arXiv subjects

Kiwamu Yoshii

Publications and source records attributed to Kiwamu Yoshii.

14 recordsLinked to original sources

arXiv2026

A semi-analytic model of the bouncing barrier for protoplanetary dust aggregates

Collisional bouncing limits the growth of dust aggregates in protoplanetary disks, but its dependence on aggregate size, collision velocity, and filling factor remains poorly understood. Here we develop a semi-analytic model for the sticking probability of colliding dust aggregates. We divide each aggregate collision into two phases: a compression phase and a separation phase. The compression phase is described with an elastoplastic contact model, which determines the maximum contact radius and repulsive energy after compression. The separation phase is treated as fracture of a stochastic network of interparticle bonds, whose fracture energy is evaluated using weakest-link statistics. The model naturally predicts that larger aggregates bounce more readily because larger contact regions are more likely to contain weak bonds. Comparison with distinct element method simulations shows that the model reproduces the simulated sticking--bouncing boundary. Furthermore, applying the calibrated model to moderately porous aggregates inferred from ALMA observations of protoplanetary disks, we find that the predicted bouncing barrier passes through the observationally inferred size--velocity range. Thus, our semi-analytic model provides a useful framework for predicting the collisional evolution of protoplanetary dust aggregates.

astro-ph.EP↗

arXiv2026

Revisiting the Stress Field Inside an Elastic Sphere Subjected to a Concentrated Load

We present a complete analytical solution for the stress field inside a homogeneous, inside a homogeneous, linearly elastic solid sphere subjected to a concentrated normal load applied on its surface. Starting from the three-dimensional linearized elastodynamic equations, the displacement and stress fields are derived using scalar and vector potential representations combined with spherical harmonic expansions. All expansion coefficients are determined explicitly by enforcing the traction boundary conditions. The static elastic solution is obtained rigorously as the long-time limit of the dynamical formulation. Closed-form expressions for all components of the stress tensor are provided, enabling direct evaluation of the principal stresses and their differences throughout the interior of the sphere. The analytical solution is further generalized to arbitrary loading positions by means of rotational transformations, allowing systematic treatment of multiple concentrated loads through superposition.

physics.class-ph↗

arXiv2026

Cohesion-induced hysteresis and breakdown of marginal stability in jammed granular materials

The dependence of mechanical properties on microscopic interactions remains a central problem in the physics of disordered solids near the jamming transition. We numerically and theoretically investigate the mechanical response of jammed cohesive granular materials using discrete element simulations and effective medium theory (EMT). We find that the shear modulus exhibits pronounced hysteresis under compression and decompression, even though the interparticle force law itself is strictly history-independent. While such hysteresis disappears for purely repulsive particles when mechanical properties are characterized in terms of pressure, it persists in cohesive packings, indicating that pressure is not a unique state variable for cohesive particles. Extending EMT to cohesive interactions, we show that the functional form of the shear modulus remains the same for both repulsive and cohesive particles, but that attractive interactions violate marginal stability. The resulting deviation from marginal stability generates excess rigidity, as predicted by a scaling relation. This prediction is quantitatively verified by numerical simulations and explains the persistent hysteresis in cohesive packings.

cond-mat.soft↗

arXiv2026

Arrested coarsening in active colloidal suspensions driven by nonreciprocal electrohydrodynamic interactions

Nonreciprocal interactions have recently attracted growing interest in nonequilibrium physics. In particular, breaking action-reaction symmetry has been proposed as a mechanism for collective motion, yet controlled experimental realizations remain scarce. Here we show that bidisperse colloidal suspensions driven by AC electric fields exhibit persistent active clusters sustained by nonreciprocal electrohydrodynamic interactions. Size-asymmetric particle pairs spontaneously self-propel due to imbalanced electrohydrodynamic attraction, producing clusters that continuously fragment and reorganize rather than coarsening into static aggregates as in monodisperse systems. Agent-based simulations reproduce the observed dynamics and identify nonreciprocal pair propulsion as the minimal ingredient for the persistent cluster dynamics. These results demonstrate that action-reaction symmetry breaking in electrohydrodynamic interactions can arrest coarsening and sustain dynamically reconfigurable collective states in dense colloidal suspensions.

cond-mat.soft↗

arXiv2026

Dynamics of particle lane formation in confined viscoelastic fluids under shear

Simple shear flow can induce flow-aligned chain formation of particles suspended in viscoelastic fluids. Although this phenomenon has been reported for decades, direct {\it in situ} measurements of the alignment dynamics and particle trajectories during chain formation remain limited. Here, we develop an {\it in situ} observation platform based on parallel rotating disks separated by a gap comparable to the particle diameter, enabling simultaneous observation of particle alignment under radially varying shear rates. The narrow gap strongly confines particle motion, thereby enhancing hydrodynamic interactions and collision events between particles. Using a viscoelastic fluid embedding zircon particles as the sample, we find that alignment occurs once the local particle Weissenberg number exceeds unity (Wi$_\mathrm{p} \geq 1$), defined using an effective shear rate based on the wall velocity and the available gap width. Particle tracking further reveals a back-and-forth shuttling motion that accompanies the alignment process. Using the image brightness in a colored fluid as a proxy for out-of-plane position, we show that the shuttling originates from vertical displacement of the particles. We further construct a minimal agent-based model in which the vertical particle position follows a Ginzburg-Landau-type double-well potential, and demonstrate that collision-driven accumulation emerges in numerical simulations. In the strongly confined geometry, alignment occurs by an effective attraction due to collision, which is reminiscent of motility-induced clustering often observed in active matter.

cond-mat.soft↗

arXiv2025

Dislocation Glides in Monolayered Granular Media: Effect of Lattice Constant

A recent study demonstrated that granular crystals containing a single dislocation exhibit dislocation glide analogous to that observed in atomic-scale crystals, resulting in plastic deformation at yield stresses several orders of magnitude lower than those of dislocation-free crystals. The yielding behavior strongly depends on the interparticle friction coefficient $μ$: dislocation glide occurs for friction coefficients below a critical value $μ_c$, while crystalline order deteriorates above $μ_c$. In this work, we use discrete element method simulations to systematically investigate how the lattice constant, which determines the interparticle spacing and is a fundamental parameter in microscopic crystalline solids, and the friction coefficient $μ$ influence the yielding behavior in monolayered granular crystals with dislocation. By decreasing the lattice constant, we find an increase in the critical friction coefficient $μ_c$, allowing dislocation glide to persist at higher friction values. Furthermore, we observe a linear scaling of yield stress with normal stress, except at extremely low friction coefficients.

cond-mat.soft↗

arXiv2024

Dislocation Glides in Granular Media

Atomic crystals with dislocations deform plastically at low stresses via dislocation glide. Whether dislocation glide occurs in macroscopic frictional granular media has remained unknown. The discrete element method is employed to simulate the structural and mechanical responses of a granular crystal with an edge dislocation. We find that dislocation glide occurs at low interparticle friction, resulting in significantly lower yield stresses than in dislocation-free crystals. Yield stress varies linearly with interparticle friction, attributed to both Peierls stress and frictional effect.

cond-mat.mtrl-sci↗

arXiv2024

Reducing segregation in vibrated binary-sized granular mixtures by excessive small particle introduction

We numerically examine binary-sized granular mixtures confined between two parallel walls subjected to vertical vibration using the discrete element method. For a size ratio of $3$ between large and small particles, we study the structure of large particles in moderately dense regimes where the combined two-dimensional packing fractions of both particle sizes exceed $1$. When the fraction of small particles is small, segregation of the large particles occurs. In contrast, as the fraction of small particles increases, an effective repulsion between the large particles emerges over distances greater than the large particle diameter, suppressing their segregation. The emergence of reduction in segregation is confirmed for another size ratio, vibrational acceleration, system size, and for a case of bidisperse size distribution. Additionally, at the size ratio of $3$, the effective repulsion induces a hexagonal phase of the large particles at packing fractions lower than in mono-component systems. This work will provide a fresh insight into granular physics, prompting further experimental and theoretical study.

cond-mat.soft↗

arXiv2024

Critical scaling for dense granular flow between parallel plates near jamming

We numerically study the flow of dense granular materials between parallel plates driven by an external force. The granular materials form a jammed solid-like state when the external force is below a critical force, while they flow like fluids above the critical force. The transition is characterized by the mass flux. The critical force depends on the average packing fraction and the distance between the plates. The scaling laws for the critical force and the mass flux are predicted theoretically based on a continuum model. They are numerically verified.

cond-mat.soft↗

arXiv2024

Pattern transition of flow dynamics in a highly water-absorbent granular bed

An aqueous sodium chloride solution was injected at a controlled rate into a granular bed in a quasi-two-dimensional cell. The granular bed was made of dried, highly water-absorbent gel particles whose swelling rate was controlled by the salinity of the injected fluid. At a high salinity level (low swelling rate), high injection rate, and short timescale, the injected fluid percolated between the gel particles in an isotropic manner. Meanwhile, at a low salinity level (high swelling rate), low injection rate, and long timescale, the gel particles clogged the flow path, resulting in anisotropic branch-like structures of the injected fluid front. The transition of the injection pattern could be understood based on the ratio of the characteristic timescales of swelling and injection. Moreover, the clogged pattern showed an oscillatory pressure drop whose amplitude was increased with higher salinity. Such an oscillatory behavior observed in an injection process in a swelling gel particle may be relevant in geological situation; i.e., such as fluid migration underground.

cond-mat.soft↗

arXiv2024

Extensive tip-splitting of injected organic liquid into an aqueous viscoelastic fluid

The injection of a fluid into another fluid causes a spatiotemporal pattern along the injection front. Viscous fingering is a well-known example when the replaced material is a viscous fluid. Notably, most fluids are, in reality, viscoelastic, i.e., they behave as an elastic solid over short timescales. For this reason, it is important to study the situation when the replaced fluid is viscoelastic. In this study, we observe extensive tip-splitting in the fingering pattern when an incompressible organic liquid was injected into an oleophilic Hele--Shaw cell filled with an aqueous viscoelastic fluid made of a wormlike micellar solution. The tip-splitting led to thin fingers with a characteristic size comparable to four times the cell thickness. We examined the material properties and suggest that the thin fingering pattern observed in our current system is due to the delamination of viscoelastic fluid from the bottom substrate surface. Our result shows that the effect of interfacial energy in the existing solid layer should be considered in the injection process.

cond-mat.soft↗

arXiv2022

Rheology of dilute granular gas mixtures where the grains interact via a square shoulder and well potential

We develop the rheology of a dilute granular gas mixture. Motivated by the interaction of charged granular particles, we assume that the grains interact via a square shoulder and well potential. Employing kinetic theory, we compute the temperature and the shear viscosity as functions of the shear rate. Numerical simulations confirm our results above the critical shear rate. At a shear rate below a critical value, clustering of the particles occurs.

cond-mat.stat-mech↗

arXiv2022

Mechanical and geometrical properties of jammed wet granular materials

We numerically investigate the mechanical and geometrical properties of dense wet granular particles with irreversible attractive interaction. The shear modulus exhibits two inflection points as the packing fraction increases, and the bulk modulus shows a non-monotonic behavior. The coordination number also exhibits two inflection points. The peak position in the pair correlation function shifts to a lower value due to attractive interaction. The Voronoi tessellation of the particle configuration reveals that the probability density function for the volume of the Voronoi cell broadens as the packing fraction approaches the jamming point.

cond-mat.soft↗

arXiv2020

Aversion of face-to-face situation of pedestrians eases crowding condition

We conducted numerical simulation for a crowd of pedestrians. Each pedestrian, modeled with three circles, has a shape whose long axis is perpendicular to the anteroposterior axis, and is designed to move fixed destination. The pedestrians have friction at the surface and soft repulsion. In this study, we newly introduced an active rotation which captures psychological effect to evade face-to-face situation. The numerical simulation revealed that active rotation induces fluidization of system leading to higher flux of pedestrian. We further confirmed that this fluidization is due to fragmentation of force chain induced by the active rotation.

physics.soc-ph↗