arXiv subjects

Toshihiro Kawakatsu

Publications and source records attributed to Toshihiro Kawakatsu.

At least 19 recordsLinked to original sources

arXiv2026

Identifying the Threshold Chain Length for Stress Overshoot in Ring-Linear Polymer Blends under Uniaxial Elongation: The Role of Multiple Threading

The rheological behavior of ring-linear polymer blends under uniaxial elongational flow has remained a subject of intense debate, particularly regarding the emergence of stress overshoot. Herein, we employ coarse-grained molecular dynamics simulations to investigate the chain-length dependence of elongational viscosity in 1:1 ring-linear blends of flexible chains with the equal molecular weight. Our results reveal a distinct threshold in the degree of threading, quantified by the number of entanglements Z = N /Ne (where N is the number of beads per chain and Ne is the entanglement chain length), for the appearance of stress overshoot: while blends with shorter chains (Z $\le$ 2) exhibit monotonic stress growth, a clear stress overshoot emerges when the chain length reaches a threshold value (Z $\approx$ 4). Consistent with previous reports, this overshoot originates from a thread-to-unthread transition. At the threshold chain length, multiple linear chains penetrate a single ring, providing sufficient topological constraints to significantly stretch the ring under elongational flow. We predict that this transition can be experimentally validated via 2D small-angle neutron scattering patterns in the plane of the stretching and perpendicular directions, offering a direct structural signature of the ring recoil process for future experimental verification.

cond-mat.soft↗

arXiv2025

Self-Consistent Field Theory for Semiflexible Gaussian Chain Model

Self-consistent field theory (SCFT) is one of the useful methods to simulate phase separated structures of multi-component polymer systems. In this article, we propose an SCFT for semiflexible polymer melts, where the basic equations for the SCFT are derived by introducing a bending stiffness into a flexible Gaussian bead-spring model and taking its continuous limit. Our SCFT is described by a coupled modified diffusion equations for the statistical weight of the chain conformation (path integral), which is a perturbation to semiflexible chains from the flexible Gaussian chain model. Using our modified diffusion equations, we investigated the influences of the bending stiffness on the conformations of symmetric semiflexible diblock copolymer in a strongly segregated lamellar structures and on the order-disorder transition.

cond-mat.soft↗

arXiv2023

Translocation of a daughter vesicle in a model system of self-reproducing vesicles

Translocation of a daughter vesicle from a mother vesicle through a pore is experimentally studied by many groups using a model system of self-reproducing vesicles. However, the theoretical formulation of the problem is not fully understood. In the present study, we present a theoretical formulation of the process based on our previous work [P. Khunpetch et al., Phys. Fluids 33, 077103 (2021)]. In our previous work, we considered the daughter vesicle as a rigid body. In the present work, however, we allow the daughter vesicle to deform during the expulsion process. We thus derive the free energy constituting of the elastic moduli of both the mother and daughter vesicles, and of pressure-driven contribution. The minimum energy path of the translocation is searched by using the string method. Our improved model successfully suggests the disappearance of the energy barrier where all the elastic moduli are in agreement with the experimental reports, while the previous work is unsuccessful to do so. The equations of motion of the daughter vesicle have been derived within the framework of the Onsager principle. We found that the translocation time of the daughter vesicle can be reduced when the pressure inside the mother vesicle increases, or the initial size of the daughter vesicle decreases.

cond-mat.soft↗

arXiv2022

Topological transition in multicyclic chains with structural symmetry inducing stress-overshoot phenomena in multicyclic/linear blends under biaxial elongational flow

Blends of multicyclic and linear polymers under biaxial elongational flow were analyzed using coarse-grained molecular dynamics simulations. The multicyclic/linear blends displayed overshoot in the normal stress difference at the start-up of biaxial elongational flow. This overshoot was steeper for multicyclic/linear blends than for our previously reported monocyclic/linear blends [T. Murashima, K. Hagita, and T. Kawakatsu, Macromolecules, 2021, 54, 7210]. Investigation of the origin of the overshoot in the multicyclic/linear blends revealed a different mechanism than that previously observed in our monocyclic/linear blends. Specifically, a "topological transition" mechanism comprising a morphological change from the open- to closed-ring state was observed in the multicyclic chains, but not in the monocyclic chains. This topological transition drastically changes the stress of the rings. Although the topological transition was also observed in asymmetric-multicyclic/linear blends, no stress overshoot appeared, owing to the asymmetry in the multicyclic chains. Therefore, we hypothesized that the structural symmetry in multicyclic chains is indispensable for overshoot behavior to occur. We determined that the topological transition in multicyclic chains with structural symmetry induces stress-overshoot behavior in multicyclic/linear blends under biaxial elongational flow.

cond-mat.soft↗

arXiv2021

Modeling of chemically active particles at an air-liquid interface

The collective motion of chemically active particles at an air-liquid interface is studied theoretically as a dynamic self-organization problem. Based on a physical consideration, we propose a minimal model for self-propelled particles by combining hydrodynamic interaction, capillary interaction, driving force by Marangoni effect, and Marangoni flow. Our model has successfully captured the features of chemically active particles, that represent dynamic self-organized states such as crystalline, chain, liquid-like and spreading states.

cond-mat.soft↗

arXiv2021

Birthing of a daughter vesicle in a model system for self-reproduction vesicles

Sakuma and Imai [Phys. Rev. Lett. 107, 198101 (2011)] established a temperature-controlled cyclic process for a model system of self-reproducing vesicles without feeding. The vesicle generates a smaller inclusion vesicle called "daughter vesicle" inside the original vesicle (we call this "mother vesicle") and then the daughter vesicle is expelled through a small pore on the mother vesicle. This self-reproducing process is called birthing. In the present study we present theoretical model on the birthing process of a single, rigid daughter vesicle through a pore. By using a simple geometric picture, we derive the free energy constituting the material properties of the bending, stretching and line tension moduli of the mother vesicle, as a function of the distance between the centers of the daughter and mother vesicles, and the size of the daughter vesicle. We see clearly the disappearance of the energy barrier by selecting appropriate moduli. The dynamics of the system is studied by employing the Onsager principle. The results indicate that translocation time decreases as the friction parameter decreases, or the initial size of the daughter vesicle decreases.

cond-mat.soft↗

arXiv2021

Molecular-shape- and size-independent power-law dependence of percolation thresholds on radius of gyration in ideal molecular systems

Three-dimensional single-component ideal gas systems composed of model homogeneous rigid molecules in various molecular shapes and sizes are simulated by a molecular Monte Carlo simulation technique. We reveal that percolation thresholds of such single-component systems result in, when the molecular volume is fixed, power-law decreasing functions of the radius of gyration (gyradius) of the molecules. The systems with the same parameter set of the molecular volume and radius of gyration, but in different molecular shapes, show the identical value of the percolation threshold. Moreover, we also reveal that a dimensionless scale-free parameter, which is the ratio between the radius of gyration and real cube root of the molecular volume, uniquely determines the percolation threshold.

cond-mat.soft↗

arXiv2020

Hybrid Particle-Field Molecular Dynamics Under Constant Pressure

Hybrid particle-field methods are computationally efficient approaches for modelling soft matter systems. So far applications of these methodologies have been limited to constant volume conditions. Here, we reformulate particle-field interactions to represent systems coupled to constant external pressure. First, we show that the commonly used particle-field energy functional can be modified to model and parameterize the isotropic contributions to the pressure tensor without interfering with the microscopic forces on the particles. Second, we employ a square gradient particle-field interaction term to model non-isotropic contributions to the pressure tensor, such as in surface tension phenomena. This formulation is implemented within the hybrid particle-field molecular dynamics approach and is tested on a series of model systems. Simulations of a homogeneous water box demonstrate that it is possible to parameterize the equation of state to reproduce any target density for a given external pressure. Moreover, the same parameterization is transferable to systems of similar coarse-grained mapping resolution. Finally, we evaluate the feasibility of the proposed approach on coarse-grained models of phospholipids, finding that the term between water and the lipid hydrocarbon tails is alone sufficient to reproduce the experimental area per lipid in constant-pressure simulations, and to produce a qualitatively correct lateral pressure profile.

cond-mat.soft↗

arXiv2019

Influence of polymer bidispersity on the effective particle-particle interactions in polymer nanocomposites

We investigate the role played by the bidispersity of polymer chains on the local structure and the potential of mean force (PMF) between silica nanoparticles (NPs) in a polystyrene melt. We use the hybrid particle-field molecular dynamics technique which allows to efficiently relax polymer nanocomposites even with high molecular weights.The NPs we investigate are either bare or grafted with polystyrene chains immersed in a melt of free polystyrene chains, whereas the grafted and the free polystyrene chains are either monodisperse or bidisperse. The two-body PMF shows that a bidisperse distribution of free polymer chains increases the strength of attraction between a pair of ungrafted NPs. If the NPs are grafted by polymer chains, the effective interaction crucially depends on bidispersity and grafting density of the polymer chains: for low grafting densities, the bidispersity of both free and grafted chains increases the repulsion between the NPs, whereas for high grafting densities we observe two different effects. An increase of bidispersity in free chains causes the rise of the repulsion between the NPs, while an increase of bidispersity in grafted chains promotes the rise of attraction. Additionally, a proper treatment of multi-body interactions improves the simpler two-body PMF calculations, in both unimodal and bimodal cases. We found that, by properly tuning the bidispersity of both free and grafted chains, we can control the structure of the composite materials, which can be confirmed by experimental observations. As a result, the hybrid particle-field approach is confirmed to be a valid tool for reproducing and predicting microscopic interactions, which determine the stability of the microscopic structure of the composite in a wide range of conditions.

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arXiv2019

Nucleation theory of polymer crystallization with conformation entropy

Based on classical nucleation theory, we propose a couple of theoretical models for the nucleation of polymer crystallization, i.e. one for a single chain system (Model S) and the other for a multi-chain system (Model M). In these models, we assume that the nucleus is composed of tails, loops and a cylindrical ordered region, and we evaluate the conformation entropy explicitly by introducing a transfer matrix. Using these two models, we evaluate the occurrence probability of critical nucleus as a function of the polymer chain stiffness. We found that the critical nucleus in Model M is easier to occur than in Model S because, for semi-flexible chains, the nucleus in Model M can grow by adding a new polymer chain into the nucleus rather than to diminish the loop and tail parts as in the case of Model S.

cond-mat.soft↗

arXiv2018

Molecular Structure and Multi-Body Potential of Mean Force in Silica-Polystyrene Nanocomposites

We perform a systematic application of the hybrid particle-field molecular dynamics technique [Milano et al, J. Chem. Phys. 2009, 130, 214106] to study interfacial properties and potential of mean force (PMF) for separating nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or grafted with Polystyrene chains, aiming to shed light on the interactions among free and grafted chains affecting the dispersion of NPs in the nanocomposite. The proposed hybrid models show good performances in catching the local structure of the chains, and in particular their density profiles, documenting the existence of the "wet-brush-to-dry-brush" transition. By using these models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene matrix are calculated. Moreover, we estimate the three-particle contribution to the total PMF and its role in regulating the phase separation on the nanometer scale. In particular, the multi-particle contribution to the PMF is able to give an explanation of the complex experimental morphologies observed at low grafting densities. More in general, we propose this approach and the models utilized here for a molecular understanding of specific systems and the impact of the chemical nature of the systems on the composite final properties.

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arXiv2018

Elongational viscosity of weakly entangled polymer melt via coarse-grained molecular dynamics simulation

We investigated the elongational flows of the weakly entangled linear polymer melt using a coarse-grained molecular dynamics simulation. We extended the uniform extensional flow (UEF) method developed by Nicholson and Rutledge (D. A. Nicholson and G. C. Rutledge, J. Chem. Phys., 145, 244903 (2016)) for application to Langevin dynamics. We succeeded in observing the elongational viscosity of the weakly entangled linear polymer melt from the equilibrium state to the steady state using the extended UEF method, whereas the conventional rectangular parallelepiped shape technique for extensional flows has failed to do so for over 20 years.

cond-mat.soft↗

arXiv2018

Thermodynamic stability of multicomponent ideal gas

We present an example counter to the widely-accepted concept on equilibrium states that "Any thermodynamic equilibrium state of two component systems is determined by specifying 4 thermodynamic variables that include at least 1 extensive variable." which is equivalent to Gibbs' phase rule. We demonstrate this fact by thought experiments on an A/B binary mixture where temperature, pressure and chemical potential of the A-species are chosen as the 3 intensive variables while the number of B-molecules is chosen as the 1 extensive variable. Our findings also apply to $M (>2)$-component systems.

cond-mat.soft↗

arXiv2018

pH Sensing by Lipids in Membranes: The Fundamentals of pH-driven Migration, Polarization and Deformations of Lipid Bilayer Assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described.

physics.bio-ph↗

arXiv2017

Modeling induction period of polymer crystallization

We study the possibility of the spinodal decomposition in the induction period of the polymer crystallization. This phenomenon was first reported in an X-ray scattering experiment, and has still been controversial due to various experiments and theories that support or deny the phenomenon. In this article, we explain the condition for the spinodal decomposition to occur in polymer melts by deriving a Ginzburg-Landau model of the free energy as a functional of the density and the orientation of the segments, where we introduce the excluded volume and the nematic interactions through a combination of the random phase approximation and the transfer matrix for the polymer conformation. We show that, upon elimination of the degrees of freedom of the orientation, the nematic interaction reduces to an effective attraction whose strength increases with the stiffness of the polymer chain. Such an attraction induces spinodal decomposion especially for stiff polymer chain case.

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arXiv2014

Onsager's Variational Principle for the Dynamics of a Vesicle in a Poiseuille Flow

We propose a systematic formulation of the migration behaviors of a vesicle in a Poiseuille flow based on Onsager's variational principle. Our model is described by a combination of the phase field theory for the vesicle and the hydrodynamics for the flow field. The time evolution equations for the phase field of the vesicle and the flow field are derived based on the Onsager's principle, where the dissipation functional is composed of viscous dissipation of the flow field, bending energy of the vesicle and the friction between the vesicle and the flow field. We performed a series of simulations on 2-dimensional systems by changing the bending elasticity of the membrane, and observed 3 types of steady states, i.e. those with bullet, snaking, and slipper shapes. We show that the transitions among these steady states can be quantitatively explained with use of the Onsager's principle, where the dissipation functional is dominated by the contribution from the friction between the vesicle and the flow field.

cond-mat.soft↗

arXiv2014

Soft Confinement for Polymer Solutions

As a model of soft confinement for polymers, we investigated equilibrium shapes of a flexible vesicle that contains a phase-separating polymer solution. To simulate such a system, we combined the phase field theory (PFT) for the vesicle and the self-consistent field theory (SCFT) for the polymer solution. We observed a transition from a symmetric prolate shape of the vesicle to an asymmetric pear shape induced by the domain structure of the enclosed polymer solution. Moreover, when a non-zero spontaneous curvature of the vesicle is introduced, a re-entrant transition between the prolate and the dumbbell shapes of the vesicle is observed. This re-entrant transition is explained by considering the competition between the loss of conformational entropy and that of translational entropy of polymer chains due to the confinement by the deformable vesicle. This finding is in accordance with the recent experimental result reported by Terasawa, et al.

physics.bio-ph↗

arXiv2011

Particle Monte Carlo simulation of string-like colloidal assembly in 2 dimensions

We simulate structural phase behavior of polymer-grafted colloidal particles by molecular Monte Carlo technique. Interparticle potential, which has a finite repulsive square-step outside a rigid core of the colloid, was previously confirmed via numerical self-consistent field calculation. This model potential is purely repulsive. We simulate these model colloids in the canonical ensemble in 2 dimensions and find that these particles containing no interparticle attraction self-assemble and align in a string-like assembly, at low temperature and high density. This string-like colloidal assembly is related to percolation phenomena. Analyzing the cluster size distribution and the average string length, we build phase diagrams and discover that the average string length diverges around the region where the melting transition line and the percolation transition line cross. This result is similar to Ising spin systems, in which the percolation transition line and the order-disorder line meet at a critical point.

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