arXiv subjects

Kazusa Beppu

Publications and source records attributed to Kazusa Beppu.

9 recordsLinked to original sources

arXiv2026

Directional Symmetry Breaking of Spherical Active Colloids by Magnetoviscous Coupling

Harnessing active matter calls for strategies that break the directional symmetry of self-propelled motion without altering the propulsion mechanism itself. Here, we show that magnetically inert spherical active colloids can be steered through the anisotropic viscous response of a ferrofluid under a uniform magnetic field. Self-propelled Janus colloids exhibit robust cross-field motion transverse to the magnetic field, although the applied magnetic field directly controls neither the particles nor their propulsion speed. Quantitative measurements reveal an emergent reorientation torque that grows with both propulsion speed and magnetic field strength. A squirmer model in a magnetoviscous medium captures these observations and shows that the torque arises from the coupling between swimmer-generated flow and anisotropic rotational viscosity. Our findings establish a hydrodynamic basis for converting viscous dissipation into directional symmetry breaking through anisotropic rheology, providing a route to field-controlled material transport by active matter.

cond-mat.soft↗

arXiv2025

Boundary-shape driven transitions in vortex and oscillatory dynamics of confined epithelial cells

Controlling the collective motion of epithelial cell populations is fundamental for understanding multicellular self-organization and for advancing tissue engineering. Under spatial confinement, cells are known to exhibit either vortex rotation or oscillatory motion depending on boundary geometry, but the mechanisms governing transitions between these states remain unclear. Here, we investigated the collective motion of MDCK cells confined within a doublet circular boundary, where the confinement aspect ratio, defined as the distance between the centers of two circles relative to their radius, can be tuned by varying the degree of overlap. When the overlap is large, cells form a stable vortex. Increasing the confinement aspect ratio destabilizes this vortex and induces oscillatory motion characterized by periodic reversals of migration direction, before ultimately transitioning into disordered dynamics. To elucidate the underlying mechanism, we developed simulations of self-propelled particles incorporating local alignment (LA) and contact inhibition of locomotion (CIL). The model successfully reproduced the experimentally observed transitions from vortices to oscillatory motion and further revealed that an appropriate balance between LA and CIL is critical for stabilizing vortex pairs with velocity reversals. Our findings demonstrate that the confinement aspect ratio serves as a minimal control parameter governing transitions in the collective dynamics of epithelial monolayers.

cond-mat.soft↗

arXiv2024

Geometric frustration and pairing order transition in confined bacterial vortices

Dense systems of active matter exhibit highly dynamic collective motion characterized by intermingled vortices, referred to as active turbulence. The interaction between these vortices is key to controlling turbulent dynamics, and a promising approach for revealing the rules governing their interaction is geometric confinement. In this study, we investigate the vortex-pairing patterns in confined bacterial suspensions as a model frustrated system in which a perfect antiferromagnetic state is prohibited. We found that three-body vortex interactions exhibited anomalous pairing-order transition from co-rotational vortex-pairing to counter-rotating patterns with frustration. Although an active matter system is in non-equilibrium, our theory based on bending energy accounts for significant features including pattern transition and shift of the transition point in frustrated systems. Moreover, the interplay between the chirality in collective motion and frustration in vortex pairing creates a collective rotational flow under the broad geometric conditions of a confined space. Our results show that frustrated vortex patterning promotes a geometric approach for arranging active turbulence in microfluidic systems.

cond-mat.soft↗

arXiv2023

Geometry-Induced Dynamics of Confined Chiral Active Matter

Controlling the motion of active matter is a central issue that has recently garnered significant attention in fields ranging from non-equilibrium physics to chemical engineering and biology. Distinct methods for controlling active matter have been developed, and physical confinement to limited space and active matter with broken rotational symmetry (chirality) are two prominent mechanisms. However, the interplay between pattern formation due to physical constraints and the ordering by chiral motion needs to be better understood. In this study, we conduct numerical simulations of chiral self-propelled particles under circular boundary confinement. The collective motion of confined self-propelled particles can take drastically different forms depending on their chirality. The balance of orientation changes between particle interaction and the boundary wall is essential for generating ordered collective motion. Our results clarify the role of the steric boundary effect in controlling chiral active matter.

cond-mat.soft↗

arXiv2023

Magnetic control of orientational order and intrinsic hydrodynamic instability in bacterial turbulence

Highly concentrated active agents tend to exhibit turbulent flows, reminiscent of classical hydrodynamic turbulence, which has attracted considerable attention lately. Controlling the so-called active turbulence has long been a challenge, and the influence of external fields on such chaotic self-organization remains largely unexplored. Here we report on active turbulence of Bacillus subtilis bacteria controlled by a uniform magnetic field via a magnetizable medium based on magnetic nanoparticles. The rod-shaped bacteria act as non-magnetic voids in the otherwise magnetic medium, allowing magnetic torques to be generated on their bodies. This leads to an externally controllable nematic alignment constraint that further controls bacterial turbulence into a nematic state. The nematic orientational ordering in the direction parallel to the magnetic field is accompanied by transverse flows owing to active stress by dipole pushers, which induce undulation of the nematic state. Remarkably, the typical length of the undulation is almost independent of the magnetic field strength. Our theoretical model based on the hydrodynamic equations for suspensions of self-propelled particles predicts the intrinsic length scale of hydrodynamic instability independent of the magnetic field. Our findings suggest that magnetic torques are a powerful approach for controlling both individual agents and their collective states in active systems.

cond-mat.soft↗

arXiv2023

Controlling topological defects and contractile flow in confined nematic cell population

Topological defects in nematically aligned cell populations play a critical role in modulating collective motion, from microbial colonies to epithelial tissues. Despite the potential of manipulating such topological defects to control diverse self-organized structures and collective dynamics, defect manipulation in active matter remains an challenging area of research. In this study, we investigated the geometric control of defect positioning and alignment in a nematic cell population by imposing spatial constraints consisting of two or three overlapping circular boundaries. The confined cell population exhibited an ordered pairing of half-integer topological defects that remained stable even when the size of the spatial constraint was altered using geometric parameters. These defects also elicited robust contractile flow that induced a negative divergence in the velocity field of collective motion. Such net contractile flow can contribute to mechanical stimulation on confined cells, as evidenced by the stretched cell nucleus. Our geometry-based approach paves the way for controlling defect pairing, providing a deeper understanding of the interplay among geometry, topology, and collective dynamics.

cond-mat.soft↗

arXiv2021

Tailoring collective motion of kinesin-driven microtubules via topographic landscapes

Biomolecular motor proteins that generate forces by consuming chemical energy obtained from ATP hydrolysis are pivotal for organizing broad cytoskeletal structures in living cells. The control of such cytoskeletal structures benefits programmable protein patterning; however, our current knowledge is limited owing to the underdevelopment of an engineering approach for controlling pattern formation. Here, we demonstrate the tailoring of assembled patterns of microtubules (MTs) driven by kinesin motors by designing the boundary shape in fabricated microwells. We found an MT bundle structure along the microwell wall and a bridging structure perpendicular to the wall. Corroborated by the theory of self-propelled rods, we further showed that the alignment of MTs defined by the boundary shape determined the transition of the assembled patterns, providing a blueprint to reconstruct bridge structures in microchannels. Our findings provide a geometric rule to tailor the self-organization of cytoskeletons and motor proteins for nanotechnological applications.

cond-mat.soft↗

arXiv2020

Edge Current and Pairing Order Transition in Chiral Bacterial Vortex

We report the selective stabilization of chiral rotational direction of bacterial vortices, from turbulent bacterial suspension, in achiral circular microwells sealed by an oil-water interface. This broken-symmetry, originating from the intrinsic chirality of bacterial swimming near hydrodynamically different top and bottom surfaces, generates a chiral edge current of bacteria at lateral boundary and grows stronger as bacterial density increases. We demonstrate that chiral edge current favors co-rotational configurations of interacting vortices, enhancing their ordering. The interplay between the intrinsic chirality of bacteria and the geometric properties of the boundary is a key-feature for the pairing order transition of active turbulence.

cond-mat.soft↗

arXiv2017

Geometry-driven collective ordering of bacterial vortices

Controlling the phases of matter is a challenge that spans from condensed materials to biological systems. Here, by imposing a geometric boundary condition, we study controlled collective motion of Escherichia coli bacteria. A circular microwell isolates a rectified vortex from disordered vortices masked in bulk. For a doublet of microwells, two vortices emerge but their spinning directions show transition from parallel to anti-parallel. A Vicsek-like model for confined self-propelled particles gives the point where two spinning patterns occur in equal probability and one geometric quantity governs the transition as seen in experiments. This mechanism shapes rich patterns including chiral configurations in a quadruplet of microwells, thus revealing a design principle of active vortices.

cond-mat.soft↗