flame flickering" or "puffing", in a certain condition. We investigated the bifurcation structure of the flame oscillation in both simulation and experiment. We performed a two-dimensional hydrodynamic simulation by employing the flame sheet model. We reproduced the flame oscillation and investigated the parameter dependences of the amplitude and frequency on the fuel-inlet diameter. We also constructed an experimental system, in which we could finely vary the fuel-inlet diameter, and we investigated the diameter-dependences of the amplitude and frequency. In our simulation, we observed the hysteresis and bistability of the stationary and oscillatory states. In our experiments, we observed the switching between the stationary and oscillatory states. As fluctuations can induce the switching in the bistable system, switching observed in our experiments suggested the bistability of the two states. Therefore, we concluded that the oscillatory state appeared from the stationary state through the subcritical Andronov-Hopf bifurcation in both the simulation and experiment. The amplitude was increased and the frequency was decreased as the fuel-inlet diameter was increased. In addition, we visualized the vortex structure in our simulation and discussed the effect of the vortex on the flame dynamics.

physics.flu-dyn↗

arXiv2022

Horizontal transport in the bouncing ball system with a sawtooth-shaped table

The system that consists of a ball bouncing off a sawtooth-shaped table vibrating vertically is considered. The horizontal motion in this system is caused by the table shape, and we plotted the mean horizontal velocity as a function of the asymmetry of the table shape. The ball is transported in the direction which the gentler slopes face. To give a description to the net asymmetric transport, we derived a simplified model using assumptions of high bounce and probabilistic collision to the left or right slopes. The simplified model exhibits a horizontal transport qualitatively similar to that observed in the original model.

nlin.CD↗

arXiv2021

Imperfect bifurcation in the rotation of a propeller-shaped camphor rotor

We investigated the bifurcation structure on the self-propelled motion of a camphor rotor at a water surface. The center of the camphor rotor was fixed by the axis, and it showed rotational motion around it. Due to the chiral asymmetry of its shape, the absolute values of the angular velocities in clockwise and counterclockwise directions were different. This asymmetry in the angular velocities implies an imperfect bifurcation. From the numerical simulation results, we discuss the condition for the occurrence of the imperfect bifurcation.

nlin.PS↗

arXiv2020

Diffusion enhancement in a levitated droplet via oscillatory deformation

Recent experimental results indicate that mixing is enhanced by a reciprocal flow induced inside a levitated droplet with an oscillatory deformation [T. Watanabe et al. Sci. Rep. 8, 10221 (2018)]. Generally, reciprocal flow cannot convect the solutes in time average, and agitation cannot take place. In the present paper, we focus on the diffusion process coupled with the reciprocal flow. We theoretically derive that the diffusion process can be enhanced by the reciprocal flow, and the results are confirmed via numerical calculation of the over-damped Langevin equation with a reciprocal flow.

nlin.PS↗

arXiv2020

Autonomous elastic microswimmer

A model of an autonomous three-sphere microswimmer is proposed by implementing a coupling effect between the two natural lengths of an elastic microswimmer. Such a coupling mechanism is motivated by the previous models for synchronization phenomena in coupled oscillator systems. We numerically show that a microswimmer can acquire a nonzero steady state velocity and a finite phase difference between the oscillations in the natural lengths. These velocity and phase difference are almost independent of the initial phase difference. There is a finite range of the coupling parameter for which a microswimmer can have an autonomous directed motion. The stability of the phase difference is investigated both numerically and analytically in order to determine its bifurcation structure.

cond-mat.soft↗

arXiv2020

Spontaneous deformation and fission of oil droplets on an aqueous surfactant solution

We investigated the spontaneous deformation and fission of a tetradecane droplet containing palmitic acid (PA) on a stearyltrimethylammonium chloride (STAC) aqueous solution. In this system, the generation and rupture of the gel layer composed of PA and STAC induce the droplet deformation and fission.To investigate the characteristics of the droplet-fission dynamics, we obtained the time series of the number of the droplets, and confirmed that the number has a peak at a certain STAC concentration. Since the fission of the droplet should be led by the deformation, we analyzed four parameters which may relate to the fission dynamics from the spatio-temporal correlation of the droplet-boundary velocity. As a result, we found that the faster deformation would be the key factor for the fission dynamics.

cond-mat.soft↗

arXiv2020

Local bifurcation structure of a bouncing ball system with a piecewise polynomial function for table displacement

The system in which a small rigid ball is bouncing repeatedly on a massive at table vibrating vertically, so-called the bouncing ball system, has been widely studied. Under the assumption that the table is vibrating with a piecewise polynomial function of time, the bifurcation diagram changes qualitatively depending on the order of the polynomial function. We elucidate the mechanism of the difference in the bifurcation diagrams by focusing on the two-period solution. In addition, we derive the approximate curve of the branch close to the period-doubling bifurcation point in the case of the piecewise cubic function of time for the table vibration. We also performed numerical calculation, and we demonstrate that the approximations well reproduce the numerical results.

nlin.CD↗

arXiv2020

Star shaped patterns caused by colloidal aggregation during the spreading process of a droplet

This research found that when an acidic solution with a low surface tension spread on the surface of a glycerol solution mixed with milk, a star shaped pattern was spontaneously formed on the surface in the horizontal plane during the spreading process. We investigated the emergence of the star shaped pattern owing to an interfacial instability in experiments using glycerol solutions with several viscosities and 2-methoxyethanol aqueous solutions, which are acidic solutions, with several concentrations. This result demonstrated that the star shaped pattern emerged in the high concentration of 2-methoxyethanol. We proposed a phenomenological model, based on our experimental results, which explains three points as follows; the spreading of the 2-methoxyethanol aqueous solution on the surface of the glycerol solution, the colloidal aggregation of the milk protein colloids caused by the denaturation that occurs when mixed with 2-methoxyethanol, and the accumulation of the aggregates toward the dent regions of the moving interface by a sweeping effect. The model reproduces the formation of the star shaped pattern which was similar to the experimental one. Furthermore, the model provided a phase diagram against the concentration of the 2-methoxyethanol solution and the viscosity of the glycerol solution as control parameters in our experiments. The phase diagram was close to that obtained from our experiments. The results suggest that the above three points are important for the formation of the star shaped pattern.

cond-mat.soft↗

arXiv2020

Spontaneous motion of a camphor particle with a triangular modification from a circle

The spontaneous motion of a camphor particle with a slight modification from a circle is investigated. The effect of the shape on the motion is examined by the perturbation method. We introduce a slight $n$-mode modification from a circle, where the profile is described by $r = R(1 + ε\cos nθ)$ in polar coordinates. The results predict that a camphor particle with an $n=3$ mode modification from a circle, i.e., a triangular modification, moves in the direction of a corner for a small particle, while it moves in the direction of a side for a large particle. The numerical simulation results well reproduce the theoretical prediction. The present study will help understand the effect of the particle shape on spontaneous motion.

nlin.PS↗

arXiv2020

Bifurcation analysis of a density oscillator using two-dimensional hydrodynamic simulation

A density oscillator exhibits limit-cycle oscillations driven by the density difference of the two fluids. We performed two-dimensional hydrodynamic simulations with a simple model, and reproduced the oscillatory flow observed in experiments. As the density difference is increased as a bifurcation parameter, a damped oscillation changes to a limit-cycle oscillation through a supercritical Hopf bifurcation. We estimated the critical density difference at the bifurcation point and confirmed that the period of the oscillation remains finite even around the bifurcation point.

nlin.PS↗

arXiv2020

Two floating camphor particles interacting through lateral capillary force

We consider a mathematical model for a two-particle system driven by the spatial gradient of a concentration field of chemicals with conservative attractive interactions in one dimension. This setup corresponds to an experimental system with floating camphor particles at a water surface. Repulsive interaction is introduced, as well as self-propelling force, through the concentration field of camphor molecules at the water surface. Here we newly adopt the attractive lateral capillary force due to the deformation of the water surface. The particles experience competing dissipative repulsion and conservative attraction. We numerically investigated the mathematical model, and found six different modes of motion. The theoretical approach revealed that some of such mode transitions can be understood in terms of bifurcation.

nlin.PS↗

arXiv2020

Experimental study on the bifurcation of a density oscillator depending on density difference

Hydrodynamic instabilities often cause spatio-temporal pattern formations and transitions between them. We investigate a model experimental system, a density oscillator, where the bifurcation from a resting state to an oscillatory state is triggered by the increase in the density difference of the two fluids. Our results show that the oscillation amplitude increases from zero and the period decreases above a critical density difference. The detailed data close to the bifurcation point provide a critical exponent consistent with the supercritical Hopf bifurcation.

nlin.PS↗

arXiv2020

On a simple model that explains inversion of a self-propelled rotor under periodic stop and release operations

We propose a simple mathematical model that describes the time evolution of a self-propelled object on a liquid surface using such variables as the object location, the surface concentration of active molecules and the hydrodynamic surface flow. The model is applied to simulate the time evolution of a rotor composed of a polygonal plate with camphor pills at its corners. We have qualitatively reproduced results of experiments, in which the inversion of rotational direction under periodic stop-and-release operations was investigated. The model correctly describes the probability of the inversion as a function of the duration of the phase when the rotor is stopped. Moreover, the model allows to introduce the rotor asymmetry unavoidable in real experiments and study its influence on the studied phenomenon. Our numerical simulations have revealed that the probability of the inversion of rotational direction is determined by the competition among the transport of the camphor molecules by the flow, the intrinsic asymmetry of the rotor, and the noise amplitude.

nlin.AO↗

arXiv2020

Diffusion in crowded colloids of particles cyclically changing their shapes

A simple model of an active colloid consisting of dumbbell-shaped particles that cyclically change their length without propelling themselves is proposed and analyzed. At nanoscales, it represents an idealization for bacterial cytoplasm or for a biomembrane with active protein inclusions. Our numerical simulations demonstrate that non-equilibrium conformational activity of particles can strongly affect diffusion and structural relaxation: while a passive colloid behaves as a glass, it gets progressively fluidized when the activity is turned on. Qualitatively, this agrees with experimental results on optical tracking of probe particles in bacterial and yeast cells where metabolism-induced fluidization of cytoplasm was observed.

nlin.AO↗

arXiv2019

Rotational motion of a camphor disk in a circular region

In a two-dimensional axisymmetric system, a symmetric self-propelled particle exhibits rotational or oscillatory motion from the consideration of the system symmetry. In the present paper, we studied the motion of a camphor disk confined in a two-dimensional circular region. By reducing the mathematical model describing the dynamics of the motion of a camphor disk and the concentration field of camphor molecules on a water surface, we analyzed the reduced equations around a bifurcation point where the rest state at the center of the system becomes unstable. As a result, we found that rotational motion is stably realized through the double-Hopf bifurcation from the rest state. The theoretical results were confirmed by numerical calculation and well corresponded to the experimental results.

nlin.AO↗

arXiv2019

Bifurcation in the angular velocity of a circular disk propelled by symmetrically distributed camphor pills

We studied rotation of a disk propelled by a number of camphor pills symmetrically distributed at its edge. The disk was put on a water surface so that it could rotate around a vertical axis located at the disk center. In such a system, the driving torque originates from surface tension difference resulting from inhomogeneous surface concentration of camphor molecules released from the pills. Here we investigated the dependence of the stationary angular velocity on the disk radius and on the number of pills. The work extends our previous study on a linear rotor propelled by two camphor pills [Phys. Rev. E, 96, 012609 (2017)]. It was observed that the angular velocity dropped to zero after a critical number of pills was exceeded. Such behavior was confirmed by a numerical model of time evolution of the rotor. The model predicts that, for a fixed friction coefficient, the speed of pills can be accurately represented by a function of the linear number density of pills. We also present bifurcation analysis of the conditions at which the transition between a standing and a rotating disk appears.

nlin.AO↗

arXiv2018

Power law observed in the motion of an asymmetric camphor boat under viscous conditions

We investigated the velocity of an asymmetric camphor boat moving on aqueous solutions with glycerol. The viscosity was controlled by using several concentrations of glycerol into the solution. The velocity decreased with an increase in the glycerol concentration. We proposed a phenomenological model, and showed that the velocity decreased with an increase in the viscosity according to power law. Our experimental result agreed with the one obtained from our model. The results provided an approximation that the characteristic decay length of the camphor concentration profile at the front of the boat was sufficiently shorter than that at the rear of the boat, which was difficult to measure directly.

nlin.PS↗