arXiv subjects

Hiroaki Katsuragi

Publications and source records attributed to Hiroaki Katsuragi.

At least 19 recordsLinked to original sources

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

Granular Rods Fall Faster in Denser Obstacle Fields

How particle shape affects transport through obstacle fields under external driving is a fundamental question in nonequilibrium physics. We simulate a dissipative rod falling under gravity through randomly placed fixed obstacles. As the obstacle density increases, the mean descent speed decreases, increases, and then decreases again before trapping. The rod can therefore fall faster in a denser obstacle field. Scaling arguments based on collision rates and rod geometry explain all three regimes, their crossovers, and the mean fall distance before trapping. These results reveal nonmonotonic driven transport arising from particle anisotropy.

cond-mat.soft↗

arXiv2026

Scaling law for the diffusion coefficient in vibration-driven crater relaxation

Impact craters relax over long timescales, and this process is commonly described by a diffusion model. The diffusion coefficient determines the relaxation rate, and it has been estimated from observed crater shapes. This coefficient, however, represents the combined effect of several physical mechanisms. Its physical basis has not been revealed yet. In this study, we isolate the contribution of seismic vibration in a controlled experiment. Using a quasi-two-dimensional setup, we first confirm that a linear diffusion equation reproduces the crater relaxation. We then measure the diffusion coefficient and obtain the scaling form through systematic experiments. We find that the diffusion coefficient is proportional to the crater diameter. This dependence is not expected for simple diffusion. We interpret this dependence as arising because the thickness of the vibro-fluidized granular layer scales with the crater depth. This scaling relation is the main result of this study. We then apply it to the Moon with a parameterized model of impact-driven seismic spreading in the regolith layer. Integrating over the impact flux yields a macroscopic coefficient nearly proportional to crater diameter for shallow-layer-like spreading. Its magnitude depends on uncertain model parameters but is consistent with observations for plausible values. These results indicate that vibration is a physically plausible contributor to crater relaxation on the Moon.

astro-ph.EP↗

arXiv2026

Anomalous phonon dispersion near yielding in athermal crystals

Vibrational properties of ordered athermal solids near yielding remain poorly understood. We show that yielding in a sheared crystal is governed not by a single localized instability but by directionally extended multimode softening that forms a cross-shaped low-frequency region in wave number space. Near yielding, the acoustic dispersion $ω\sim k$ is replaced by $ω\sim k^2$ along the soft direction, and the vibrational density of states crosses over from Debye to non-Debye scaling, with a diverging length scale. We analytically derive these scaling laws.

cond-mat.mtrl-sci↗

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. $δ/R$ (the sinking depth $δ$ 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 $μ_\mathrm{d}$. We demonstrate that $μ_\mathrm{d}$ decreases with increasing the slope angle and the slip ratio. Furthermore, systematic measurements over a wide range of sphere densities reveal that $μ_\mathrm{d}$ increases linearly with $δ/R$ : $μ_\mathrm{d}=β(δ/R)+μ_0$. The value of $μ_0$ is linearly decreasing with slip ratio and its coefficient $β(\simeq0.41)$ 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↗

arXiv2026

Constitutive flow law for hydrogel granular rafts near the brittle-ductile transition

Spatially varying flow laws have been identified in dry granular flow, yet their applicability to unjammed suspensions remains unclear. This study demonstrates that the quasistatic suspension flow combines dry granular rheology with nonlocal effects in the shear band and damped viscous flow in the outer creep region. Through rotary shear experiments on a hydrogel granular raft, we observe that the flow decays from the interface in the quasistatic regime, where the particles remain mobile even below the yield stress. These findings suggest the universal flow law across the transition between jammed/brittle granular behavior and unjammed/ductile viscous flow.

cond-mat.soft↗

arXiv2025

Origin of slow earthquake statistics in low-friction soft granular shear

Slow earthquakes differ from regular earthquakes in their slower moment release and size distribution dominated by smaller events. However, the physical origin of these slow earthquake statistics remains controversial. In this work, we experimentally demonstrate that their characteristics emerge from low-friction soft granular shear. To model slow-earthquake fault materials under hydrothermal conditions, we use a low-friction soft hydrogel particle layer floating on lubricating fluid and conduct stick-slip experiments. The observed slip events follow the same laws of both moment release rate and size distribution as with slow earthquakes, contrasting with frictional rigid granular shear. Slip size is determined by the competing effects of shear localization and pressure enhancement with decreasing porosity. These findings indicate that low friction and particle softness in sheared granular systems with sparse contact structures cause slow earthquake statistics, which may be driven by pore fluid dynamics and shear localization within hazardous fault zones.

physics.geo-ph↗

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↗

arXiv2025

Liquid water transport model in hydrophilic granular : Preliminary validation with drying rate of hierarchical granular

The drying rate profile of granular beds can be divided into the constant rate period (CRP), which is characterized by a nearly constant drying rate, and the falling rate period (FRP), in which the drying rate rapidly decays. In order to explain this behavior quantitatively, we proposed a simple one-dimensional power law model in which the product of the water permeability and the pressure gradient is assumed to be proportional to the cube of the saturation. To test this model, we measured the drying rates of glass beads and hierarchical granular materials produced by sintering and breaking glass beads. Our results and those of previous experiments showed consistency with the power law. The obtained proportional constant of the experimental power law also shows a rough agreement with that estimated from previous studies on water permeability and capillary pressure. Drying behavior in FRP also agrees with our model in some points. The remnant deviation of the model from experimental results may be attributed to the inhomogeneity of granular media, which was qualitatively verified.

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

Air jet impact craters on granular surfaces: a universal scaling

Craters form as the lander's exhaust interacts with the planetary surfaces. Understanding this phenomenon is imperative to ensure safe landings. We investigate crater morphology, where a turbulent air jet impinges on the granular surfaces. To reveal the fundamental aspect of this phenomenon, systematic experiments are performed with various air jet velocities, nozzle positions, and grain properties. The resultant crater morphology is characterized by an aspect ratio. We find a universal scaling law in which the aspect ratio is scaled by the dimensionless variable consisting of air velocity at the nozzle, speed of sound in air, nozzle diameter, nozzle tip distance from the surface, grain diameter, the density of grains, and density of air. The obtained scaling reveals the crossover of the length scales governing crater aspect ratio, providing a useful guideline for ensuring safe landings. Moreover, we report a novel drop shaped subsurface cratering phenomenon.

physics.flu-dyn↗

arXiv2024

Tip angle dependence for resistive force into dry granular materials at shallow cone penetration

In relation to the interaction of the earth's surface with machines and organisms, and its engineering applications, there has been a recent increase in interest in the penetration resistive force into granular materials at shallow depths. Previous studies have proposed various models for penetration resistive forces into dry granular materials. This study focuses on the model which has a coefficient depending on the angle of repose and the increase of resistive force in proportion to the penetration volume. In the previous studies, the model has been validated for several geometries such as cylinders, cones, and spheres. However, for cones, the model has only been validated under conditions of a tip angle close to the angle of repose. In this study, the effect of cone tip angle on penetration resistive force is investigated under several conditions with different angles of repose. This study carries out cone penetration simulations using the discrete element method. For the cone geometry, five tip angles ranging from sharp to blunt (tip angles are 15, 30, 45, 60, 75 deg) are used. The simulation results show that the penetration resistive forces for blunt cones are much higher than that computed by the model. To solve the discrepancy between the model and simulation results, this study modifies the model by assuming that the stagnant zone formed in front of the cone penetrating the granular material behaves as an effective cone. Thereby, the proposed model can calculate penetration resistive forces more accurately for cones with a wider range of tip angles than in the previous model.

cond-mat.soft↗

arXiv2024

Drop impact on wet granular beds: water-content effects on the cratering

Drop impact events on wet granular bed show rich variety by changing the substrate composition. We observe the drop impact onto dry/wet granular substrates with different grain size (50-400 μm) and water content (0-22 vol %). Although the impactor condition is fixed (impact velocity: 4.0 m/s, water drop radius: 1.8 mm), the experiment reveals that the post-impact behaviors of both impactor and target are strongly affected by the substrate composition. We sort these behaviors into several phases regarding liquid splashing and crater shapes left after the event. As these phases show relevance each other, we measure the mechanical characteristics of the substrates and find that the onset of splashing and particle ejection are explained by a fracture of the substrate. Furthermore, we discuss several timescales of the event to understand more detailed phase separations. Consequently, we find that the splashing phase and the crater shape are determined by a competition of the timescales of impact, penetration, and contact.

cond-mat.soft↗

arXiv2023

Energy dissipation of a sphere rolling up a granular slope: slip and deformation of granular surface

We experimentally investigate the dynamics of a sphere rolling up a granular slope. During the rolling-up motion, the sphere experiences slipping and penetration (groove formation) on the surface of the granular layer. The former relates to the stuck motion of the rolling sphere, and the latter causes energy dissipation due to the deformation of the granular surface. To characterize these phenomena, we measured the motion of a sphere rolling up a granular slope of angle $α$. The initial velocity $v_0$, initial angular velocity $ω_0$, angle of slope $α$, and density of the sphere $ρ_s$ were varied. As a result, the penetration depth can be scaled solely by the density ratio between the sphere and granular layer. By considering the rotational equation of motion, we estimate the friction due to the slips. Besides, by considering energy conservation, we define and estimate the friction due to groove formation. Moreover, the translational friction is proportional to the penetration depth. Using these results, we can quantitatively predict the sphere's motion including stuck behavior.

cond-mat.soft↗

arXiv2023

Fracturing-induced fluidization of vibrated fine-powder column

We experimentally investigate the effect of vertical vibrations on the brittle behavior of fine cohesive powders consisting of glass beads of 5 microns in diameter. This is an attempt to understand the sole role of vibrations in fluidizing Geldart's group C powders, which is known for posing difficulty while fluidization. We find that the cohesive powder column can be compacted, fractured, and effectively fluidized by increasing the strengths of external vibrations. This process of vibration-induced fracturing is summarized in a full experimental phase diagram showing four distinct phases of the vibrated powder column: consolidation (CS), static fracture (SF), dynamic fracture (DF), and convective fracture (CF). We find that the boundary separating the consolidated and fracture regimes depends on the dimensionless shaking strength, S. However, in the DF regime, the decompaction wave propagation speed normalized to gravitational speed is found to be independent of S. In order to reach our ultimate goal of effective fluidization of group C powders, we explore geometrical parameters like container shapes, sizes, and base conditions. We find that the circular cylinder with hemispherical base condition is the most effective container in order to achieve effective fluidization of group C powders when vibrated.

cond-mat.soft↗

arXiv2022

Decompaction-wave propagation in a vibrated fine powder bed

We experimentally study the crack formation and decompaction-wave propagating in a vibrated powder bed consisting of glass beads of 5 μm in diameter. The vibrated powder bed exhibits three distinct phases depending on the vibration conditions: consolidation (CS), static fracture (SF), and dynamic fracture (DF). Particularly, we found an upward wave propagation in the DF regime when the powder bed is strongly vibrated. As a remarkable feature, we found that in fine cohesive powders, the decompaction-wave propagation speed normalized to gravitational speed is independent of the shaking strength. This result implies that the wave propagation speed is governed by the balance between gravity and cohesion effect rather than vibration strength. We also explore the universality of wave propagation phenomenon in coarser and low-density granular powders.

cond-mat.soft↗

arXiv2021

Grain size effect on the compression and relaxation of a granular column: solid particles vs dust agglomerates

We studied experimentally the effect of grain size and maximum load on the compaction and subsequent relaxation of a granular column when it is subjected to vertical uniaxial compression. The experiments were performed using two different types of grains: 1) solid glass beads, and 2) porous beads that consist of agglomerates of glass powder. We found that the compression force increases non-linearly with time, with sudden drops for the case of glass beads and periodic undulations for dust particles. Whereas the grain size effect is small in the average force load, the fluctuations become larger as the grain size increases. On the other hand, the relaxation process is well described by the Maxwell model with three different relaxation time scales.

cond-mat.soft↗

arXiv2021

Undulating compression and multi-stage relaxation in a granular column consisting of dust particles or glass beads

For fundamentally characterizing the effect of hierarchical structure in granular matter, a set of compression-relaxation tests for dust particles and glass beads confined in a cylindrical cell was performed. Typical diameter of both grains is approximately 1~mm. However, dust particles are produced by binding tiny ($\sim 5$~{\textmu}m) glass beads. The granular columns were compressed with a piston until reaching a maximum load force of 20~N with a constant compression rate $v$ ($0.17 \leq v \leq 2000$~{\textmu}m~s$^{-1}$). After that, the piston was stopped and the relaxation process was quantified. From the experimental results, we found that the compression force $F$ nonlinearly increases with the increase of compression stroke $z$ depending on particles. Besides, periodic undulation and sudden force drops were observed on $F(z)$ in dust particles and glass beads, respectively. The relaxation process was characterized by an exponential decay of stress followed by a logarithmic dependence one in both kinds of particles. These experimental findings are the main point in this study. To understand the underlying physics governing the compression mechanics, we assumed empirical forms of $F(z)$; $F\propto z^α$ for dust particles and $F \propto \exp(z/z_G)$ for glass beads ($α=2.4$ and $z_G=70$~{\textmu}m). Then, we found that the growing manners of periodic undulation and force drops were identical to those of mean compression forces, i.e., power law in dust particles and exponential in glass beads. In addition, the undulation amplitude and wavelength decreased as $v$ increased in dust-particles compression. On the basis of experimental results and the difference between dust particles and glass beads, we also discuss the origin of undulation and the physical meaning of granular-compression models used in engineering fields.

cond-mat.soft↗