arXiv subjects

Kojiro Otoguro

Publications and source records attributed to Kojiro Otoguro.

5 recordsLinked to original sources

arXiv2026

A behavioral principle underlying attacker-defender interactions in soccer

Soccer is widely popular for its simple rules and complex yet coordinated play that unfolds on the pitch. Nevertheless, the fundamental mechanisms governing such play are not well understood: what shapes player interactions on the pitch? What short-term goals guide players' decisions about their movements over the next few seconds? We address these questions by focusing on one-on-one settings in open play, in which the attacker, in possession of the ball and typically dribbling, faces a defender aiming to stop or delay the attacker's actions over a short period. Here we develop a mathematical model of attacker-defender interactions and analyze 306 professional soccer games. Synthesizing the large-scale dataset with an analysis of the model reveals a simple behavioral principle that may underlie these interactions: the defender seeks to minimize their future relative speed to the attacker, whereas the attacker initiates their movements to preempt the defender's objective. This principle, relative-speed minimization, provides a consistent and unified account of the empirical data. Since our framework depends little on soccer-specific details, this principle may govern diverse pursuit-evasion scenarios as well as other invasion team sports.

physics.soc-ph↗

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↗

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↗

arXiv2023

Precipitation induced filament pattern of injected fluid controlled by structured cell

Mixing of two fluids can lead to the formation of a precipitate. If one of the fluids is injected into a confined space filled with the other, a created precipitate disrupts the flow locally and forms complex spatiotemporal patterns. The relevance of controlling these patterns has been highlighted in the engineering and geological contexts. Here, we show that such injection patterns can be controlled consistently by injection rate and obstacles. Our experimental results revealed filament patterns for high injection and low reaction rates, and the injection rate can control the number of active filaments. Furthermore, appropriately spaced obstacles in the cells can straighten the motion of the advancing tip of the filament. A mathematical model based on a moving boundary adopting the effect of precipitation reproduced the phase diagram and the straight motion of filaments in structured cells. Our study clarifies the impact of the nonlinear permeability response on the precipitate density and that of the obstacles in the surrounding medium on the motion of the injected fluid with precipitation.

cond-mat.soft↗