arXiv subjects

Yasuya Nakayama

Publications and source records attributed to Yasuya Nakayama.

17 recordsLinked to original sources

arXiv2025

Microrheology with rotational Brownian motion

Passive rotational microrheology (RMR) for evaluating the dynamic modulus $G^*$ of a suspending fluid through the rotational Brownian motion of a spherical probe particle is validated using direct numerical simulations (DNS) of Brownian motion in a viscoelastic fluid. Two methods of RMR are compared: an inertialess RMR based on the Generalized Stokes--Einstein relation for rotational diffusion (RGSER) and the full RMR based on the generalized Langevin equation for rotation, which accounts for fluid and particle inertia. Our analysis, performed using DNS of the fluctuating Oldroyd-B fluid, reveals that inertialess RMR accurately estimates $G^*$ for $ωλ\alt 1$, but deviates significantly at high frequencies. In contrast, the full RMR improves $G^*$ estimation accuracy up to the frequency $ω\approx τ_{s}^{-1}=η_{s}/ρ_{f}a^{2}$, where fluid inertia becomes relevant. However, in the ballistic regime ($t \ll τ_{s}$), particle inertia dominates, making accurate $G^*$ evaluation challenging even with the full RMR. This study clarifies the applicability range of RMR. Additionally, rotational Brownian motion is turned out to be insensitive to periodic boundary conditions, which allows direct application to various mesoscale molecular simulations, including coarse-grained molecular dynamics, dissipative particle dynamics, and fluid dynamics simulations. In conclusion, rotational microrheology offers a promising approach for detailed rheological analysis in complex systems and conditions.

cond-mat.soft↗

arXiv2025

Nonlinear Dielectric Decrement of Electrolyte Solutions: an Effective Medium Approach

Hypothesis. The dielectric constant of an electrolyte solution, which determines electrostatic interactions between colloids and interfaces, depends nonlinearly on the salinity and also on the type of salt. The linear decrement at dilute solutions is due to the reduced polarizability in the hydration shell around an ion. However, the full hydration volume cannot explain the experimental solubility, which indicates the hydration volume should decrease at high salinity. Volume reduction of the hydration shell is supposed to weaken dielectric decrement and thus should be relevant to the nonlinear decrement. Simulations. According to the effective medium theory for the permittivity of heterogeneous media, we derive an equation which relates the dielectric constant with the dielectric cavities created by the hydrated cations and anions, and the effect of partial dehydration at high salinity is taken into account. Findings. Analysis of experiments on monovalent electrolytes suggests that weakened dielectric decrement at high salinity originates primarily from the partial dehydration. Furthermore, the onset volume fraction of the partial dehydration is found to be salt-specific, and is correlated with the solvation free energy. Our results suggest that while the reduced polarizability of the hydration shell determines the linear dielectric decrement at low salinity, ion-specific tendency of dehydration is responsible for nonlinear dielectric decrement at high salinity.

cond-mat.soft↗

arXiv2025

Brownian motion and microrheology in complex fluids under periodic boundary conditions

We review the theoretical aspects of determining linear rheology using passive microrheology from simulations under periodic boundary conditions (PBC). It is common to impose periodic boundary conditions when evaluating bulk properties by molecular simulation. The Brownian motion of the probe particles is affected by PBCs, and thus their effects must be considered when microrheological analysis is applied to evaluate the dynamic modulus. First, we review the theory of microrheology based on the generalized Langevin equation (GLE) in an unbounded domain. Then, we briefly discuss the effect of periodic boundary conditions on the diffusion coefficient. After that, we explain the effect of PBCs on the mean-square displacement of Brownian particles and their impact on microrheology. The passive microrheology technique under PBCs provides a new approach for rheology evaluation in molecular simulations of complex liquids in addition to the traditional Green--Kubo formula and non-equilibrium molecular dynamics (NEMD).

cond-mat.soft↗

arXiv2021

Prediction of shear-thickening of particle suspensions in viscoelastic fluids by direct numerical simulation

To elucidate the key factor for the quantitative prediction of the shear-thickening in suspensions in viscoelastic fluids, direct numerical simulations of many-particle suspensions in a multi-mode Oldroyd-B fluid are performed using the smoothed profile method. Suspension flow under simple shear flow is solved under periodic boundary conditions by using Lees--Edwards boundary conditions for particle dynamics and a time-dependent oblique coordinate system that evolves with mean shear flow for fluid dynamics. Semi-dilute many-particle suspensions up to a particle volume fraction of 0.1 are investigated. The presented numerical results regarding the bulk rheological properties of the shear-thickening behavior agree quantitatively with recent experimental results of semi-dilute suspensions in a Boger fluid. The presented result clarifies that an accurate estimation of the first normal stress difference of the matrix in the shear-rate range where the shear-thickening starts to occur is crucial for the quantitative prediction of the suspension shear-thickening in a Boger fluid matrix at around the Weissenberg number $\rm{Wi}=1$ by an Oldroyd-B model. Additionally, the effect of suspension microstructures on the suspension viscosity is examined. The paper concludes with a discussion on how the flow pattern and the elastic stress development change with the volume fraction and Weissenberg number.

physics.flu-dyn↗

arXiv2020

Effects of viscoelasticity on shear-thickening in dilute suspensions in a viscoelastic fluid

We investigate previously unclarified effects of fluid elasticity on shear-thickening in dilute suspensions in an Oldroyd-B viscoelastic fluid using a novel direct numerical simulation based on the smoothed profile method. Fluid elasticity is determined by the Weissenberg number Wi and by viscosity ratio $1-β=η_p/(η_s+η_p)$ which measures the coupling between the polymer stress and flow: $η_p$ and $η_s$ are the polymer and solvent viscosity, respectively.As $1-β$ increases, while the stresslet does not change significantly compared to that in the $β\to 1$ limit, the growth rate of the normalized polymer stress with Wi was suppressed.Analysis of flow and conformation dynamics around a particle for different $β$ reveals that at large $1-β$, polymer stress modulates flow, leading to suppression of polymer stretch.This effect of $β$ on polymer stress development indicates complex coupling between fluid elasticity and flow, and is essential to understand the rheology and hydrodynamic interactions in suspensions in viscoelastic media.

cond-mat.soft↗

arXiv2018

Improving mixing characteristics with a pitched tip in kneading elements in twin-screw extrusion

In twin-screw extrusion, the geometry of a mixing element mainly determines the basic flow pattern, which eventually affects the mixing ability as well as the dispersive ability of the mixing element. The effects of geometrical modification, with both forward and backward pitched tips, of a conventional forwarding kneading disks element (FKD) in the pitched-tip kneading disks element on the flow pattern and mixing characteristics are discussed. Numerical simulations of fully-filled, non-isothermal polymer melt flow in the melt-mixing zone were performed, and the flow pattern structure and the tracer trajectories were investigated. The pitched tips largely affects the inter-disc fluid transport, which is mainly responsible for mixing. These changes in the local flow pattern are analyzed by the distribution of the strain-rate state. The distribution of the finite-time Lyapunov exponent reveals a large inhomogeneity of the mixing in FKD is suppressed both by the forward and backward tips. By the forward tips on FKD, the mixing ability is relatively suppressed compared to FKD, whereas for the backward tips on FKD, the mixing ability is enhanced while maintaining the same level of dispersion efficiency as FKD. From these results, the pitched tips on the conventional KD turns out to be effective at reducing the inhomogeneity of the mixing and tuning the overall mixing performance.

physics.flu-dyn↗

arXiv2017

Strain Mode of General Flow: Characterization and Implications for Flow Pattern Structures

Understanding the mixing capability of mixing devices based on their geometric shape is an important issue both for predicting mixing processes and for designing new mixers. The flow patterns in mixers are directly connected with the modes of the local strain rate, which is generally a combination of elongational flow and planar shear flow. We develop a measure to characterize the modes of the strain rate for general flow occurring in mixers. The spatial distribution of the volumetric strain rate (or non-planar strain rate) in connection with the flow pattern plays an essential role in understanding distributive mixing. With our measure, flows with different types of screw elements in a twin-screw extruder are numerically analyzed. The difference in flow pattern structure between conveying screws and kneading disks is successfully characterized by the distribution of the volumetric strain rate. The results suggest that the distribution of the strain rate mode offers an essential and convenient way for characterization of the relation between flow pattern structure and the mixer geometry.

physics.flu-dyn↗

arXiv2016

Effects of pitched tips of novel kneading disks on melt mixing in twin-screw extrusion

In mixing highly viscous materials, like polymers, foods, and rubbers, the geometric structure of the mixing device is a determining factor for the quality of the mixing process. In pitched-tip kneading disks (ptKD), a novel type of mixing element, based on conventional kneading disks (KD), the tip angle is modified to change the channel geometry as well as the drag ability of KD. We discuss the effects of the tip angle in ptKD on mixing characteristics based on numerical simulation of the flow in the melt-mixing zone under different feed rates and a screw rotation speed. It turns out that the passage of fluid through the high-stress regions increases in ptKD compared to conventional KD regardless of the directions and sizes of the tip angle, while the fluctuation in residence time stays at the same level as the conventional KD. Furthermore, pitched tips of backward direction increase the mean applied stress on the fluid elements during its residence in the melt-mixing zone, suggesting the enhancement of dispersive mixing quality in ptKD. These understandings of the role of the tip angle on KD can give a basic guide in selecting and designing suitable angle parameters of ptKD for different mixing purposes.

physics.flu-dyn↗

arXiv2015

Differential capacitance of the electric double layer: The interplay between ion finite size and dielectric decrement

We study the electric double layer by combining the effects of ion finite size and dielectric decrement. At high surface potential, both mechanisms can cause saturation of the counter-ion concentration near a charged surface. The modified Grahame equation and differential capacitance are derived analytically for a general expression of a permittivity epsilon(n) that depends on the local ion concentration, n, and under the assumption that the co-ions are fully depleted from the surface. The concentration at counter-ion saturation is found for any epsilon(n), and a criterion predicting which of the two mechanisms (steric vs. dielectric decrement) is the dominant one is obtained. At low salinity, the differential capacitance as function of surface potential has two peaks (so-called camel-shape). Each of these two peaks is connected to a saturation of counter-ion concentration caused either by dielectric decrement or by their finite size. Because these effects depend mainly on the counter-ion concentration at the surface proximity, for opposite surface-potential polarity either the cations or anions play the role of counter-ions, resulting in an asymmetric camel-shape. At high salinity, we obtain and analyze the crossover in the differential capacitance from a double-peak shape to a uni-modal one. Finally, several nonlinear models of the permittivity decrement are considered, and we predict that the concentration at dielectrophoretic saturation shifts to higher concentration than those obtained by the linear decrement model.

cond-mat.soft↗

arXiv2013

Hydrodynamic Interactions of Self-Propelled Swimmers

The hydrodynamic interactions of a suspension of self-propelled particles are studied using a direct numerical simulation method which simultaneously solves for the host fluid and the swimming particles. A modified version of the "Smoothed Profile" method (SPM) is developed to simulate microswimmers as squirmers, which are spherical particles with a specified surface-tangential slip velocity between the particles and the fluid. This simplified swimming model allows one to represent different types of propulsion (pullers and pushers) and is thus ideal to study the hydrodynamic interactions among swimmers. We use the SPM to study the diffusive behavior which arises due to the swimming motion of the particles, and show that there are two basic mechanisms responsible for this phenomena: the hydrodynamic interactions caused by the squirming motion of the particles, and the particle-particle collisions. This dual nature gives rise to two distinct time- and length- scales, and thus to two diffusion coefficients, which we obtain by a suitable analysis of the swimming motion. We show that the collisions between swimmers can be interpreted in terms of binary collisions, in which the effective collision radius is reduced due to the collision dynamics of swimming particles in viscous fluids. At short time-scales, the dynamics of the swimmer is analogous to that of an inert tracer particle in a swimming suspension, in which the diffusive motion is caused by fluid-particle collisions. Our results, along with the simulation method we have introduced, will allow us to gain a better understanding of the complex hydrodynamic interactions of self-propelled swimmers.

cond-mat.soft↗

arXiv2012

Quantifying hierarchical mixture quality in polymer composite materials: structure and inhomogeneity in multiple scales

Mixture quality plays a crucial role in the physical properties of multi-component immiscible polymer mixtures including nanocomposites and polymer blends. Such complex mixtures are often characterized by hierarchical internal structures, which have not been accounted for by conventional mixture quantifications. We propose a way to characterize the mixture quality of complex mixtures with hierarchical structures. Starting from a concentration field, which can be typically obtained from TEM/SEM images, the distribution of the coarsegrained concentration is analyzed to obtain the scale-dependent inhomogeneity of a mixture. The hierarchical nature of a mixture is characterized by multiple characteristic scales of the scale-dependent inhomogeneity. We demonstrate how the proposed method works to characterize sizes and distributions in different dispersions. This method is generally applicable to various complex mixtures.

cond-mat.soft↗

arXiv2012

Dynamic shear responses of polymer-polymer interfaces

In multi-component soft matter, interface properties often play a key role in determining the properties of the overall system. The identification of the internal dynamic structures in non-equilibrium situations requires the interface rheology to be characterized. We have developed a method to quantify the rheological contribution of soft interfaces and evaluate the dynamic modulus of the interface. This method reveals that the dynamic shear responses of interfaces in bilayer systems comprising polypropylene and three different polyethylenes can be classified as having hardening and softening effects on the overall system: a interface between linear long polymers becomes more elastic than the component polymers, while large polydispersity or long-chain-branching of one component make the interface more viscous. We find that the chain lengths and architectures of the component polymers, rather than equilibrium immiscibility, play an essential role in determining the interface rheological properties.

cond-mat.soft↗

arXiv2010

Melt-Mixing by Novel Pitched-Tip Kneading Disks in a Co-Rotating Twin-Screw Extruder

Melt-mixing in twin-screw extruders is a key process in the development of polymer composites. Quantifying the mixing performance of kneading elements based on their internal physical processes is a challenging problem. We discuss melt-mixing by novel kneading elements called "pitched-tip kneading disk (ptKD)". The disk-stagger angle and tip angle are the main geometric parameters of the ptKDs. We investigated four typical arrangements of the ptKDs, which are forward and backward disk-staggers combined with forward and backward tips. Numerical simulations under a certain feed rate and screw revolution speed were performed, and the mixing process was investigated using Lagrangian statistics. It was found that the four types had different mixing characteristics, and their mixing processes were explained by the coupling effect of drag flow with the disk staggering and pitched-tip and pressure flows, which are controlled by operational conditions. The use of a pitched-tip effectively to controls the balance of the pressurization and mixing ability.

physics.flu-dyn↗

arXiv2008

Velocity autocorrelation function of fluctuating particles in incompressible fluids. Toward direct numerical simulation of particle dispersions

Motions of fluctuating Brownian particles in an incompressible viscous fluid have been studied by coupled simulations of Brownian particles and host fluid. We calculated the velocity autocorrelation functions of Brownian particles and compared them with the theoretical results. Extensive discussions have been made on the time scales for which our numerical model is valid.

cond-mat.soft↗

arXiv2008

Simulating (electro)hydrodynamic effects in colloidal dispersions: smoothed profile method

Previously, we have proposed a direct simulation scheme for colloidal dispersions in a Newtonian solvent [Phys.Rev.E 71,036707 (2005)]. An improved formulation called the ``Smoothed Profile (SP) method'' is presented here in which simultaneous time-marching is used for the host fluid and colloids. The SP method is a direct numerical simulation of particulate flows and provides a coupling scheme between the continuum fluid dynamics and rigid-body dynamics through utilization of a smoothed profile for the colloidal particles. Moreover, the improved formulation includes an extension to incorporate multi-component fluids, allowing systems such as charged colloids in electrolyte solutions to be studied. The dynamics of the colloidal dispersions are solved with the same computational cost as required for solving non-particulate flows. Numerical results which assess the hydrodynamic interactions of colloidal dispersions are presented to validate the SP method. The SP method is not restricted to particular constitutive models of the host fluids and can hence be applied to colloidal dispersions in complex fluids.

cond-mat.soft↗

arXiv2006

Direct Numerical Simulations of Electrophoresis of Charged Colloids

We propose a numerical method to simulate electrohydrodynamic phenomena in charged colloidal dispersions. This method enables us to compute the time evolutions of colloidal particles, ions, and host fluids simultaneously by solving Newton, advection-diffusion, and Navier--Stokes equations so that the electrohydrodynamic couplings can be fully taken into account. The electrophoretic mobilities of charged spherical particles are calculated in several situations. The comparisons with approximation theories show quantitative agreements for dilute dispersions without any empirical parameters, however, our simulation predicts notable deviations in the case of dense dispersions.

cond-mat.soft↗

arXiv2003

A Smooth Interface Method for Simulating Liquid Crystal Colloid Dispersions

A new method is presented for mesoscopic simulations of particle dispersions in liquid crystal solvents. It allows efficient first-principle simulations of the dispersions involving many particles with many-body interactions mediated by the solvents. Demonstrations have been performed for the aggregation of colloid dispersions in two-dimensional nematic and smectic-C* solvents neglecting hydrodynamic effects, which will be taken into account in the near future.

cond-mat.soft↗