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Takeshi Kawasaki

Publications and source records attributed to Takeshi Kawasaki.

50 records · Page 3Linked to original sources
arXiv2018

Discontinuous shear-thickening in Brownian suspensions

Discontinuous shear-thickening in dense suspensions naturally emerges from the activation of frictional forces by shear flow in non-Brownian systems close to jamming. Yet, this physical picture is incomplete as most experiments study soft colloidal particles subject to thermal fluctuations. To characterise discontinuous shear-thickening in colloidal suspensions we use computer simulations to provide a complete description of the competition between athermal jamming, frictional forces, thermal motion, particle softness, and shear flow. We intentionally neglect hydrodynamics, electrostatics, lubrication, and inertia, but can nevertheless achieve quantitative agreement with experimental findings. In particular, shear-thickening corresponds to a crossover between frictionless and frictional jamming regimes which is controlled by thermal fluctuations and particle softness and occurs at a softness dependent Péclet number. We also explore the consequences of our findings for constant pressure experiments, and critically discuss the reported emergence of `S-shaped' flow curves. Our work provides the minimal ingredients to quantitatively interpret a large body of experimental work on discontinuous shear-thickening in colloidal suspensions.

cond-mat.soft↗
arXiv2017

Acoustic resonance in periodically sheared glass

Using molecular dynamics simulation, we study acoustic resonance in low-temperature glass by applying a small periodic shear at a boundary wall. Shear wave resonance occurs as the frequency $ω$ approaches $ω_\ell= πc_\perp\ell/L$ ($\ell=1, 2, 3,...)$. Here, $c_\perp$ is the transverse sound speed and $L$ is the cell length. At resonance, large-amplitude sound waves appear after many cycles even for very small applied strains. They then induce plastic events, which are heterogeneous in space and intermittent on time scales longer than the oscillation period $2π/ω$. From these irreversible particle motions, there arises strong dissipation suppressing the growth of sounds. After many resonant cycles, we observe a phenomenon of forced aging, where the shear modulus (measured after switching off the oscillation) is increased significantly.Sometimes, exceptionally large plastic events and system-size sliding motions induce a transition from resonant to off-resonant states. At resonance, translational diffusion becomes appreciable as well as aging due to enhanced configurational changes.

cond-mat.soft↗
arXiv2017

Identifying time scales for violation/preservation of Stokes-Einstein relation in supercooled water

The violation of Stokes--Einstein (SE) relation $D\sim (η/T)^{-1}$ between the shear viscosity $η$ and the translational diffusion constant $D$ at temperature $T$ is of great importance for characterizing anomalous dynamics of supercooled water. Determining which time scales play key roles in the SE violation remains elusive without the measurement of $η$. Here we provide comprehensive simulation results of the dynamic properties involving $η$ and $D$ in TIP4P/2005 supercooled water. This enabled the thorough identification of the appropriate time scales for SE relation $Dη/T$. In particular, it is demonstrated that the temperature dependence of various time scales associated with structural relaxation, hydrogen bond breakage, stress relaxation, and dynamic heterogeneities can be definitely classified into only two classes. That is, we propose the generalized SE relations that exhibit "violation" or "preservation". The classification depends on the examined time scales that are coupled or decoupled with the diffusion. On the basis of the classification, we explain the physical origins of the violation in terms of the increase in the plateau modulus and the nonexponentiality of stress relaxation. This implies that the mechanism of SE violation is attributed to the attained solidity upon supercooling, which is in accord with the growth of non-Gaussianity and spatially heterogeneous dynamics.

cond-mat.soft↗
arXiv2016

Unveiling Dimensionality Dependence of Glassy Dynamics: 2D Infinite Fluctuation Eclipses Inherent Structural Relaxation

By using large-scale molecular dynamics simulations, the dynamics of two-dimensional (2D) supercooled liquids turns out to be dependent on the system size, while the size dependence is not pronounced in three dimensional (3D) systems. It is demonstrated that the strong system-size effect in 2D amorphous systems originates from the enhanced fluctuations at long wavelengths, which are similar to those of 2D crystal phonons. This observation is further supported by the frequency dependence of the vibrational density of states, consisting of the Debye approximation in the low-wavenumber-limit. However, the system-size effect in the intermediate scattering function becomes negligible when the length scale is larger than the vibrational amplitude. This suggests that the finite-size effect in a 2D system is transient and also that the structural relaxation itself is not fundamentally different from that in a 3D system. In fact, the dynamic correlation lengths estimated from the bond-breakage function, which do not suffer from those enhanced fluctuations, are not size dependent in either 2D or 3D systems.

cond-mat.stat-mech↗
arXiv2016

Cluster Glass Transition of Ultrasoft-Potential Fluids at High Density

Using molecular dynamics simulation, we investigate the slow dynamics of a supercooled binary mixture of soft particles interacting with a generalized Hertzian potential. At low density, it displays typical slow dynamics near its glass transition temperature. At higher densities, particles bond together, forming clusters, and the clusters undergo the glass transition. The number of particles in a cluster increases one by one as the density increases. We demonstrate that there exist the multiple cluster-glass phases characterized by a different number of particles per cluster, each of which is separated by distinct minima. Surprisingly, a so-called higher order singularity of the mode-coupling theory signaled by a logarithmic relaxation is observed in the vicinity of the boundaries between monomer and cluster glass phases. The system also exhibits rich and anomalous dynamics in the cluster glass phases, such as the decoupling of the self- and collective dynamics.

cond-mat.soft↗
arXiv2016

Macroscopic yielding in jammed solids is accompanied by a non-equilibrium first-order transition in particle trajectories

We use computer simulations to analyse the yielding transition during large-amplitude oscillatory shear of a simple model for soft jammed solids. Simultaneous analysis of global mechanical response and particle-scale motion demonstrates that macroscopic yielding, revealed by a smooth crossover in mechanical properties, is accompanied by a sudden change in the particle dynamics, which evolves from non-diffusive motion to irreversible diffusion as the amplitude of the shear is increased. We provide numerical evidence that this sharp change corresponds to a non-equilibrium first-order dynamic phase transition, thus establishing the existence of a well-defined microscopic dynamic signature of the yielding transition in amorphous materials in oscillatory shear.

cond-mat.soft↗
arXiv2016

Excitation of vibrational soft modes in disordered systems using active oscillation

We propose a new method to characterize the spatial distribution of particles' vibrations in solids with much lower computational costs compared to the usual normal mode analysis. We excite the specific vibrational mode in a two dimensional athermal jammed system by giving a small amplitude of active oscillation to each particle's size with an identical driving frequency. The response is then obtained as the real time displacements of the particles. We show remarkable correlations between the real time displacements and the eigen vectors obtained from conventional normal mode analysis. More importantly, from these real time displacements, we can measure the participation ratio and spatial polarization of particles' vibrations. From these measurements, we find three distinct frequency regimes which characterize the spatial distribution and correlation of particles' vibrations in jammed amorphous solids. Furthermore, we can possibly apply this method to a much larger system to examine the low frequency behaviour of amorphous solids with a much higher resolution of the frequency space.

cond-mat.soft↗
arXiv2015

Diverging viscosity and soft granular rheology in non-Brownian suspensions

We use large scale computer simulations and finite size scaling analysis to study the shear rheology of dense three-dimensional suspensions of frictionless non-Brownian particles in the vicinity of the jamming transition. We perform simulations of soft repulsive particles at constant shear rate, constant pressure, and finite system size, and study carefully the asymptotic limits of large system sizes and infinitely hard particle repulsion. Extending earlier analysis by about two orders of magnitude, we first study the asymptotic behavior of the shear viscosity in the hard particle limit. We confirm its asymptotic power law divergence at the jamming transition, but show that a precise determination of the critical density and critical exponent is difficult due to the `multiscaling' behavior of the viscosity. Additionally, finite-size scaling analysis suggests that this divergence is accompanied by a growing correlation length scale, which also diverges algebraically. We then study the effect of soft repulsion, and propose a natural extension of the standard granular rheology to account for softness effects, which we validate from simulations. Close to the jamming transition, this `soft granular rheology' offers a detailed description of the non-linear rheology of soft particles, which differs from earlier empirical scaling forms.

cond-mat.soft↗
arXiv2014

Thinning or thickening? Multiple rheological regimes in dense suspensions of soft particles

The shear rheology of dense colloidal and granular suspensions is strongly nonlinear, as these materials exhibit shear-thinning and shear-thickening, depending on multiple physical parameters. We numerically study the rheology of a simple model of soft repulsive particles at large densities, and show that nonlinear flow curves reminiscent of experiments on real suspensions can be obtained. By using dimensional analysis and basic elements of kinetic theory, we rationalize these multiple rheological regimes and disentangle the relative impact of thermal fluctuations, glass and jamming transitions, inertia and particle softness on the flow curves. We characterize more specifically the shear-thickening regime and show that both particle softness and the emergence of a yield stress at the jamming transition compete with the inertial effects responsible for the observed thickening behaviour. This allows us to construct a dynamic state diagram, which can be used to analyze experiments.

cond-mat.soft↗
arXiv2013

Spatiotemporal heterogeneity of local free volumes in highly supercooled liquid

We discuss the spatiotemporal behavior of local density and its relation to dynamical heterogeneity in a highly supercooled liquid by using molecular dynamics simulations of a binary mixture with different particle sizes in two dimensions. To trace voids heterogeneously existing with lower local densities, which move along with the structural relaxation, we employ the minimum local density for each particle in a time window whose width is set along with the structural relaxation time. Particles subject to free volumes correspond well to the configuration rearranging region of dynamical heterogeneity. While the correlation length for dynamical heterogeneity grows with temperature decrease, no growth in the correlation length of heterogeneity in the minimum local density distribution takes place. A comparison of these results with those of normal mode analysis reveals that superpositions of lower-frequency soft modes extending over the free volumes exhibit spatial correlation with the broken bonds. This observation suggests a possibility that long-ranged vibration modes facilitate the interactions between fragile regions represented by free volumes, to induce dynamical correlations at a large scale.

cond-mat.soft↗
arXiv2012

Slow relaxations and stringlike jump motions in fragile glass-forming liquids: Breakdown of the Stokes-Einstein relation

We perform molecular dynamics simulation on a glass-forming liquid binary mixture with the soft-core potential in three dimensions. We investigate crossover of the configuration changes caused by stringlike jump motions. With lowering the temperature $T$, the motions of the particles composing strings become larger in sizes and displacements, while those of the particles surrounding strings become smaller. Then, the contribution of the latter to time-correlation functions tends to be long-lived as $T$ is lowered. As a result, the relaxation time $τ_α$ and the viscosity $η$ grow more steeply than the inverse diffusion constant $D^{-1}$ at low $T$, leading to breakdown of the Stokes-Einstein relation. At low $T$, the diffusion occurs as activation processes and may well be described by short-time analysis of rare jump motions with broken bonds and large displacements. Some characteristic features of the van Hove self-correlation function arise from escape jumps over high potential barriers.

cond-mat.soft↗
arXiv2012

Relationship between bond-breakage correlations and four-point correlations in heterogeneous glassy dynamics: Configuration changes and vibration modes

We investigate the dynamic heterogeneities of glassy particle systems in the theoretical schemes of bond breakage and four-point correlation functions. In the bond-breakage scheme, we introduce the structure factor S_b(q,t) and the susceptibility chi_b(t) to detect the spatial correlations of configuration changes. Here chi_b(t) attains a maximum at t =t_b^max as a function of time t, where the fraction of the particles with broken bonds phi_b(t)$ is about 1/2. In the four-point scheme, treating the structure factor S_4(q,t) and the susceptibility chi_4(t), we detect superpositions of the heterogeneity of bond breakage and that of thermal low-frequency vibration modes. While the former grows slowly, the latter emerges quickly to exhibit complex space-time behavior. In two dimensions, the vibration modes extending over the system yield significant contributions to the four-point correlations, which depend on the system size logarithmically. A maximum of chi_4(t) is attained at t= t_4^max, where these two contributions become of the same order. As a result, t_4^max is considerably shorter than t_b^max.

cond-mat.soft↗
arXiv2011

Visualization of stacking faults in fcc crystals in plastic deformations

Using molecular dynamics simulation, we investigate the dynamics of stacking faults in fcc crystals in uniaxial stretching in a Lennard-Jones binary mixture composed of 4096 particles in three dimensions. We visualize stacking faults using a disorder variable $D_j(t)$ for each particle $j$ constructed from local bond order parameters based on spherical harmonics (Steinhardt order parameters). Also introducing a method of bond breakage, we examine how stacking faults are formed and removed by collective particle motions. These processes are relevant in plasticity of fcc crystals.

cond-mat.soft↗