arXiv subjects

H. Katsuragi

Publications and source records attributed to H. Katsuragi.

At least 19 recordsLinked to original sources

arXiv2026

Time-resolved sedimentation of dense potato-starch suspensions measured by optical coherence tomography

We demonstrate optical coherence tomography (OCT) as a measurement technique for dense, optically opaque suspensions. Conventional optical methods cannot access the interior of such suspensions. OCT resolves individual potato-starch particles (${\sim}20~\mathrm{\mu m}$) as distinct scatterers, even though the suspension appears opaque to the eye. By tracking the vertical centroid position of the particle-laden layer $\langle Z \rangle(t)$ and the supernatant boundary $Z_\mathrm{sup}(t)$ in the same OCT image sequence, we obtain the instantaneous settling velocity $V(t)$ and the time-evolving effective volume fraction $\phi_\mathrm{eff}(t)$ simultaneously and continuously in time. To our knowledge, this is the first measurement to combine settling velocity and particle concentration into a single continuous trajectory within one sedimentation run. Conventional batch measurements yield only one velocity value per run. We applied this method to dense potato-starch suspensions, varying the initial volume fraction $\phi_0$ from 0.30 to 0.50 and the solvent density $\rho_\mathrm{L}$ from 1.0 to $1.3{\times}10^3~\mathrm{kg~m^{-3}}$ using aqueous sodium polytungstate solutions. The normalized velocity $V/V_\mathrm{Stokes}$ plotted against $\phi_\mathrm{eff}$ collapses onto a common trend consistent with both the Krieger--Dougherty model and the Richardson--Zaki law over $\phi_\mathrm{eff} \simeq 0.30$--$0.52$, confirming that the method captures physically reasonable hindered-settling behavior. These results establish OCT as a viable tool for probing internal dynamics in dense suspensions that were previously inaccessible to optical measurement.

cond-mat.soft↗

arXiv2026

Scaling of microcraters with molten rims derived from laser-induced cratering experiments

Microcraters with molten rims are widely observed in returned samples from the Moon and asteroids. These features reflect the conditions of small-scale impact events, but the impact velocities required for their formation are not well understood. In this study, we use well-controlled laser irradiation on rock surfaces as an experimental analog to investigate the formation of these molten-rim craters. Specifically, we derive a scaling relation between crater volume, total irradiation energy, and energy loss associated with thermal diffusion. The experimental results indicate that molten-rim structures of microcraters can be formed in the range of $10^2$--$10^3$~m/s impact velocity. This value is consistent with some previous studies which suggested that the origin of microcraters is secondary impact events. These findings provide experimental constraints on the formation mechanisms of molten-rim microcraters and offer a new perspective on impact processes recorded on planetary surfaces. The estimated result is also consistent with the impact velocity required to produce the observed wavy rim protrusions due to a Rayleigh-Taylor hydrodynamic instability ($\sim 10^2$~m/s).

astro-ph.EP↗

arXiv2026

Skimming transition in flexible granular sweeping

A flexible body placed in a steady flow bends to reduce drag. This self-streamlining is a hallmark of fluid-structure interaction (FSI). Granular-structure interaction is equally ubiquitous in nature. However, it remains poorly understood. Thus, we investigate inertial granular-structure interaction (IGSI). Specifically, ejection induced by a flexible plate sweeping a granular bed is experimentally examined. We find that a faster sweep results in less ejection, particularly for a flexible plate. To understand the underlying physics of this behavior, a dimensionless number Sk is introduced as the ratio of the plate elastic timescale to the sweep timescale. At $\mathrm{Sk} \lesssim 1$, the plate deflection follows the self-streamlining law of FSI and induces substantial ejection. At $\mathrm{Sk} \gtrsim 1$, on the other hand, the plate skims the bed and the mass of ejected grains decreases sharply. Sk organizes IGSI as the granular counterpart of FSI.

cond-mat.soft↗

arXiv2023

Disordering two-dimensional magnet-particle configurations using bidispersity

In various types of many-particle systems, bidispersity is frequently used to avoid spontaneous ordering in particle configuration. In this study, the relation between bidispersity and disorder degree of particle configuration is investigated. By using magnetic dipole-dipole interaction, magnet particles are dispersed in a two-dimensional cell without physical contact between them. In this magnetic system, bidispersity is introduced by mixing large and small magnets. Then, the particle system is compressed to produce a uniform particle configuration. The compressed particle configuration is analyzed by using Voronoi tessellation for evaluating the disorder degree which strongly depends on bidispersity. Specifically, standard deviation and skewness of the Voronoi cell area distribution are measured. As a result, we find that the peak of standard deviation is observed when the numbers of large and small particles are almost identical. Although the skewness shows non-monotonic behavior, zero skewness state (symmetric distribution) can be achieved when the numbers of large and small particles are identical. In this ideally random (disordered) state, the ratio between pentagonal, hexagonal, and heptagonal Voronoi cells become roughly identical, while hexagons are dominant in monodisperse (ordered) condition. The relation between Voronoi cell analysis and the global bond orientational order parameter is also discussed.

cond-mat.soft↗

arXiv2022

Grain-size dependence of water retention in a model aggregated soil

We experimentally examined the amount of water retention in a model soil composed of aggregated glass beads. The model soil was characterized by two size parameters: size of aggregates $D$ and size of monomer particles (composing aggregates) $d$. In the experiment, water was sprinkled on the model-soil system that has an open top surface and drainable sieve bottom. When the sprinkled water amount exceeded a threshold (retainable limit), draining flux balanced with the sprinkled flux. The weight variations of retained and drained water were measured to confirm this balanced (steady) state and quantify the retained water. We defined the weight of the retained water in this steady state as $W_0$ and examined the relationship among $W_0$, $d$ and $D$. As a result, it was revealed that $W_0$ increases as $d$ decreases simply due to the capillary effects. Regarding $D$ dependence, it turned out that $W_0$ becomes the maximum around $D\simeq 500$~$μ$m. The value of $D$ maximizing water retention is determined by the void formation due to the aggregated structure, capillary effect, and gravity.

cond-mat.soft↗

arXiv2022

Impact drag force exerting on a projectile penetrating into a hierarchical granular bed

Impact of a solid object onto a small-body surface can be modeled by the solid impact onto a hierarchically structured granular target. Impact drag force model for the hierarchically structured granular target is developed based on the experiment. We perform a set of granular impact experiments in which mechanical strength and porosity of target grains are systematically varied. Tiny glass beads ($5$~$μ$m in diameter) are agglomerated to form porous grains of $2$--$4$~mm in diameter. Then, the grains are sintered to control their strength. A polyethylene sphere ($12.7$~mm in diameter) is dropped onto a hierarchical granular target consisting of these porous grains. Motion of the penetrating sphere is captured by a high-speed camera and analyzed. We find that impact drag force produced by the hierarchically structured granular target can be modeled by the sum of inertial drag and depth-proportional drag. The depth-proportional drag in hierarchical granular impact is much greater than that of the usual granular target consisting of rigid grains. The ratio between grain strength and impact dynamic pressure is a key dimensionless parameter to characterize this extraordinary large depth-proportional drag. Grain fracturing plays an important role in the impact dynamics when the impact dynamic pressure is sufficiently larger than the grain strength. This implies that the effect of grain fracturing should be considered also for the impact on a small body. Perhaps, effective strength of the surface grains can be estimated based on the kinematic observation of the intrusion or touchdown of the planetary explorator.

astro-ph.EP↗

arXiv2022

History-dependent deformation of a rotated granular pile governed by granular friction

We experimentally examined the history dependence of the rotation-induced granular deformation. As an initial state, we prepared a quasi-two-dimensional granular pile whose apex is at the rotational axis and its initial inclination is at the angle of repose. The rotation rate was increased from $0$ to $620$~(rpm) and then decreased back to $0$. During the rotation, deformation of the rotated granular pile was captured by a camera. From the acquired image data, granular friction coefficient $μ$ was measured as a function of the ratio between centrifugal force and gravity, $Γ$. To systematically evaluate the variation of $μ$ both in the increasing (spinning up) and decreasing (spinning down) rotation-rate regimes, surface profiles of the deformed granular piles were fitted to a model considering the force balance among gravity, friction, and centrifugal force at the surface. We found that $μ$ value grows in the increasing $Γ$ regime. However, when $Γ$ was reduced, $μ$ cannot recover its initial value. A part of the history-dependent behaviors of the rotated granular pile can be understood by the force balance model.

cond-mat.soft↗

arXiv2021

History-dependent growth and reduction of the ripples formed on a swept granular track

When a solid object or wheel is repeatedly dragged on a dry sandy surface, ripple patterns are formed. Although the conditions to form ripple patterns have been studied well, methods to eliminate the developed ripple patterns have not been understood thus far. Therefore, history-dependent stability of the ripple patterns formed on a sandy surface is investigated in this study. First, the ripple patterns are formed by sweeping the flat sandy surface with a flexible plow at a constant speed. Then, the sweeping speed is reduced, and the variation of ripple patterns is measured. As a result, we find that the ripple patterns show hysteresis. Specifically, the increase in amplitude of ripples is observed when the reduced velocity is close to the initial velocity forming the ripple pattern. In addition, splitting of ripples is found when the reduced velocity is further decreased. From a simple analysis of the plow's motion, we discuss the physical mechanism of the ripple splitting.

cond-mat.soft↗

arXiv2021

Deformation of a rotated granular pile governed by body-force-dependent friction

Although the gravity dependence of granular friction is crucial to understand various natural phenomena, its precise characterization is difficult. We propose a method to characterize granular friction under various gravity (body force) conditions controlled by centrifugal force; specifically, the deformation of a rotated granular pile was measured. To understand the mechanics governing the observed nontrivial deformation of this pile, we introduced an analytic model considering local force balance. The excellent agreement between the experimental data and theoretical model suggests that the deformation is simply governed by the net body force (sum of gravity and centrifugal force) and friction angle. The body-force dependence of granular friction was precisely measured from the experimental results. The results reveal that the grain shape affects the degree of body-force dependence of the granular friction.

cond-mat.soft↗

arXiv2021

Measurement of surface deformation and cohesion of a granular pile under the effect of centrifugal force

An experimental apparatus measuring free-surface deformation of a centrifuged granular pile is developed. By horizontally rotating a quasi two-dimensional granular pile whose apex is located at the vertical rotation axis, the resultant force of gravity and centrifuge yields the deformation of the granular pile. In this setup, centrifugal force depends on distance from the rotation axis whilst gravitational force is constant everywhere. Therefore, free-surface deformation by various centrifuge degrees can be systematically examined using this apparatus. In the system, a small unit consisting of a camera and computer is rotated with the granular sample to record the rotation-induced deformation. To evaluate the validity of the system, deformation of a rotated water surface is first measured and analyzed. The obtained data are properly explained by the theoretical parabolas without any fitting parameter. Next, we measure the deformation of non-cohesive and cohesive granular piles using the developed apparatus. Both granular samples show the significant deformation of granular pile and finally develop steep granular slopes on the side walls. However, details of the deformation processes depend on the cohesion strength. To quantitatively characterize the difference, the effective strength by cohesion and granular local-slope variations are analyzed based on the experimental results.

cond-mat.soft↗

arXiv2020

Force chain structure in a rod-withdrawn granular layer

When a rod is vertically withdrawn from a granular layer, oblique force chains can be developed by effective shearing. In this study, the force-chain structure in a rod-withdrawn granular layer was experimentally investigated using a photoelastic technique. The rod is vertically withdrawn from a two-dimensional granular layer consisting of bidisperse photoelastic disks. During the withdrawal, the development process of force chains is visualized by the photoelastic effect. By systematic analysis of photoelastic images, force chain structures newly developed by the rod withdrawing are identified and analyzed. In particular, the relation between the rod-withdrawing force $F_\mathrm{w}$, total force-chains force $F_\mathrm{t}$, and their average orientation $θ$ are discussed. We find that the oblique force chains are newly developed by withdrawing. The force-chain angle $θ$ is almost constant (approximately $20^{\circ}$ from the horizontal), and the total force $F_\mathrm{t}$ gradually increases by the withdrawal. In addition, $F_\mathrm{t}\sinθ$ shows a clear correlation with $F_\mathrm{w}$.

cond-mat.soft↗

arXiv2019

Scaling laws for the oblique impact cratering on an inclined granular surface

Although a large number of astronomical craters are actually produced by the oblique impacts onto inclined surfaces, most of the laboratory experiments mimicking the impact cratering have been performed by the vertical impact onto a horizontal target surface. In previous studies on the effects of oblique impact and inclined terrain, only one of the impact angle $φ$ or target inclination angle $θ$ has been varied in the experiments. Therefore, we perform impact-cratering experiments by systematically varying both $φ$ and $θ$. A solid projectile of diameter $D_{\rm i}=6$~mm is impacted onto a sand surface with the range of impact velocity $v_{\rm i}=7$--$97$~m~s$^{-1}$. From the experimental result, we develop scaling laws for the crater dimensions on the basis of $Π$-group scaling. As a result, the crater dimensions such as cavity volume, diameter, aspect ratio, and depth-diameter ratio can be scaled by the factors $\sin φ$ and $\cos θ$ as well as the usual impact parameters ($v_{\rm i}$, $D_{\rm i}$, density of projectile, and surface gravity). Finally, we consider the possible application of the obtained scaling laws to the estimate of impact conditions (e.g., impact speed and impact angle) in natural crater records.

astro-ph.EP↗

arXiv2019

Bouncing of a projectile impacting a dense potato-starch suspension layer

When a solid projectile is dropped onto a dense non-Brownian-particle suspension, the action of an extremely large resistance force on the projectile results in its drastic deceleration, followed by a rebound. In this study, we perform a set of simple experiments of dropping a solid-projectile impact onto a dense potato-starch suspension. From the kinematic data of the projectile motion, the restitution coefficient and timescale of the rebound are measured. By assuming linear viscoelasticity, the effective transient elasticity and viscosity can be estimated. We additionally estimate the Stokes viscosity on a longer timescale by measuring the slow sinking time of the projectile. The estimated elastic modulus and viscosity are consistent with separately measured previous results. In addition, the effect of mechanical vibration on the viscoelasticity is examined. As a result, we find that the viscoelasticity of the impacted dense suspension is not significantly affected by the mechanical vibration.

cond-mat.soft↗

arXiv2018

Impact-induced collapse of an inclined wet granular layer

The collapse of an inclined cohesive granular layer triggered by a certain perturbation can be a model for not only landslides on Earth but also relaxations of asteroidal surface terrains. To understand such terrain dynamics, we conduct a series of experiments of a solid-projectile impact onto an inclined wet granular layer with various water contents and inclination angles. As a result, we find two types of outcomes: "crater formation" and "collapse". The "collapse" phase is observed when the inclination angle is close to the maximum stable angle and the impact-induced vibration at the bottom of wet granular layer is sufficiently strong. To explain the collapse condition, we propose a simple block model considering the maximum stable angle, inclination angle, and impact-induced vibrational acceleration. Additionally, the attenuating propagation of the impact-induced vibrational acceleration is estimated on the basis of three-dimensional numerical simulations with discrete element method using dry particles. By combining wet-granular experiments and dry-granular simulations, we find that the impact-induced acceleration attenuates anisotropically in space. With a help of this attenuation form, the physical conditions to induce the collapse can be estimated using the block model.

cond-mat.soft↗

arXiv2018

Shape dependence of resistance force exerted on an obstacle placed in a gravity-driven granular silo flow

Resistance force exerted on an obstacle in a gravity-driven slow granular silo flow is studied by experiments and numerical simulations. In a two-dimensional granular silo, an obstacle is placed just above the exit. Then, steady discharge flow is made and its flow rate can be controlled by the width of exit and the position of obstacle. During the discharge of particles, flow rate and resistance force exerting on the obstacle are measured. Using the obtained data, a dimensionless number characterizing the force balance in granular flow is defined by the relation between the discharge flow rate and resistance-force decreasing rate. The dimensionless number is independent of flow rate. Rather, we find the weak shape dependence of the dimensionless number. This tendency is a unique feature for the resistance force in granular silo flow. It characterizes the effective flow width interacting with the obstacle in granular silo flow.

cond-mat.soft↗

arXiv2017

The Physics of Protoplanetesimal Dust Agglomerates. X. Mechanical properties of dust aggregates probed by a solid-projectile impact

Dynamic characterization of mechanical properties of dust aggregates has been one of the most important problems to quantitatively discuss the dust growth in protoplanetary disks. We experimentally investigate the dynamic properties of dust aggregates by low-speed ($\lesssim 3.2$ m s$^{-1}$) impacts of solid projectiles. Spherical impactors made of glass, steel, or lead are dropped onto a dust aggregate of packing fraction $ϕ=0.35$ under vacuum conditions. The impact results in cratering or fragmentation of the dust aggregate, depending on the impact energy. The crater shape can be approximated by a spherical segment and no ejecta are observed. To understand the underlying physics of impacts into dust aggregates, the motion of the solid projectile is acquired by a high-speed camera. Using the obtained position data of the impactor, we analyze the drag-force law and dynamic pressure induced by the impact. We find that there are two characteristic strengths. One is defined by the ratio between impact energy and crater volume and is $\simeq 120$ kPa. The other strength indicates the fragmentation threshold of dynamic pressure and is $\simeq 10$ kPa. The former characterizes the apparent plastic deformation and is consistent with the drag force responsible for impactor deceleration. The latter corresponds to the dynamic tensile strength to create cracks. Using these results, a simple model for the compaction and fragmentation threshold of dust aggregates is proposed. In addition, the comparison of drag-force laws for dust aggregates and loose granular matter reveals the similarities and differences between the two materials.

astro-ph.EP↗

arXiv2017

Obstacle-shape effect in a two-dimensional granular silo flow field

We conducted simple experiment and numerical simulation of two-dimensional granular discharge flow driven by gravity under the influence of an obstacle. According to the previous work (Zuriguel {\it et al.,\,Phys.\,Rev.\,Lett.}\,{\bf 107}: 278001, 2011), the clogging of granular discharge flow can be suppressed by putting a circular obstacle at a proper position. In order to investigate the details of obstacle effect in granular flow, we focused on particle dynamics in this study. From the experimental and numerical data, we found that the obstacle remarkably affects the horizontal-velocity distribution and packing fraction at the vicinity of the exit. In addition to the circular obstacle, we utilized triangular, inverted-triangular, and horizontal-bar obstacles to discuss the obstacle-shape effect in granular discharge flow. Based on the investigation of dynamical quantities such as velocity distributions, granular temperature, and volume fraction, we found that the triangular obstacle or horizontal bar could be very effective to prevent the clogging. From the obtained result, we consider that the detouring of particles around the obstacle and resultant low packing fraction at the exit region effectively prevent the clogging in a certain class of granular discharge flow.

cond-mat.soft↗

arXiv2016

Statistical properties of gravity-driven granular discharge flow under the influence of an obstacle

Two-dimensional granular discharge flow driven by gravity under the influence of an obstacle is experimentally investigated. A horizontal exit of width $W$ is opened at the bottom of vertical Hele-Shaw cell filled with stainless-steel particles to start the discharge flow. In this experiment, a circular obstacle is placed in front of the exit. Thus, the distance between the exit and obstacle $L$ is also an important parameter. During the discharge, granular-flow state is acquired by a high-speed camera. The bulk discharge-flow rate is also measured by load cell sensors. The obtained high-speed-image data are analyzed to clarify the particle-level granular-flow dynamics. Using the measured data, we find that the obstacle above the exit affects the granular-flow field. Specifically, the existence of obstacle results in large horizontal granular temperature and small packing fraction. This tendency becomes significant when $L$ is smaller than approximately 6$D_g$ when $W \simeq 4 D_g$, where $D_g$ is diameter of particles.

cond-mat.soft↗