arXiv subjects

Takeshi Kawasaki

Publications and source records attributed to Takeshi Kawasaki.

At least 37 records · Page 2Linked to original sources

arXiv2023

Anomalous fluctuations in homogeneous fluid phase of active Brownian particles

Giant number fluctuations (GNF) are an anomaly universally observed in active fluids with polar or nematic order. In this paper, we show that GNF arise in the fluid phase of active Brownian particles (ABP), where the polar order is absent. GNF in ABP extends over a large but finite length which characterizes the growing velocity correlations. To suppress unwanted phase separation and allow ones to explore the disordered fluid phase at large activities, we impart the inertia, or the mass, to the ABP. A linearized hydrodynamic theory captures our findings, but only qualitatively. We find numerically a nontrivial scaling relation for the density correlation function, which the linearized theory cannot explain. The results suggest ubiquitousness of the anomalous fluctuations even in the disordered homogeneous fluid phase in the absence of the directional order.

cond-mat.soft↗

arXiv2022

What Do Deep Neural Networks Find in Disordered Structures of Glasses?

Glass transitions are widely observed in various types of soft matter systems. However, the physical mechanism of these transitions remains {elusive}, despite years of ambitious research. In particular, an important unanswered question is whether the glass transition is accompanied by a divergence of the correlation lengths of the characteristic static structures. In this study, we develop a deep-neural-network-based method that is used to extract the characteristic local meso-structures solely from instantaneous {particle} configurations without any {information} about the dynamics. We first train a neural network to classify configurations of liquids and glasses correctly. Then, we obtain the characteristic structures by quantifying the grounds for the decisions made by the network using Gradient-weighted Class Activation Mapping (Grad-CAM). We considered two qualitatively different glass-forming binary systems, and through comparisons with several established structural indicators, we demonstrate that our system can be used to identify characteristic structures that depend on the details of the systems. Moreover, the extracted structures are remarkably correlated with the nonequilibrium aging dynamics in thermal fluctuations.

cond-mat.dis-nn↗

arXiv2022

Aging or DEAD: origin of the non-monotonic response to weak self-propulsion in active glasses

Among amorphous states, glass is defined by relaxation times longer than the observation time. This nonergodic nature makes the understanding of glassy systems an involved topic, with complex aging effects or responses to further out-of-equilibrium external drivings. In this respect active glasses made of self-propelled particles have recently emerged as stimulating systems which broadens and challenges our current understanding of glasses by considering novel internal out-of-equilibrium degrees of freedom. In previous experimental studies we have shown that in the ergodicity broken phase, the dynamics of dense passive particles first slows down as particles are made slightly active, before speeding up at larger activity. Here, we show that this nonmonotonic behavior also emerges in simulations of soft active Brownian particles and explore its cause. We refute that the Deadlock by Emergence of Active Directionality (DEAD) model we proposed earlier describes our data. However, we demonstrate that the nonmonotonic response is due to activity enhanced aging, and thus confirm the link with ergodicity breaking. Beyond self-propelled systems, our results suggest that aging in active glasses is not fully understood.

cond-mat.soft↗

arXiv2021

Geometrical properties of mechanically annealed systems near the jamming transition

Geometrical properties of two-dimensional mixtures near the jamming transition point are numerically investigated using harmonic particles under mechanical training. The configurations generated by the quasi-static compression and oscillatory shear deformations exhibit anomalous suppression of the density fluctuations, known as hyperuniformity, below and above the jamming transition. For the jammed system trained by compression above the transition point, the hyperuniformity exponent increases. For the system below the transition point under oscillatory shear, the hyperuniformity exponent also increases until the shear amplitude reaches the threshold value. The threshold value matches with the transition point from the point-reversible phase where the particles experience no collision to the loop-reversible phase where the particles' displacements are non-affine during a shear-cycle before coming back to an original position. The results demonstrated in this paper are explained in terms of neither of universal criticality of the jamming transition nor the nonequilibrium phase transitions.

cond-mat.soft↗

arXiv2021

Emergent elasticity linked to topological phase transitions controlled via molecular chirality and steric anisotropy

Self-organisation into spatially modulated structures has different nature from phase transition into uniform states. Skyrmions and half-skyrmions (merons) are representatives of such structures and are utilised in designing magnetoelectric, optical, and mechanoresponsive materials by controlling topological phases. However, skyrmions and half-skyrmions in molecular solids are rarely studied, though there is a universality in theoretical descriptions between magnetic and molecular systems with chiral interactions. Here we develop a simple physical system for controlling topological phases in a solid with chirality. We reveal that emergence of elastic fields from anisotropic steric interactions and intermolecular twisting is a key to control helical and half-skyrmion structures. Utilising the coupling between the emergent elastic fields and molecular orientations, we successfully control topological phases by temperature, external electromagnetic fields, and anisotropic stresses. The concept of the emergent elasticity provides a control system for designing molecular and macromolecular solids with tunable electro- and magneto-mechanical properties.

cond-mat.mtrl-sci↗

arXiv2021

Dynamic susceptibilities in dense soft athermal spheres under a finite-rate shear

The mechanical responses of dense packings of soft athermal spheres under a finite-rate shear are studied by means of molecular dynamics simulations. We investigate the volume fraction and shear rate dependence of the fluctuations in the shear stress and the interparticle contact number. In particular, we quantify them by defining the susceptibility as the ratio of the global to local fluctuations. The obtained susceptibilities form ridges on the volume fraction-shear rate plane, which are reminiscent of the Widom lines around the critical point in an equilibrium phase transition.

cond-mat.soft↗

arXiv2020

Glass stability changes the nature of yielding under oscillatory shear

We perform molecular dynamics simulations to investigate the effect of a glass preparation on its yielding transition under oscillatory shear. We use swap Monte Carlo to investigate a broad range of glass stabilities from poorly annealed to highly stable systems. We observe a qualitative change in the nature of yielding, which evolves from ductile to brittle as glass stability increases. Our results disentangle the relative role of mechanical and thermal annealing on the mechanical properties of amorphous solids, which is relevant for various experimental situations from the rheology of soft materials to fatigue failure in metallic glasses.

cond-mat.dis-nn↗

arXiv2020

Shear jamming and shear melting in mechanically trained frictionless particles

We investigate criticality near the jamming transition in both quiescent systems and those under shear by considering the effect of mechanical training on the jamming transition and nonlinear rheology. We simulate frictionless soft particles undergoing athermal quasi-static shear using initial configurations trained with athermal quasi-static cyclic volume deformations. The jamming transition density of the initial configuration $φ_{\rm J0}$ is systematically altered by tuning the ``depth'' of mechanical training. We exert a steady shear on these configurations and observe either shear jamming (gain of stiffness due to shear) or shear melting (loss of stiffness due to shear), depending on the depth of training and proximity to the jamming transition density. We also observe that the characteristic strains, at which shear jamming or melting occur, diverge at a unique density $φ_{\rm JS}$. This is due to the shift of the jamming transition density from $φ_{\rm J0}$ to $φ_{\rm JS}$ under shear, associated with loss of memory of the initial configuration. Finally, we thoroughly investigate nonlinear rheology near the jamming transition density, and contrary to previous works, we find a nonlinear ``softening'' takes place below as well as above the jamming transition density.

cond-mat.soft↗

arXiv2020

Critical scaling of diffusion coefficients and size of rigid clusters of soft athermal particles under shear

We numerically investigate the self-diffusion coefficient and correlation length of the rigid clusters (i.e., the typical size of the collective motions) in sheared soft athermal particles. Here we find that the rheological flow curves on the self-diffusion coefficient are collapsed by the proximity to the jamming transition density. This feature is in common with the well-established critical scaling of flow curves on shear stress or viscosity. We furthermore reveal that the divergence of the correlation length governs the critical behavior of the diffusion coefficient, where the diffusion coefficient is proportional to the correlation length and the strain rate for a wide range of the strain rate and packing fraction across the jamming transition density.

cond-mat.soft↗

arXiv2020

Active microrheology of a bulk metallic glass

The glass transition remains unclarified in condensed matter physics. Investigating the mechanical properties of glass is challenging because any global deformation that may result in shear rejuvenation requires an astronomical relaxation time. Moreover, it is well known that a glass is heterogeneous and a global perturbation cannot explore local mechanical/transport properties. However, an investigation based on a local probe, i.e. microrheology, may overcome these problems. Here, we establish active microrheology of a bulk metallic glass: a probe particle driven into host medium glass. This is a technique amenable for experimental investigations. We show that upon cooling the microscopic friction exhibits a second-order phase transition; this sheds light on the origin of friction in heterogeneous materials. Further, we provide distinct evidence to demonstrate that a strong relationship exists between the microscopic dynamics of the probe particle and the macroscopic properties of the host medium glass. These findings establish active microrheology as a promising technique for investigating the local properties of bulk metallic glass.

cond-mat.soft↗

arXiv2019

Local Density Fluctuation Governs the Divergence of Viscosity underlying Elastic and Hydrodynamic Anomalies in a 2D Glass-Forming Liquid

If a liquid is cooled rapidly to form a glass, its structural relaxation becomes retarded, producing a drastic increase in viscosity. In two dimensions, strong long-wavelength fluctuations persist, even at low temperature, making it difficult to evaluate the microscopic structural relaxation time. This Letter shows that, in a 2D glass-forming liquid, relative displacement between neighbor particles yields a relaxation time that grows in proportion to the viscosity. In addition to thermal elastic vibrations, hydrodynamic fluctuations are found to affect the long-wavelength dynamics, yielding a logarithmically diverging diffusivity in the long-time limit.

cond-mat.soft↗

arXiv2019

Classification of the reversible-irreversible transitions in particle trajectories across the jamming transition point

The reversible-irreversible (RI) transition of particle trajectories in athermal colloidal suspensions under cyclic shear deformation is an archetypal nonequilibrium phase transition which attracts much attention recently. In the low-density limit, the RI transition is predicted to belong to a universality class of the absorbing state transitions, whereas at the high densities well above the jamming transition density, $φ_{\rm J}$, the transition is discontinuous and is closely related to the yielding transition. The transition between the two limiting cases is largely unexplored. In this paper, we study the RI transition of athermal frictionless colloidal particles over a wide range of densities, with emphasis on the region below $φ_{\rm J}$, by using oscillatory sheared molecular dynamics simulation. We reveal that the nature of the RI transitions in the intermediate densities is very rich. As demonstrated by the previous work by Schreck $et\ al.$ [Phys. Rev. E. ${\bf 88}$, 052205 (2013)], there exist the point-reversible and the loop-reversible phases depending on the density and the shear strain amplitude. We find that, between the two reversible phases, a quasi-irreversible phase where the particles' trajectories are highly non-affine and diffusive. The averaged number of contacts of particles is found to characterize the phase boundaries. We also find that the system undergoes the yielding transition below but in the vicinity of $φ_{\rm J}$ when the strain with a small but finite strain rate is applied. This yielding transition line matches with the RI transition line separating the loop-reversible from the irreversible phases. Surprisingly, the nonlinear rheological response called "softening" has been observed even below $φ_{\rm J}$. These findings imply that geometrical properties encoded in the sheared configurations control the dynamical transitions.

cond-mat.soft↗

arXiv2019

Classification of mobile- and immobile-molecule timescales for the Stokes-Einstein and Stokes-Einstein-Debye relations in supercooled water

Molecular dynamics simulations have been performed on TIP4P/2005 supercooled water to investigate the molecular diffusion and shear viscosity at various timescales and assess the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations. For this purpose, we calculated various time correlation functions, such as the mean-squared displacement, stress relaxation function, density correlation function, hydrogen-bond correlation function, rotational correlation function of molecular orientation, non-Gaussian parameter, and four-point correlation function. Our study of the SE and SED relations indicates that the transport coefficients and timescales obtained using these time correlation functions may be classified into two distinct classes: those governed by either mobile or immobile molecules, due to dynamical heterogeneity. In particular, we show that the stress relaxation time, hydrogen-bond lifetime, and large-angle rotational relaxation time are coupled with translational diffusion, and are characterized by mobile molecules. In contrast, the structural $α$-relaxation time, small-angle rotational relaxation time, and characteristic timescales of four-point correlation functions are decoupled with translational diffusion, and are governed by immobile molecules. This decoupling results in a violation of the SE relation. These results indicate that the identification of timescales that appropriately characterize transport coefficients, such as translational diffusion constant and shear viscosity, provides a deep insight into the violation of the SE and SED relations in glass-forming liquids.

cond-mat.soft↗

arXiv2019

Spurious violation of the Stokes-Einstein-Debye relation in supercooled water

The theories of Brownian motion, the Debye rotational diffusion model, and hydrodynamics together provide us with the Stokes--Einstein--Debye (SED) relation between the rotational relaxation time of the $\ell$-th degree Legendre polynomials $τ_\ell$, and viscosity divided by temperature, $η/T$. Experiments on supercooled liquids are frequently performed to measure the SED relations, $τ_{\ell}k_{\rm B}T/η$ and $D_{\rm t}τ_{\ell}$, where $D_{\rm t}$ is the translational diffusion constant. However, the SED relations break down, and its molecular origin remains elusive. Here, we assess the validity of the SED relations in TIP4P/2005 supercooled water using molecular dynamics simulations. Specifically, we demonstrate that the higher-order $τ_\ell$ values exhibit a temperature dependence similar to that of $η/T$, whereas the lowest-order $τ_\ell$ values are decoupled with $η/T$, but are coupled with the translational diffusion constant. We reveal that the SED relations are so spurious that they significantly depend on the degree of Legendre polynomials.

cond-mat.soft↗

arXiv2019

Universal relaxation dynamics of sphere packings below jamming

We show that non-Brownian suspensions of repulsive spheres below jamming display a slow relaxational dynamics with a characteristic time scale that diverges at jamming. This slow time scale is fully encoded in the structure of the unjammed packing and can be readily measured via the vibrational density of states. We show that the corresponding dynamic critical exponent is the same for randomly generated and sheared packings. Our results show that a wide variety of physical situations, from suspension rheology to algorithmic studies of the jamming transition are controlled by a unique diverging timescale, with a universal critical exponent.

cond-mat.soft↗

arXiv2019

Glassy dynamics of a model of bacterial cytoplasm with metabolic activities

Recent experiments have revealed that cytoplasms become glassy when their metabolism is suppressed, while they maintain fluidity in a living state. The mechanism of this active fluidization is not clear, especially for bacterial cytoplasms, since they lack traditional motor proteins, which can cause directed motions. We introduce a model of bacterial cytoplasm focusing on the impact of conformational change in proteins due to metabolism. In the model, proteins are treated as particles under thermal agitation, and conformation changes are treated as changes in particle volume. Simulations revealed that a small change in volume fluidizes the glassy state, accompanied by a change in fragility, as observed experimentally.

cond-mat.soft↗

arXiv2018

Theory of Shear Modulus in Glasses

We construct a linear response theory of applying shear deformations from boundary walls in the film geometry in Kubo's theoretical scheme. Our method is applicable to any solids and fluids. For glasses, we assume quasi-equilibrium around a fixed inherent state. Then, we obtain linear-response expressions for any variables including the stress and the particle displacements, even though the glass interior is elastically inhomogeneous. In particular, the shear modulus can be expressed in terms of the correlations between the interior stress and the forces from the walls. It can also be expressed in terms of the inter-particle correlations, as has been shown in the previous literature. Our stress relaxation function includes the effect of the boundary walls and can be used for inhomogeneous flow response. We show the presence of long-ranged, long-lived correlations among the fluctuations of the forces from the walls and the displacements of all the particles in the cell. We confirm these theoretical results numerically in a two-dimensional model glass. As an application, we describe propagation of transverse sounds after boundary wall motions using these time-correlation functions We also find resonant sound amplification when the frequency of an oscillatory shear approaches that of the first transverse sound mode.

cond-mat.soft↗

arXiv2018

Slow dynamics coupled with cluster formation in ultrasoft-potential glasses

We numerically investigate slow dynamics of a binary mixture of ultrasoft particles interacting with the generalized Hertzian potential. If the softness parameter, $α$, is small, the particles at high densities start penetrating each other, form clusters, and eventually undergo the glass transition. We find multiple cluster-glass phases characterized by different number of particles per cluster, whose boundary lines are sharply separated by the cluster size. Anomalous logarithmic slow relaxation of the density correlation functions is observed in the vicinity of these glass-glass phase boundaries, which hints the existence of the higher-order dynamical singularities predicted by the mode-coupling theory. Deeply in the cluster glass phases, it is found that the dynamics of a single particle is decoupled from that of the collective fluctuations.

cond-mat.soft↗