arXiv subjects

Akira Furukawa

Publications and source records attributed to Akira Furukawa.

At least 19 recordsLinked to original sources

arXiv2026

Near-field Hydrodynamics Disentangles Angular Correlations in Confined Active Suspensions

Spatial confinement profoundly impacts the transport and self-organization of active matter across diverse biological systems. While the collective orders in confined active matter have been extensively characterized, how geometric constraints reshape near-field flows and the resulting inter-particle correlations remains largely unexplored. In this study, we combine experiments and hydrodynamic simulations to investigate inter-particle correlations within quasi-two-dimensional Chlamydomonas reinhardtii suspensions. We reveal two disentangled modes characterizing cell pairs: a dipolar mode and an entrainment mode, which exhibit a density- and distance-dependent competition. Combining single-cell flow field analysis, hydrodynamic simulations, and active-passive mixtures, we link these two modes to singular hydrodynamics and lubrication-induced entrainment. Our results demonstrate that spatiotemporal correlations in confined active matter are fundamentally rooted in the interplay of these two hydrodynamic mechanisms.

cond-mat.soft↗

arXiv2025

Neural-Network-Assisted Boltzmann Approach for Dilute Microswimmer Suspensions

We introduce a neural-network-assisted Boltzmann framework that learns the binary-collision map of microswimmers directly from data and uses it to evaluate collision integrals efficiently. Using a representative model swimmer, the learned map quantitatively predicts translational and rotational diffusivities and enables a linear-stability analysis of isotropy against polar ordering in dilute suspensions. The resulting predictions closely match direct simulations. The present framework is agnostic to active matter models and broadly applicable: once two-body collision data are obtained -- either from simulations or experiments -- the same surrogate can be used to evaluate kinetic transport across dilute conditions where binary collisions dominate. Because the workflow relies only on pre- and post-collision statistics, the present approach provides a general data-driven route linking particle-scale interactions to macroscopic transport and collective behavior in active suspensions.

cond-mat.soft↗

arXiv2025

Anomalous rheology of puller-type microswimmer suspensions

We explore the mechanism underlying the anomalous rheology of puller-type microswimmer suspensions through direct hydrodynamic simulations. Puller-type swimmers generate contractile flow fields along their swimming direction, leading to hydrodynamic interactions that cause the swimmers to align vertically. Our simulations reveal that this alignment effect, along with the resultant orientational order of swimming motion, becomes particularly pronounced near boundary walls, where local swimmer density is amplified, predominantly controlling the overall swimming dynamics and rheological properties of the suspension. These findings contrast with our previous simulations of pusher-type swimmers, which hydrodynamically interact through extensile flow fields, whereby they exhibit weak orientational order in the bulk region, which primarily determines their steady-state properties. Furthermore, we demonstrate that the steady-state behavior near the walls is strongly influenced by the aspect ratio of the microswimmers and the degree of confinement between the walls. Our results highlight the crucial role of microswimmer characteristics, such as shape and swimming mechanisms, in determining the rheological properties of active suspensions.

cond-mat.soft↗

arXiv2024

Heterogeneous Solvent Dissipation Coupled with Particle Rearrangement in Shear Thinning Non-Brownian Suspensions

Dense non-Brownian suspensions exhibit significant shear thinning, although a comprehensive understanding of the full scope of this phenomenon remains elusive. This study numerically reveals intimate heterogenous coupled dynamics between many-body particle motions and solvent hydrodynamics in shear-thinning non-Brownian suspensions. We demonstrate the spatially correlated viscous dissipation and particle motions; they share the same characteristic length, which decreases with increasing shear rate. We further show that, at lower shear rates, significant particle density changes are induced against the incompressibility of the solvent, suggesting the cooperative creation and annihilation of gaps and flow channels. We discuss that hydrodynamic interactions may substantially restrict particle rearrangements even in highly dense suspensions, influencing the quantitative aspects of macroscopic rheology.

cond-mat.soft↗

arXiv2024

Quantification of the volume-fraction reduction of sheared fragile glass-forming liquids and its impact on rheology

This study determines the volume-fraction reduction of sheared fragile glass-forming liquids. We consider a group of hypothetical systems that consist of particles with anisotropic particle-size modulations yet have almost the same average particle configuration as actual systems under shear flow. Our molecular dynamics (MD) simulations demonstrate that one specific hypothetical system can reproduce the relaxation dynamics of an actual sheared system, and we identify the shear-flow effect on the particle size with anisotropic size-modulation of this specific system. Then, based on the determination of the particle size and the resultant volume fraction, we rationalize how slight decreases in the volume fraction significantly reduce the viscosity snf provide a nonlinear constitutive equation. Notably, the obtained rheological predictions, including the crossover shear rate from Newtonian to non-Newtonian behavior, can be expressed only in terms of experimental observables, showing a good agreement with the MD simulation results. Our perspective on the volume fraction under shear flow may provide new insights into the conventional concept of free-volume.

cond-mat.soft↗

arXiv2023

Microrheology of active suspensions

We study the microrheology of active suspensions through direct hydrodynamic simulations using model pusher-like microswimmers. We demonstrate that the friction coefficient of a probe particle is notably reduced by hydrodynamic interactions (HIs) among a moving probe and the swimmers. When a swimmer approaches a probe from the rear (front) side, the repulsive HIs between them are weakened (intensified), which results in a slight front rear asymmetry in swimmer orientation distribution around the probe, creating a significant additional net driving force acting on the probe from the rear side. The present drag-reduction mechanism qualitatively differs from that of the viscosity-reduction observed in sheared bulk systems and depends on probing details. This study provides insights into our fundamental knowledge of hydrodynamic effects in active suspensions and serves as a practical example illuminating distinctions between micro- and macrorheology measurements.

cond-mat.soft↗

arXiv2023

Hydrodynamic interactions in anomalous rheology of active suspensions

We explore a mechanism of the anomalous rheology of active suspensions by hydrodynamic simulations using model pusher swimmers. Our simulations demonstrate that hydrodynamic interactions under shear flow systematically orient swimmers along the extension direction, which is responsible for determining the global swimming states and the resulting significant viscosity reduction. The present results indicate the essential role of hydrodynamic interactions in the elementary processes controlling the rheological properties in active suspensions. Furthermore, such processes may be the substance of the previously proposed scenario for anomalous rheology based on the interplay between the rotational diffusivities and the external shear flow.

cond-mat.soft↗

arXiv2021

Transverse viscous transport in classical solids

The transverse velocity time correlation function ${\tilde C}_{\rm T}(k,ω)$ with $k$ and $ω$ being the wavenumber and the frequency, respectively, is a fundamental quantity in determining the transverse mechanical and transport properties of materials. In ordinary liquids, a nonzero value of ${\tilde C}_{\rm T}(k,0)$ is inevitably associated with viscous material flows. Curiously, even in solids where significant material flows are precluded due to frozen positional degrees of freedom, molecular dynamics simulations reveal that ${\tilde C}_{\rm T}(k,0)$ certainly takes a nonzero value, and in consequence, the time integration of the velocity field shows definite diffusive behavior with diffusivity ${\tilde C}_{\rm T}(k,0)/3$. We demonstrate that this diffusive behavior can be attributed to a solid-specific viscous transport. The resultant viscosity is interpreted as the renormalized viscosity accounting for the nonlinear inertia effect.

cond-mat.soft↗

arXiv2019

The emergence of cooperativity accompanying vitrification: Insights from density fluctuation dynamics

We discuss the emergence and growth of the cooperativity accompanying vitrification based on the density fluctuation dynamics for fragile glass-forming liquids. (i) The relaxation of density fluctuations proceeds by the particle (density) exchange process, and is diffusive; that is, the allowable kinetic paths are strongly restricted by the local conservation law. (ii) In normal liquid states, this exchange process is less cooperative, and the diffusion coefficient of density fluctuations $D_c$ is given as $D_c\sim λ^2/τ_α$, where $λ$ is the particle size and $τ_α$ is the structural relaxation time. On the other hand, in supercooled states the restriction on the kinetic path is more severe with increasing the degree of supercooling, which makes the exchange process more cooperative, resulting in $D_c \sim ξ_{\rm d}^2/τ_α$ with $ξ_{\rm d}$ being the cooperative length scale. (iii) The molecular dynamics simulation results show that the self-diffusion coefficient of the tagged particle, $D_s$, almost coincides with $D_c$, suggesting that the collective density diffusion and the particle diffusion closely share the same mechanism: in normal states $D_s$ determines $D_c$, but vice versa in supercooled states. This immediately leads to the idea that the breakdown of the Stokes-Einstein relation is not the anomaly in the single-particle dynamics but reflects the increase in the cooperativity in the density diffusion at the length scale of $ξ_{\rm d}$.

cond-mat.soft↗

arXiv2018

Qualitative difference in rheology between fragile and network-forming strong liquids

We elucidate a qualitative difference in rheology between fragile and network-forming strong liquids. In a flow field, the structural configuration is distorted in accordance with the flow symmetry, whereas the form of the interaction potential remains unchanged. The role of this mismatch in the relaxation mechanism under the flow field is crucial for understanding the shear-thinning mechanism and differs between strong and fragile glass formers. In fragile glass formers, shear thinning can be attributed to the shear-induced reduction of the {\it effective density}. In contrast, in strong glass formers, the shear-induced reduction of the {\it effective activation energy} is a possible origin of a significant acceleration in relaxation. Our simple predictions of the crossover shear rate, $\dotγ_{\rm c}$, from Newtonian to non-Newtonian behaviors can be expressed in terms of experimental observables: in fragile liquids, $\dotγ_{\rm c}=(ρ\partial τ_α/\partial ρ)^{-1}$, where $ρ$ and $τ_α$ are the density and structural relaxation time, respectively, and in strong liquids, $\dotγ_{\rm c}= (τ_αΔE_0 /T)^{-1} $, where $T$ and $ΔE_0$ are the temperature and equilibrium activation energy, respectively. These predictions are consistent with the results of molecular dynamics simulations for four different glass formers: two fragile and two strong ones. This different route to the non-Newtonian flow response is related to differences in the role of density in the relaxation dynamics.

cond-mat.soft↗

arXiv2017

Growing length scale accompanying the vitrification: A perspective based on non-singular density fluctuations

In glass forming liquids close to the glass transition point, even a very slight increase in the macroscopic density results in a dramatic slowing down of the macroscopic relaxation. Concomitantly, the local density itself fluctuates in space. Therefore, one can imagine that even very small local density variations control the local glassy nature. Based on this perspective, a model for describing growing length scale accompanying the vitrification is introduced, in which we assume that in a subsystem whose density is above a certain threshold value, $ρ_{\rm c}$, owing to steric constraints, particle rearrangements are highly suppressed for a sufficiently long time period ($\sim$ structural relaxation time). We regard such a subsystem as a glassy cluster. Then, based on the statistics of the subsystem-density, we predict that with compression (increasing average density $ρ$) at a fixed temperature $T$ in supercooled states, the characteristic length of the clusters, $ξ$, diverges as $ξ\sim(ρ_{\rm c}-ρ)^{-2/d}$, where $d$ is the spatial dimensionality. This $ξ$ measures the average persistence length of the steric constraints in blocking the rearrangement motions and is determined by the subsystem density. Additionally, with decreasing $T$ at a fixed $ρ$, the length scale diverges in the same manner as $ξ\sim(T-T_{\rm c})^{-2/d}$, for which $ρ$ is identical to $ρ_{\rm c}$ at $T=T_{\rm c}$. The exponent describing the diverging length scale is the same as the one predicted by some theoretical models and indeed has been observed in some simulations and experiments. However, the basic mechanism for this divergence is different; that is, we do not invoke thermodynamic anomalies associated with the thermodynamic phase transition as the origin of the growing length scale. We further present arguements for the cooperative properties based on the clusters.

cond-mat.soft↗

arXiv2015

Probing colloidal gels at multiple lengthscales: the role of hydrodynamics

Colloidal gels are out-of-equilibrium structures, made up of a rarefied network of colloidal particles. Comparing experiments to numerical simulations, with hydrodynamic interactions switched off, we demonstrate the crucial role of the solvent for gelation. Hydrodynamic interactions suppress the formation of larger local equilibrium structures of closed geometry, and instead lead to the formation of highly anisotropic threads, which promote an open gel network. We confirm these results with simulations which include hydrodynamics. Based on three-point correlations, we propose a scale-resolved quantitative measure for the anisotropy of the gel structure. We find a strong discrepancy for interparticle distances just under twice the particle diameter between systems with and without hydrodynamics, quantifying the role of hydrodynamics from a structural point of view.

cond-mat.soft↗

arXiv2014

Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semidilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.

cond-mat.soft↗

arXiv2013

Nonequilibrium critical Casimir interactions in binary fluids

Colloids immersed in a critical binary liquid mixture are subject to critical Casimir forces (CCFs) because they confine its concentration fluctuations and influence the latter via effective surface fields. To date, CCFs have only been studied in thermodynamic equilibrium. However, due to the critical slowing down, the order parameter around a particle can easily be perturbed by any motion of the colloid or by solvent flow. This leads to significant but largely unexplored changes in the CCF. Here we study the drag force on a single colloidal particle moving in a near-critical fluid mixture and the relative motion of two colloids due to the CCF acting on them. In order to account for the kinetic couplings among the order parameter field, the solvent velocity field, and the particle motion, we use a fluid particle dynamics method. These studies extend the understanding of CCFs from thermal equilibrium to non-equilibrium processes, which are relevant to current experiments, and show the emergence of significant effects near the critical point.

cond-mat.soft↗

arXiv2009

Defect structures in nematic liquid crystals around charged particles

We numerically study the orientation deformations in nematic liquid crystals around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous electric field. If the dielectric anisotropy varepsilon_1 is positive, Saturn ring defects are formed around the particles. For varepsilon_1<0, novel "ansa" defects appear, which are disclination lines with their ends on the particle surface. We find unique defect structures around two charged particles. To lower the free energy, oppositely charged particle pairs tend to be aligned in the parallel direction for varepsilon_1>0 and in the perpendicular plane for varepsilon_1<0 with respect to the background director . For identically charged pairs the preferred directions for varepsilon_1>0 and varepsilon_1<0 are exchanged. We also examie competition between the charge-induced anchoring and the short-range anchoring. If the short-range anchoring is sufficiently strong, it can be effective in the vicinity of the surface, while the director orientation is governed by the long-range electrostatic interaction far from the surface.

cond-mat.stat-mech↗

arXiv2004

Numerical simulations studies of the convective instability onset in a supercritical fluid

Numerical simulation studies in 2D with the addition of noise are reported for the convection of a supercritical fluid,3He, in a Rayleigh-Be'nard cell where the fluid parameters and cell height L are the same as in published laboratory experiments. The noise addition is to accelerate the instability onset after starting the heat flow across the fluid, so as to bring simulations into better agreement with experimental observations. Homogeneous temperature noise and spatial lateral periodic temperature variations in the top plate were programmed into the simulations. A speed-up in the instability onset was obtained, which was most effective through the spatial temperature variations with a period of 2L, close to the wavelength of a pair of convections rolls. For a small amplitude of 0.5 microK, this perturbation gave a semiquantitative agreement with experimental observations. Results for various noise amplitudes are presented and discussed in relation to predictions by El Khouri and Carle`s.

physics.flu-dyn↗

arXiv2004

Viscoelastic Phase Separation in Shear Flow

We numerically investigate viscoelastic phase separation in polymer solutions under shear using a time-dependent Ginzburg-Landau model. The gross variables in our model are the polymer volume fraction and a conformation tensor. The latter represents chain deformations and relaxes slowly on the rheological time giving rise to a large viscoelastic stress. The polymer and the solvent obey two-fluid dynamics in which the viscoelastic stress acts asymmetrically on the polymer and, as a result, the stress and the diffusion are dynamically coupled. Below the coexistence curve, interfaces appear with increasing the quench depth and the solvent regions act as a lubricant. In these cases the composition heterogeneity causes more enhanced viscoelastic heterogeneity and the macroscopic stress is decreased at fixed applied shear rate. We find steady two-phase states composed of the polymer-rich and solvent-rich regions, where the characteristic domain size is inversely proportional to the average shear stress for various shear rates. The deviatoric stress components exhibit large temporal fluctuations. The normal stress difference can take negative values transiently at weak shear.

cond-mat.soft↗

arXiv2004

Spatio-temporal structures in sheared polymer systems

We investigate spatio-temporal structures in sheared polymer systems by solving a time-dependent Ginzburg-Landau model in two dimensions. (i) In polymer solutions above the coexistence curve, crossover from linear to nonlinear regimes occurs with increasing the shear rate. In the nonlinear regime the solution behaves chaotically with large-amplitude composition fluctuations. A characteristic heterogeneity length is calculated in the nonlinear regime. (ii) We also study dynamics of shear-band structures in wormlike micellar solutions under the condition of fixed stress. The average shear rate exhibits large temporal fluctuations with occurrence of large disturbances in the spatial structures.

cond-mat.soft↗