arXiv subjects

Takashi Uneyama

Publications and source records attributed to Takashi Uneyama.

At least 37 records · Page 2Linked to original sources

arXiv2024

Dislocation Glides in Granular Media

Atomic crystals with dislocations deform plastically at low stresses via dislocation glide. Whether dislocation glide occurs in macroscopic frictional granular media has remained unknown. The discrete element method is employed to simulate the structural and mechanical responses of a granular crystal with an edge dislocation. We find that dislocation glide occurs at low interparticle friction, resulting in significantly lower yield stresses than in dislocation-free crystals. Yield stress varies linearly with interparticle friction, attributed to both Peierls stress and frictional effect.

cond-mat.mtrl-sci↗

arXiv2024

Brownian simulations for fracture of star polymer phantom networks

Based on a recent simulation study [Masubuchi et al., Macromolecules, 56, 9359 (2023)], the cycle rank plays a significant role in determining the fracture characteristics of network polymers. However, the study only considered energy-minimized networks without the effects of thermal agitation. We conducted Brownian dynamics simulations at various stretch rates to address this gap. The results showed that even with Brownian motion, the strain and stress at the break obtained for different node functionalities and conversion ratios exhibited master curves if plotted against cycle rank. These master curves were dependent on the strain rate, with the curves tending to approach those observed in energy-minimized simulations as the strain rate decreased, even though the fracture process was affected by the competition against Brownian motion, elongation, and bond degradation.

cond-mat.soft↗

arXiv2024

Reverse non-equilibrium molecular dynamics simulations of a melt of Kremer-Grest type model under fast shear

Although the reverse non-equilibrium molecular dynamics (RNEMD) simulation method has been widely employed, the range of applicability is yet to be discussed. In this study, for the first time, we systematically examine the method against an unentangled melt of the Kremer-Grest type chain. The simulation results indicate that as the shear rate increases, the temperature and density become inhomogeneous. However, the average viscosity remains consistent with the results obtained using the SLLOD method under homogeneous temperature and density. We also confirm that the temperature-density inhomogeneity does not significantly affect polymer conformation.

cond-mat.soft↗

arXiv2024

Phantom chain simulations for fracture of star polymer networks with various strand densities

Despite many attempts, the relation between fracture and structure of polymer networks is yet to be clarified. For this problem, a recent study for phantom chain simulations [Macromolecules, 56, 9359 (2023)] has demonstrated that the fracture characteristics obtained for polymer networks with various node functionalities and conversion ratios lie on master curves if they are plotted against cycle rank. In this study, we extended the simulation to the effect of prepolymer concentration on the relationships between cycle rank and fracture characteristics within the concentration range of 1<c/c^*<8, concerning the overlapping concentration c^*. We created networks from sols of star-branched phantom bead-spring chains via end-linking reaction between different chains through Brownian dynamics simulations with varying the number of branching arms f from 1 to 8, and the conversion ratio ϕ_c from 0.6 to 0.95. For the resultant networks, cycle rank ξ was consistent with the mean-field theory. The networks were uniaxially stretched with energy minimization until break to obtain modulus G, strain at break ε_b, stress at break σ_b, and work for fracture W_b. With the branch point density \u{psion}_br, G/\u{psion}_br, ε_b, σ_b/\u{psion}_br, and W_b/\u{psion}_br of the data for various f and ϕ_c draw master curves if plotted against ξ. The master curves depend on c; as c increases, all the mechanical characteristics monotonically increase. If we plot σ_b/\u{psion}_br and W_b/\u{psion}_br against G/\u{psion}_br, the data for various f and ϕ_c lie on master curves but depending on c. Consequently, the fracture characteristics are not solely described by modulus for the examined energy-minimized phantom chain networks.

cond-mat.soft↗

arXiv2024

Normal Modes of Rouse-Ham Symmetric Star Polymer Model

The Rouse-Ham model is a simple yet useful dynamics model for an unentangled branched polymer. In this work, we study the normal modes of the Rouse-Ham type coarse-grained symmetric star polymer model. We model a star polymer by connecting multiple arm beads to a center bead by harmonic springs. In the Rouse-Ham model, the dynamics of the bead positions can be decomposed into the normal modes, which are chosen to be orthogonal to each other. Due to the existence of degenerate eigenvalues, the eigenmodes do not directly correspond to the normal modes. We propose several methods to construct the normal modes for the coarse-grained symmetric star polymer model. We show that we can construct the normal modes by using a simple permutation or the Hadamard matrix. These methods give symple and highly symmetric orthogonal modes, but work just for a special number of arms. We also show that we can construct the normal modes by using the discrete Fourier transform (DFT) matrix. This method is applicable for an arbitrary number of arms.

cond-mat.soft↗

arXiv2024

Brownian Yet Non-Gaussian Diffusion of a Light Particle in Heavy Gas: Lorentz Gas Based Analysis

Non-Gaussian diffusion was recently observed in gas mixtures with mass and fraction contrast [F. Nakai et al, Phys. Rev. E 107, 014605 (2023)]. The mean square displacement of a minor gas particle with a small mass is linear in time, while the displacement distribution deviates from the Gaussian distribution, which is called the Brownian yet non-Gaussian diffusion. In this work, we theoretically analyze this case where the mass contrast is sufficiently large. Major heavy particles can be interpreted as immobile obstacles, and a minor light particle behaves like a Lorentz gas particle within an intermediate time scale. Despite the similarity between the gas mixture and the conventional Lorentz gas system, the Lorentz gas description cannot fully describe the Brownian yet non-Gaussian diffusion. A successful description can be achieved through an ensemble average of the statistical quantities of the Lorentz gas over the initial speed.

cond-mat.stat-mech↗

arXiv2024

Dissipative particle dynamics simulation plus slip-springs for entangled polymers with various slip-spring densities

Slip-spring models are valuable tools for simulating entangled polymers, bridging the gap between bead-spring models with excluded volume and network models with presumed reptation motion. This study focuses on the DPD-SS (Dissipative Particle Dynamics - Slip-Spring) model, which introduces slip-springs into the standard DPD polymer model with soft-core interactions. By systematically adjusting the fugacity of slip-springs, the density of slip-springs within the system is varied. Simulation results demonstrate the compatibility of models with different slip-spring densities in terms of diffusion and linear relaxation modulus when the average number of slip-springs per chain is the same. The conversion between DPD-SS models concerning length and time is achieved through Rouse scaling, which utilizes the average number of DPD beads between consecutive anchoring points of slip-springs. Additionally, the modulus conversion is accomplished through the plateau modulus that takes account of fluctuations around entanglement. Notably, diffusion and relaxation modulus from the DPD-SS model align with those reported for standard Kremer-Grest and DPD models featuring strong repulsive interactions.

cond-mat.soft↗

arXiv2023

Comparison of Linear Viscoelastic Behaviors of Langevin- and Jump-Rouse Models

The Rouse model with harmonic springs and the Langevin equation (Langevin-Rouse model) is widely used to describe the linear viscoelasticity of unentangled polymer melts. A similar model, in which the Langevin equation is replaced by discrete local jump dynamics (the jump-Rouse model), is also used to describe the dynamics of some systems such as entangled polymer melts. Intuitively, we expect that the Langevin- and jump-Rouse models give similar linear viscoelastic behaviors. However, the Langevin- and jump-Rouse models are not equivalent, and their linear viscoelastic behaviors can be different. In this work, we compare the shear relaxation moduli of the Langevin- and jump-Rouse model in detail. We develop a jump rate model in which the resampling ratio is tunable. By using this jump rate model, we can smoothly connect the jump-Rouse model and the Langevin-Rouse model. We perform kinetic Monte Carlo simulations to calculate the shear relaxation modulus data of the jump-Rouse model. We compare the simulation results with various numbers of beads and resampling ratios, and show that the shear relaxation moduli of the Langevin- and jump-Rouse models are similar but slightly different. We analyze the short-time relaxation behavior of the jump-Rouse model, and show that the local jumps give a single Maxwell type relaxation in the short-time region.

cond-mat.soft↗

arXiv2023

Modeling for heterogeneous oxidative aging of polymers using coarse-grained molecular dynamics

This study presents a coarse-grained molecular dynamics simulation model to investigate the process of oxidative aging in polymers. The chemical aging effect is attributed to the auto-oxidation mechanism, which is initiated by radicals, leading to the chain scission and crosslinking of polymer chains. In this study, we integrate a thermal oxidation kinetic model in the oxygen excess regime into the Kremer-Grest model, thereby enabling a reactive coarse-grained molecular dynamics simulation to capture the process of oxidative degradation. Our simulation reveals that when the timescale of the characteristic reaction step of oxidative degradation is shorter than the longest relaxation time of polymer chains, the scission sites exhibit spatial heterogeneity. This innovative simulation model possesses the potential to enhance our comprehension of polymer aging phenomena, thus making noteworthy contributions to the realm of polymer science and degradation chemistry.

cond-mat.soft↗

arXiv2023

Phantom-Chain Simulations for the Effect of Node Functionality on the Fracture of Star-Polymer Networks

The influence of node functionality (f) on the fracture of polymer networks remains unclear. While many studies have focused on multi-functional nodes with f>4, recent research suggests that networks with f=3 exhibit superior fracture properties compared to those with f=4. To clarify this discrepancy, we conducted phantom chain simulations for star-polymer networks varying f between 3 and 8. Our simulations utilized equimolar binary mixtures of star branch prepolymers with a uniform arm length. We employed a Brownian dynamics scheme to equilibrate sols and induce gelation through end-linking reactions. We prevented the formation of odd-order loops owing to the binary reaction and second-order loops algorithmically. We stored network structures at various conversion ratios (ϕ_c) and minimized energy to reduce computation costs induced by structural relaxation. We subjected the networks to stretching until fracture to determine stress and strain at break and work for fracture, ε_b, σ_b, and W_b. These fracture characteristics are highly dependent on ϕ_c for networks with small f but relatively insensitive for those with large f. Thus, the networks with small f exhibit greater fracture properties than those with large f at high ϕ_c, whereas the opposite relationship occurs at low ϕ_c. We analyzed ε_b, σ_b, and W_b concerning the cycle rank ξ and the broken strand fraction ϕ_bb. We found ε_b, σ_b/ϕ_bb, and W_b/ϕ_bb monotonically decrease with increasing ξ, and the data for various f and ϕ_c superpose with each other to draw master curves. These results imply that the mechanical superiority of the networks with small f comes from their smaller ξ that gives higher ε_b, σ_b/ϕ_bb, and W_b/ϕ_bb than the networks with large f.

cond-mat.soft↗

arXiv2023

Estimating relaxation times from a single trajectory

Complex systems such as protein conformational fluctuations and supercooled liquids exhibit a long relaxation time and are considered to posses multiple relaxation times. We analytically obtain the exact correlation function for stochastic processes with multiple relaxation times. We show that the time-averaged correlation function calculated by a trajectory whose length is shorter than the longest relaxation time exhibits an apparent aging behavior. We propose a method to extract relaxation times from a single trajectory. This method successfully extracts relaxation times of a stochastic process with multiple states when a state can be characterized by the values of the trajectory. As an application of this method, we estimate several relaxation times smaller than the longest relaxation time in conformational fluctuations of a small protein.

cond-mat.soft↗

arXiv2023

Increase in rod diffusivity emerges even in Markovian nature

Rod-shaped particles embedded in certain matrices have been reported to exhibit an increase in their center of mass diffusivity upon increasing the matrix density. This increase has been considered to be caused by a kinetic constraint in analogy with tube models. We investigate a mobile rod-like particle in a sea of immobile point obstacles using a kinetic Monte Carlo scheme equipped with a Markovian process, that generates gas-like collision statistics, so that such kinetic constraints do essentially not exist. Even in such a system, provided the particle's aspect ratio exceeds a threshold value of about 24, the unusual increase in the rod diffusivity emerges. This result implies that the kinetic constraint is not a necessary condition for the increase in the diffusivity.

cond-mat.stat-mech↗

arXiv2023

Effects of attractive inter-particle interaction on cross-transport coefficient between mass and heat in binary fluids

In some binary fluids, mass transport is observed under a temperature gradient. This phenomenon is called the Soret effect. In this study, we discuss the influence of inter-particle interaction. We considered equimolar binary Lennard-Jones fluids with a mass contrast, whereas the interaction was common for all the particle pairs with various cut-off lengths. We performed molecular dynamics simulations of such fluids under equilibrium to obtain the cross-transport coefficients L1q between the fluxes of mass and heat. The simulation revealed that this quantity strongly depends on the cut-off length. Further, we decomposed the heat flux into kinetic and potential contributions and calculated the cross-correlations between decomposed fluxes and the mass flux. The result indicates that the potential contribution dominates L1q, implying that the Soret coefficient is altered by the inter-particle interaction.

cond-mat.stat-mech↗

arXiv2023

Extension of Moving Particle Simulation including rotational degrees of freedom for dilute fiber suspension

We develop a novel Moving Particle Simulation (MPS) method to accurately reproduce the motion of fibers floating in sheared liquids. In conventional MPS schemes, if a fiber suspended in a liquid is represented by a one-dimensional array of MPS particles, it is entirely aligned to the flow direction due to the lack of shear stress difference between fiber-liquid interfaces. To address this problem, we employ the micropolar fluid model to introduce rotational degrees of freedom into the MPS particles. The translational motion of liquid and solid particles and the rotation of solid particles are calculated with the explicit MPS algorithm. The fiber is modeled as an array of micropolar fluid particles bonded with stretching and bending potentials. The motion of a single rigid fiber is simulated in a three-dimensional shear flow generated between two moving solid walls. We show that the proposed method is capable of reproducing the fiber motion predicted by Jeffery's theory being different from the conventional MPS simulations.

physics.flu-dyn↗

arXiv2023

Fluctuating Diffusivity Emerges even in Binary Gas Mixtures

Diffusivity in some soft matter and biological systems changes with time, called the fluctuating diffusivity. In this work, we propose a novel origin for fluctuating diffusivity based on stochastic simulations of binary gas mixtures. In this system, the fraction of one component is significantly small, and the mass of the minor component molecule is different from that of the major component. The minor component exhibits fluctuating diffusivity when its mass is sufficiently smaller than that of the major component. We elucidate that this fluctuating diffusivity is caused by the time scale separation between the relaxation of the velocity direction and the speed of the minor component molecule.

cond-mat.stat-mech↗

arXiv2022

Stress Tensor of Single Rigid Dumbbell by Virtual Work Method

We derive the stress tensor of a rigid dumbbell by using the virtual work method. In the virtual work method, we virtually apply a small deformation to the system, and relate the change of the energy to the work done by the stress tensor. A rigid dumbbell consists of two particles connected by a rigid bond of which length is constant (the rigid constraint). The energy of the rigid dumbbell consists only on the kinetic energy. Also, only the deformations which do not violate the rigid constraint are allowed. Thus we need the dynamic equations which is consistent with the rigid constraint to apply the virtual deformation. We rewrite the dynamic equations for the underdamped SLLOD-type dynamic equations into the forms which are consistent with the rigid constraint. Then we apply the virtual deformation to a rigid dumbbell based on the obtained dynamic equations. We derive the stress tensor for the rigid dumbbell model from the change of the kinetic energy. Finally, we take the overdamped limit and derive the stress tensor and the dynamic equation for the overdamped rigid dumbbell. We show that the Green-Kubo type linear response formula can be reproduced by combining the stress tensor and the dynamic equation at the overdamped limit.

cond-mat.soft↗

arXiv2022

Application of Projection Operator Method to Coarse-Grained Dynamics with Transient Potential

We show that the coarse-grained dynamics model with the time-dependent and fluctuating potential (transient potential) can be derived from the microscopic Hamiltonian dynamics. The concept of the transient potential was first introduced rather phenomenologically, and its relation to the underlying microscopic dynamics has not been clarified yet. This is in contrast to the generalized Langevin equation, of which relation to the microscopic dynamics is well-established. In this work, we show that the dynamic equations with the transient potential can be derived for the coupled oscillator model, without any approximations. It is known that the dynamics of the coupled oscillator model can be exactly described by the generalized Langevin type equations. This fact implies that the dynamic equations with the transient potential can be utilized as a coarse-grained dynamics model in a similar way to the generalized Langevin equation. Then we show that the dynamic equations for the transient potential can be also formally derived for the microscopic Hamiltonian dynamics, without any approximations. We use the projection operator method for the coarse-grained variables and transient potential. The dynamic equations for the coarse-grained positions and momenta are similar to those in the Hamiltonian dynamics, but the interaction potential is replaced by the transient potential. The dynamic equation for the transient potential is the generalized Langevin equation with the memory effect. Our result justifies the use of the transient potential to describe the coarse-grained dynamics. We propose several approximations to obtain simplified dynamics model. We show that, under several approximations, the dynamic equation for the transient potential reduces to the relatively simple Markovian dynamic equation for the potential parameters.

cond-mat.soft↗

arXiv2022

Linear Response Theory of Scale-Dependent Viscoelasticity for Overdamped Brownian Particle Systems

We show the linear response theory of spatial-scale-dependent relaxation moduli for overdamped Brownian particle systems. We employ the Irving-Kirkwood stress tensor field as the microscopic stress tensor field. We show that the scale-dependent relaxation modulus tensor, which characterizes the response of the stress tensor field to the applied velocity gradient field, can be expressed by using the correlation function of the Irving-Kirkwood stress tensor field. The spatial Fourier transform of the relaxation modulus tensor gives the wavenumber-dependent relaxation modulus. For isotropic and homogeneous systems, the relaxation modulus tensor has only two independent components. The transverse and longitudinal deformation modes give the wavenumber-dependent shear relaxation modulus and the wavenumber-dependent bulk relaxation modulus. As simple examples, we derive the explicit expressions for the relaxation moduli for two simple models the non-interacting Brownian particles and the harmonic dumbbell model.

cond-mat.soft↗