arXiv subjects

Hiroyuki Ebata

Publications and source records attributed to Hiroyuki Ebata.

9 recordsLinked to original sources

arXiv2026

Directional Cluster Migration Driven by Escape-Rate Asymmetry in Multi-Compartment Granular Systems

Granular materials are inherently out-of-equilibrium systems due to energy dissipation through inelastic collisions and friction. When driven by mechanical agitation such as vibration, they exhibit rich collective behaviors including segregation, clustering, and spontaneous oscillations. Here, we report directional stepwise migration of particle clusters from one compartment to the next in a vertically vibrated granular system composed of small and large particles. To clarify the underlying mechanism, we directly measured how the flux of both particle species depends on the instantaneous particle populations. The measurements reveal an asymmetric interaction between particle species: the flux of small particles is enhanced by the presence of large particles, whereas that of large particles is suppressed by small particles. A minimal flux model incorporating these measured fluxes reproduces the observed directional dynamics and provides an experimentally grounded framework for collective transport in vibrated granular systems.

cond-mat.soft↗

arXiv2026

A pushing-pulling captive bubble method for repeatable measurement of dynamic contact angles underwater

Accurate measurement of dynamic contact angles in aqueous environments is essential for evaluating surface wettability. However, conventional captive bubble methods often suffer from limitations such as bubble instability and interference from needle wetting. In this study, we develop a pushing-pulling captive bubble method that enables stable and repeatable measurement of dynamic contact angles underwater without directly changing the bubble volume. In this method, a bubble is pushed against and detached from a surface by controlled vertical motion. This procedure allows stable observation of the contact line while suppressing bubble deformation and lateral movement. Dynamic contact angles were measured in both air and water using three types of surfaces: smooth surfaces, sandpaper-polished surfaces prepared to exhibit the Wenzel state in air and the reversed gas-liquid Wenzel state in water, and microstructured surfaces exhibiting hydrophobicity in air. For smooth and Wenzel surfaces, the dynamic contact angles measured in air and water showed similar values. Moreover, the modified captive bubble method exhibited reproducibility comparable to that observed in conventional captive bubble methods under the present experimental conditions. For microstructured surfaces, dynamic contact angle measurements in water had previously been difficult because an air layer remained trapped on the surface. In this study, ultrasonic degassing enabled dynamic contact angle measurements under fully wetted conditions, revealing behavior that differed significantly from that observed in air.

cond-mat.soft↗

arXiv2026

Depth and slip ratio dependencies of friction for a sphere rolling on a granular slope

We experimentally investigate the dynamics of a sphere rolling down a granular slope by varying the initial velocity, slope angle, and sphere density. The results show that the sphere rolls down with constant deceleration while sinking into the granular bed. $\delta/R$ (the sinking depth $\delta$ normalized to the sphere radius $R$) is scaled by the sphere density normalized by the bulk density of the granular layer. To evaluate the translational energy dissipation, we introduce an effective friction coefficient $\mu_\mathrm{d}$. We demonstrate that $\mu_\mathrm{d}$ decreases with increasing the slope angle and the slip ratio. Furthermore, systematic measurements over a wide range of sphere densities reveal that $\mu_\mathrm{d}$ increases linearly with $\delta/R$ : $\mu_\mathrm{d}=\beta(\delta/R)+\mu_0$. The value of $\mu_0$ is linearly decreasing with slip ratio and its coefficient $\beta(\simeq0.41)$ does not vary significantly. The results suggest that the normalized depth and slip ratio determine the effective friction of a rolling sphere.

cond-mat.soft↗

arXiv2024

A link between anomalous viscous loss and boson peak in soft jammed solids

Soft jammed solids exhibit intriguing mechanical properties, while their linear response is elusive. In particular, foams and emulsions generally reveal anomalous viscous loss with the loss and storage modulus following $G^{\prime \prime} \propto \sqrt{\omega}$ and $G^{\prime} \propto \omega^0$. In this study, we offer a comprehensive microscopic understanding of this behavior. Using microrheology experiment, we measured $G^* = G^{\prime} + i G^{\prime \prime}$ of concentrated emulsions in a wide range of frequencies. In theory, we applied a linear response formalism for microrheology to a soft sphere model that undergoes the jamming transition. We find that the theory quantitatively explains the experiments without the need for parameter adjustments. Our analysis reveals that the anomalous viscous loss results from the boson peak, which is a universal vibrational property of amorphous solids and reflects the marginal stability in soft jammed solids. We discuss that the anomalous viscous loss is universal in systems with various interparticle interactions as it stems from the universal boson peak, and it even survives below the jamming density where thermal fluctuation is pronounced and the dynamics becomes inherently nonlinear.

cond-mat.soft↗

arXiv2024

Non-Equilibrium Fluidization of Dense Active Suspension

We investigate dense suspensions of swimming bacteria prepared in a nutrient-exchange chamber. Near the pellet concentration, nonthermal fluctuations showed notable agreement between self and collective behaviors, a phenomenon not previously observed at equilibrium. The viscosity of active suspensions dramatically decreased compared to their inactive counterparts, where glassy features, such as non-Newtonian viscosity and dynamic heterogeneity, disappeared. Instead, the complex shear modulus showed a power-law rheology,$G^*(\omega)\propto\left(-i\omega\right)^\frac{1}{2}$, indicating the role of bacterial activity in driving the system towards a critical jamming state.

cond-mat.soft↗

arXiv2023

Convection of mono-disperse particles in a highly filled rotating cylinder

We investigate the occurrence of spontaneous convection in a coaxial cylinder highly filled with mono-disperse spheres. To analyze the flow field non-invasively, initial pulses consisting of colored particles are placed at equal intervals. By analyzing the spatio-temporal distribution of these pulses, we obtained axial velocity profiles for both the surface and subsurface regions. Our advection-diffusion equations with steady advection terms incorporate experimentally obtained axial velocity profiles in the surface layer, while the rest of the components are estimated using azimuthal symmetry and volume conservation. The validity of our model is confirmed by comparing experimental data with numerical solutions for both the spatio-temporal distribution and cross-sectional profile of the colored particles.

cond-mat.soft↗

arXiv2023

Segregation patterns in rotating cylinders determined by the size difference, density ratio, and cylinder diameter

Granular materials often segregate under mechanical agitation, which differs from the expectation of mixing. It is well known that a bidisperse mixture of granular materials in a partially filled rotating cylinder exhibits alternating bands depending on the combination of the two species. The dynamic angle of repose, which is the angle that a steady avalanche makes with the horizontal, has been considered the dominant parameter that determines the segregated state. However, the previously known angle of repose condition was not always satisfied in different experimental cases. To clarify the experimental conditions, we conducted an exhaustive parameter search with three dimensionless parameters: the particle size difference normalized by the average particle size, the specific density ratio, and the ratio of the cylinder diameter to the average particle size. Additional experiments were conducted to explore the effect of the rotational speed of the cylinder. This systematic approach enabled us to predict the segregated state. Moreover, we discovered that the band width can be effectively scaled by combining these three parameters.

cond-mat.soft↗

arXiv2023

Self-replicating segregation patterns in horizontally vibrated binary mixture of granules

When granular mixtures of different sizes are fluidized, each species spontaneously separates and condenses to form patterns. Although granular segregation has been extensively studied, the inability to directly observe the time evolution of the internal structure hinders the understanding of the mechanism of segregation dynamics driven by surface flow. In this study, we report rich band dynamics, including a self-replicating band, in a horizontally shaken granular mixture in a quasi-two-dimensional container where the granules formed steady surface waves. Direct observation of surface flow and segregated internal structure revealed that coupling among segregation, surface flow, and hysteresis in the fluidity of granules is key to understanding complex band dynamics.

cond-mat.soft↗

arXiv2020

Why epithelial cells collectively move against a traveling signal wave

The response of cell populations to external stimuli plays a central role in biological mechanical processes such as epithelial wound healing and developmental morphogenesis. Wave-like propagation of a signal of ERK MAP kinase has been shown to direct collective migration in one direction; however, the mechanism based on continuum mechanics under a traveling wave is not fully understood. To elucidate how the traveling wave of the ERK kinase signal directs collective migration, we constructed the mechanical model of the epithelial cell monolayer by considering the signal-dependent coordination of contractile stress and cellular orientation. The proposed model was studied by using an optogenetically-controlled cell system where we found that local signal activation induces changes in cell density and orientation with the direction of propagation. The net motion of the cell population occurred relative to the wave, and the migration velocity showed a maximum in resonance with the velocity of the ERK signal wave. The presented mechanical model was further validated in \textit{in vitro} wound healing process.

physics.bio-ph↗