arXiv subjects

Takashi Uneyama

Publications and source records attributed to Takashi Uneyama.

At least 19 recordsLinked to original sources

arXiv2026

A unified description of flow-induced scission of wormlike micelles under shear and extensional flows

We investigate flow-induced scission of wormlike micelles under different flow kinematics using dissipative particle dynamics simulations of surfactant solutions in shear, uniaxial extensional, planar extensional, and biaxial extensional flows. The average lifetime of wormlike micelles is used to quantify the degree of scission. While flow-induced scission occurs in all flow types when the deformation rate is sufficiently high, the dependence of the average lifetime on the deformation rate differs among flow types. To provide a unified description, we introduce an effective extension rate determined by the velocity gradient tensor and micellar orientation. When expressed in terms of this effective extension rate, the average lifetimes obtained under all flow types considered collapse onto a single curve. These results demonstrate that a unified description of flow-induced scission requires not only the strength and kinematics of the imposed flow, but also the micellar orientation relative to the extensional direction.

cond-mat.soft↗

arXiv2026

Stress Drops Associated with Surface Crack Formation in Photo-aged Polypropylene during Three-Point Bending

Using three-point bending, this study investigates surface-crack formation in photo-aged polypropylene (PP) that has a depth-dependent aging gradient. PP undergoes embrittlement under ultraviolet (UV) irradiation, and because the photo-oxidation proceeds inward from the irradiated surface, the embrittlement develops non-uniformly across the specimen thickness. PP specimens were mildly photo-aged by UV irradiation and had not yet developed visible surface cracks. Each specimen was bent in two configurations: with the UV-irradiated ("aged") surface on the tensile side, and with the opposite ("reverse") surface on the tensile side. When the aged surface was on the tensile side, the stress-strain curves exhibited several discrete stress drops, and in-situ side-view observation confirmed that the formation of each new surface crack coincided with a stress drop. In contrast, no clear stress drops were observed when the reverse surface was on the tensile side. These results show that the through-thickness gradient of embrittlement is directly reflected in the bending stress-strain response. Uniaxial tensile testing, the standard method for evaluating mechanical properties, formally assumes a nominally uniform deformation across the cross-section and therefore reflects the spatially averaged response. Three-point bending, by contrast, imposes the largest tensile strain at the specimen surface and thus selectively probes the embrittled surface layer, making it an effective method for detecting the surface embrittlement of photo-aged polymers.

cond-mat.soft↗

arXiv2026

A Generalized Mechanical Model for the Cycle Rank Dependence of Stretch at Break in Phantom Chain Star Polymer Networks

A simple mechanical model was recently proposed to explain the universality of stretch at break (λ_b) as a function of cycle-rank density (ξ) in phantom-chain network simulations [J Non-Newtonian Fluid Mech., 349, 105620 (2026)]. Here, that model is reformulated as a series of the bottleneck strand and the surrounding network, yielding λ_b-1=(λ_bs-1)[1+ν_h/(1+cξ)]. In this formula, λ_bsis the stretch at break of the bottleneck strand, ν_h is the number of stiff units in series along the rupture path, and c is a geometric constant for the parallel redundancy of the medium. Since c and ν_hare difficult to separate over the examined range of ξ, c is fixed, and λ_bs and ν_hare treated as fitting parameters. The formula is applied to phantom-chain simulations of networks with various conditions. In all cases, it reasonably captures the data, and the two parameters represent network characteristics.

cond-mat.soft↗

arXiv2026

Statistics of rupture in phantom chain network simulations

Phantom chain simulations have shown that the mean rupture properties of star polymer networks collapse onto master curves against the cycle rank density $ξ$. This study revisits this universality with a much larger ensemble than in earlier studies to discuss the statistics. Phantom Gaussian networks were made by end-linking star prepolymers, and 1,000 realizations were collected for each of 30 conditions with functionality $f=3$--$8$ and conversion $p=0.60$--$0.95$, giving 30,000 networks in total. For each realization, the breaking stretch $λ_b$, the breaking stress $σ_b$, the breaking energy $W_b$, and the cycle rank $ξ$ were recorded. The master curves are unchanged by the larger sample, demonstrating that the earlier conclusions reported for the averages of smaller ensembles hold. However, the individual realizations are inherently random, and their statistical properties, rather than the individual values, are examined. At fixed $f,p$, the fluctuation of $ξ$ is small, varying by less than 0.01, whereas $λ_b$, $σ_b$, and $W_b$ scatter by 0.05--0.3. The fluctuation of $ξ$ is almost uncorrelated with that of the breaking properties. In addition, the scatter has a definite structure; its magnitude decreases with the mean cycle rank density $ξ$, the $λ_b$--$σ_b$ correlation grows with $ξ$, and the distributions deviate from Gaussian. The $λ_b$ distribution is skewed to the right at small $ξ$, whereas $σ_b$ is skewed to the left at large $ξ$. These rupture statistics were discussed in the framework of extreme-value statistics to demonstrate that the observed trends are opposite to those of the random fuse model, in which strength decreases with size and weakest-link statistics appear for weak disorder. The difference may reflect the source of fluctuation, i.e., the cross-linking in the present networks.

cond-mat.soft↗

arXiv2026

Anomalous statistics in the Langevin equation with fluctuating diffusivity: from Brownian yet non-Gaussian diffusion to anomalous diffusion and ergodicity breaking

Diffusive motion is a fundamental transport mechanism in physical and biological systems, governing dynamics across a wide range of scales -- from molecular transport to animal foraging. In many complex systems, however, diffusion deviates from classical Brownian behaviour, exhibiting striking phenomena such as Brownian yet non-Gaussian diffusion (BYNGD) and anomalous diffusion. BYNGD describes a frequently observed statistical feature characterised by the coexistence of linear mean-square displacement (MSD) and non-Gaussian displacement distributions. Anomalous diffusion, in contrast, involves a nonlinear time dependence of the MSD and often reflects mechanisms such as trapping, viscoelasticity, heterogeneity, or active processes. Both phenomena challenge the conventional framework based on constant diffusivity and Gaussian statistics. This review focuses the theoretical modelling of such behaviour via the Langevin equation with fluctuating diffusivity (LEFD) -- a flexible stochastic framework that captures essential features of diffusion in heterogeneous media. LEFD not only accounts for BYNGD but also naturally encompasses a wide range of anomalous transport phenomena, including subdiffusion, ageing, and weak ergodicity breaking. Ergodicity is discussed in terms of the correspondence between time and ensemble averages, as well as the trajectory-to-trajectory variability of time-averaged observables. The review further highlights the empirical relevance of LEFD and related models in explaining diverse experimental observations and underscores their value to uncovering the physical mechanisms governing transport in complex systems.

cond-mat.stat-mech↗

arXiv2026

Effect of flow kinematics on extensional viscosity of dilute polymer solutions

We investigate the effect of flow kinematics on the extensional viscosity of dilute polymer solutions by conducting dissipative particle dynamics simulations under uniaxial, planar, and biaxial extensional flows. At high extension rates, dilute polymer solutions exhibit strain hardening under these flows, while the quantitative behavior depends on the flow type. To elucidate the physical origin of this flow-kinematics dependence, we relate the extensional viscosity to polymer conformation using an analytical expression derived from a single-chain model. The resulting relation allows us to separate the contribution of flow-induced polymer conformational changes and the purely kinematic contribution determined by the structure of the velocity gradient tensor. When polymers remain almost unperturbed by extensional flows, differences in the extensional viscosity are governed primarily by the purely kinematic effects. In contrast, as polymers are stretched, the gyration radius in the extensional direction becomes the dominant factor, and differences in the stretching degree in this direction lead to corresponding variations in the extensional viscosity.

cond-mat.soft↗

arXiv2025

Relation between extensional viscosity and polymer conformation in dilute polymer solutions

We investigate extensional viscosity and polymer conformation in dilute polymer solutions under uniaxial extensional flow using dissipative particle dynamics simulations. At high extension rates, polymers are significantly stretched by extensional flows, and the extensional viscosity growth function exhibits strain hardening. To reveal their quantitative relation, we adopt an analysis method based on the Rouse-type model. We demonstrate that the extensional viscosity growth function is determined by the instantaneous gyration radii in the parallel and perpendicular directions to the extensional direction and their time derivatives. Our approach also provides a unified description of the steady-state extensional viscosity of dilute polymer solutions for various chain lengths and concentrations in terms of the polymer gyration radius.

cond-mat.soft↗

arXiv2025

Effects of Stirring Time on Formation of Microplastics Fragmented from Photo-aged Polypropylene

This paper examines the evolution of microplastic (MP) size distributions fragmented from photo-aged polypropylene (PP) in stirred water. PP specimens fragmented into MPs with their size of 1-30 um after UV irradiation and stirring in laboratory settings. These laboratory-fragmented MPs were dispersed into the water during the stirring process. A series of MP size distributions was analyzed from optical microscope images of obtained MPs. The MP size distribution was described by an exponential function in the short stirring time domain, whereas it changed to a power-law function as the stirring time increased. The fragmentation rate of MPs and nanoplastics (NPs) decreased with increasing stirring time. The obtained MP exhibited higher crystallinity than the photo-aged PP specimen after stirring. This result implies that MP fragmentation, as observed under controlled laboratory conditions, is related with the chemi-crystallization of PP.

cond-mat.soft↗

arXiv2025

A Review on Molecular Simulations for the Rupture of Polymer Networks

Molecular simulations provide a powerful means to unravel the complex relationships between network architecture and the mechanical response of polymer networks, with a particular emphasis on rupture and fracture phenomena. Although simulation studies focused on polymer network rupture remain relatively limited compared to the broader field, recent advances have enabled increasingly nuanced investigations that bridge molecular structures and macroscopic failure behaviors. This review surveys the evolution of molecular simulation approaches for polymer network rupture, from early studies on related materials to state-of-the-art methods. Key challenges, including mismatched spatial and temporal scales with experiments, the validity of coarse-grained models, the choice of simulation protocols and boundary conditions, and the development of meaningful structural descriptors, are critically discussed. Special attention is paid to the assumptions underlying universality, limitations of current methodologies, and the ongoing need for theoretically sound and experimentally accessible network characterization. Continued progress in computational techniques, model development, and integration with experimental insights will be essential for a deeper, predictive understanding of polymer network rupture.

cond-mat.soft↗

arXiv2025

Information-Theoretical Approach to Relaxation Time Distribution in Rheology: Log-Normal Relaxation Spectrum Model

The relaxation modulus of a viscoelastic fluid can be decomposed into multiple Maxwell models and characterized by the relaxation spectrum for the relaxation time. It is empirically known that the logarithmic relaxation time is useful to express the relaxation spectrum. We use information geometry to analyze the relaxation modulus and shown that the logarithmic relaxation time is the most natural variable for the relaxation spectrum. Then we use information theory to estimate the most probable functional form for the relaxation spectrum. We show that the log-normal distribution is the information-theoretically most probable relaxation spectrum. We analyze the properties of the log-normal relaxation spectrum model and compare it with the fractional Maxwell model. The fractional Maxwell model with a small power-law exponent can be approximated as the log-normal relaxation spectrum model with a large standard deviation. We also compare the log-normal relaxation spectrum model with experimental linear viscoelasticity data for a high-density polyethylene, both at melt and solid states.

cond-mat.soft↗

arXiv2025

Influence of Stretching Boundary Conditions on Fracture in Phantom Star Polymer Networks: From Volume to Cross-sectional Area Conservation

This study systematically investigates the effect of stretching boundary conditions, ranging from conservation of cross-sectional area to conservation of volume, on the rupture behavior of phantom star polymer networks using energy-minimizing coarse-grained molecular simulations. By continuously varying the deformation parameter, the simulations reveal that true stress and rupture characteristics, such as strain and stress at break and work for rupture, systematically decrease as the boundary condition approaches cross-sectional area conservation. In contrast, nominal stress and the corresponding rupture characteristics exhibit near-independence from boundary conditions, indicating that bond tension remains largely unaffected for phantom networks under the examined conditions. These results clarify that volume expansion primarily drives deviations in true stress and highlight a critical distinction between true and nominal stress-strain definitions. The difference between true and nominal stress-strain relations also affected the scaling exponent for strand length dependence on stretch at break. The findings stress the importance of specifying both deformation boundary conditions and stress-strain definitions in polymer network simulations for accurate interpretation of mechanical properties.

cond-mat.soft↗

arXiv2025

Steady-state extensional viscosity of wormlike micellar solutions via dissipative particle dynamics simulations

We investigate the steady-state extensional viscosity of wormlike micellar solutions using dissipative particle dynamics simulations. As the extension rate increases, the steady-state extensional viscosity initially increases and subsequently decreases after reaching a maximum, as observed in experiments. We reveal that this nonmonotonic behavior arises from the competition between micellar stretching and scission under uniaxial extensional flow. We further propose a relation that connects the extensional viscosity to micellar structures and kinetics. This relation provides a unified description of the extensional viscosity of unentangled wormlike micellar solutions for various temperatures, concentrations, and extension rates.

cond-mat.soft↗

arXiv2025

Direct observation of the compression behavior of polystyrene microbeads in a diamond anvil cell

The pressure dependence of the bulk modulus of glassy polystyrene (PS) was measured in the relatively high-pressure regime, up to 6 GPa, at ambient temperature. For the measurements, PS microbeads were immersed in a pressure medium consisting of a mixture of methanol and ethanol, and the sample was placed in a diamond anvil cell capable of generating high and hydrostatic pressure. The volume change of the PS beads was observed under an optical microscope. The results demonstrated that the volume change in this study is consistent with an equation of state determined from the earlier studies in the low-pressure range up to 0.2 GPa. The bulk modulus was obtained as the derivative of the microbead volume with respect to pressure, and compared with the earlier data obtained from Brillouin spectroscopy.

cond-mat.soft↗

arXiv2025

Dislocation Glides in Monolayered Granular Media: Effect of Lattice Constant

A recent study demonstrated that granular crystals containing a single dislocation exhibit dislocation glide analogous to that observed in atomic-scale crystals, resulting in plastic deformation at yield stresses several orders of magnitude lower than those of dislocation-free crystals. The yielding behavior strongly depends on the interparticle friction coefficient $μ$: dislocation glide occurs for friction coefficients below a critical value $μ_c$, while crystalline order deteriorates above $μ_c$. In this work, we use discrete element method simulations to systematically investigate how the lattice constant, which determines the interparticle spacing and is a fundamental parameter in microscopic crystalline solids, and the friction coefficient $μ$ influence the yielding behavior in monolayered granular crystals with dislocation. By decreasing the lattice constant, we find an increase in the critical friction coefficient $μ_c$, allowing dislocation glide to persist at higher friction values. Furthermore, we observe a linear scaling of yield stress with normal stress, except at extremely low friction coefficients.

cond-mat.soft↗

arXiv2025

Nonmonotonic concentration dependence of the self-diffusion coefficient of surfactants in wormlike micellar solutions

We investigate the concentration dependence of surfactant diffusion in wormlike micellar solutions using dissipative particle dynamics simulations. The simulations show that the self-diffusion coefficient of surfactants exhibits a nonmonotonic dependence on the surfactant concentration, as observed in previous experiments. We quantitatively reveal that this nonmonotonic behavior results from the competition between micellar center-of-mass diffusion and surfactant diffusion within micelles by decomposing the mean-square displacement of surfactants into the corresponding contributions. Furthermore, our detailed analyses demonstrate how the competition between the two diffusion mechanisms is governed by the aggregation number distribution, the dynamics of individual surfactants and micelles, and the kinetics of micellar scission and recombination.

cond-mat.soft↗

arXiv2025

Coarse-Grained Molecular Dynamics Simulations for Oxidative Aging of Polymers under Various O2 Concentration

Modeling of polymer oxidative aging has been actively studied since the 1990s. Insights from these studies suggest that the transport of oxygen and radicals significantly influences aging heterogeneity, alongside chemical reaction kinetics. A recent simulation study [Ishida et al., Macromolecules, 56(21), 8474-8483, 2023] demonstrated that mesoscale heterogeneity arises when the H-abstraction reaction occurs faster than the relaxation times of polymer chains. In this study, the simulations were extended by modeling the rate of oxygen addition to polymer radicals (k_2) to reflect the effects of the O2 concentration. Three key aspects of oxidative aging behavior were found to be influenced by the O2 addition rate: (i) reaction kinetics, (ii) the degree of heterogeneity, and (iii) amount of crosslinking. Namely, reducing O2 concentration slows the conversion of polymer radicals into H-abstractable peroxyl radicals. This deceleration delays H-abstraction reactions, increases the number of polymer radicals, and promotes crosslinking reactions between two polymer radicals.

cond-mat.soft↗

arXiv2024

Radius of Gyration in Shear Gradient Direction Governs Steady Shear Viscosity of Rouse-Type Model

We analyze the stress tensor and the gyration tensor of an unentangled polymer melt under flow by using a Rouse-type single chain model. We employ the bead-spring type single chain model, in which beads interact each other via nonlinear potentials such as the finite-extensible nonlinear elasticity (FENE) potential. Beads are assumed to obey the Langevin equation with a constant friction coefficient. We derive simple yet general relations between the stress tensor and the gyration tensor for this Rouse-type model, without any additional approximations. Various formulae for rheological quantities in terms of the gyration tensor can be derived from the general relations. For example, the steady shear viscosity is governed by the gyration radius in the shear gradient direction.

cond-mat.soft↗

arXiv2024

Coarse-Grained Simulation Model for Crystalline Polymer Solids by using Breakable Bonds

We propose a highly coarse-grained simulation model for crystalline polymer solids with crystalline lamellar structures. The mechanical properties of a crystalline polymer solid are mainly determined by the crystalline lamellar structures. This means that coarse-grained models rather than fine-scale molecular models are suitable to study mechanical properties. We model a crystalline polymer solid by using highly coarse-grained particles, of which size is comparable to the crystalline layer thickness. One coarse-grained particle consists of multiple subchains, and is much larger than monomers. Coarse-grained particles are connected by bonds to form a network structure. Particles are connected by soft but ductile bonds, to form a rubber-like network. Particles in the crystalline region are connected by hard but brittle bonds. Brittle bonds are broken when large deformations are applied. We perform uniaxial elongation simulations based on our coarse-grained model. As the applied strain increases, crystalline layers are broken into pieces and non-affine and collective motions of broken pieces are observed. Our model can successfully reproduce yield behaviors which are similar to typical crystalline polymer solids.

cond-mat.soft↗