arXiv subjects

Yusuke Koide

Publications and source records attributed to Yusuke Koide.

17 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

Lagrangian velocity statistics of homogeneous isotropic turbulence in dilute polymer solutions

We conduct direct numerical simulations of homogeneous isotropic turbulence in dilute polymer solutions to investigate the Lagrangian velocity statistics. We show how polymers modulate the power spectral density of the Lagrangian velocity and the Lagrangian integral timescale by varying the Reynolds number, forcing method, and polymer relaxation time. As the polymer relaxation time increases, the attenuation of the power spectral density extends successively from high to low frequencies, and the Lagrangian integral timescale increases. To clarify the mechanism underlying the modulation of the Lagrangian velocity statistics, we decompose the Lagrangian velocity into the contributions from vortices at different length scales. Using this scale-decomposition analysis, we demonstrate that the observed modulation of the Lagrangian velocity statistics results from polymer-induced suppression of vortices that proceeds from smaller to larger scales.

physics.flu-dyn↗

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

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

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

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

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

Relationship between the power spectral density of the Lagrangian velocity and the hierarchy of coherent vortices in turbulence

We conduct direct numerical simulations of developed turbulence in a periodic cube to investigate the formation mechanism of the power spectral density of the Lagrangian velocity. We compare the power spectral density of the Lagrangian velocity of turbulent flows with different forcing methods and Reynolds numbers. This systematic comparison demonstrates that universal behavior is observed in a narrow high-frequency regime, whereas non-universality originating from the forcing method broadly appears in a low-frequency regime. To reveal the formation mechanism of the spectra in terms of the hierarchy of coherent structures in turbulence, we propose a scale-decomposition method for the Lagrangian velocity, which enables us to evaluate the contribution of vortices at different scales. This scale-decomposition analysis directly demonstrates that the largest-scale flows driven by the external force can contaminate the Kolmogorov scaling of the Lagrangian velocity spectra formed by small-scale vortices in the inertial range, thus leading to the narrow Lagrangian inertial range. Furthermore, we provide evidence that this remarkable effect by the largest-scale flows is specific to the Lagrange velocity by demonstrating that the power spectral density of the Eulerian velocity is less sensitive to the forcing method.

physics.flu-dyn↗

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

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 {\phi}_c from 0.6 to 0.95. For the resultant networks, cycle rank {\xi} was consistent with the mean-field theory. The networks were uniaxially stretched with energy minimization until break to obtain modulus G, strain at break {\epsilon}_b, stress at break {\sigma}_b, and work for fracture W_b. With the branch point density \u{psion}_br, G/\u{psion}_br, {\epsilon}_b, {\sigma}_b/\u{psion}_br, and W_b/\u{psion}_br of the data for various f and {\phi}_c draw master curves if plotted against {\xi}. The master curves depend on c; as c increases, all the mechanical characteristics monotonically increase. If we plot {\sigma}_b/\u{psion}_br and W_b/\u{psion}_br against G/\u{psion}_br, the data for various f and {\phi}_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↗

arXiv2016

A Multi-Line Ammonia Survey of the Galactic Center Region with the Tsukuba 32-m Telescope - I. Observations and Data

We present survey data of the NH3 (J, K) = (1, 1)--(6, 6) lines, simultaneously observed with the Tsukuba 32-m telescope, in the main part of the central molecular zone of the Galaxy. The total number of on-source positions was 2655. The lowest three transitions were detected with S/N > 3 at 2323 positions (93% of all the on-source positions). Among 2323, the S/N of (J, K ) = (4, 4), (5, 5), and (6, 6) exceeded 3.0 at 1426 (54%), 1150 (43%), and 1359 (51%) positions, respectively. Simultaneous observations of the lines enabled us to accurately derive intensity ratios with less systematic errors. Boltzmann plots indicate there are two temperature components: cold ($\sim$ 20 K) and warm ($\sim$ 100 K). Typical intensity ratios of Tmb(2,2)/Tmb(1,1), Tmb(4,4)/Tmb(2,2), Tmb(5,5)/Tmb(4,4), and Tmb(6,6)/Tmb(3,3) were 0.71, 0.45, 0.65, and 0.17, respectively. These line ratios correspond to diversity of rotational temperature, which results from mixing of the two temperature components.

astro-ph.GA↗