# gst: the effect of noise on the dynamics of microswimmers in externally-driven fluid flows.

AA << have quantified the effect of noise on swimmer dynamics in a steady, two-dimensional hyperbolic fluid flow. In such a flow, swimmers are ultimately forced to escape to the left or the right, with their transient dynamics near the passive unstable fixed point determining which way they go. >>

<< Without noise, a swimmer’s fate is sealed based on its position relative to the SwIM (swimming invariant manifolds) in the xθ phase space. With noise, the swimmer’s motion is a stochastic process. >>

AA << calculated the steady-state orientation distributions of diffusive, run-and-tumble, or mixed swimmers in the hyperbolic flow. The fluctuations give some swimmers greater opportunity to cross the SwIM and exit on the opposite side than they would have without noise. There is however a maximal distance that swimmers can get on either side of the passive fixed point and still be able to swim back to the other side—this is where the stable BIMs (burning invariant manifolds) block inward swimming particles. >>

<< Fluctuations make it increasingly likely that a swimmer close to one of these BIMs does indeed end up crossing it, causing irreversible changes to the fluctuating swimmers’ trajectories (assuming negligible translational diffusion).  >>️

Simon A. Berman, Kevin A. Mitchell. Swimmer dynamics in externally-driven fluid flows: The role of noise. arXiv: 2108.10488v1 [physics.flu-dyn]. Aug 24, 2021.

https://arxiv.org/abs/2108.10488

keywords: gst, swimmer, swimming particle, fluid dynamics, chaotic dynamics, rotational diffusion, random fluctuation, tumbling, noise

# gst: transitional dynamics among gyrotactic (prolate spheroid) swimmers in turbulence

<< In this study, (AA) consider small, elongated, gyrotactic, swimming particles in homogenous isotropic turbulence (..). Many motile phytoplankton species are gyrotactic, i.e., their swimming direction results from the competition between shear-induced viscous torque and the stabilizing torque due to bottom-heaviness. (..)  Moreover, both single phytoplankton cells and multicellular phytoplankton chains can have elongated shapes, which makes the study of gyrotactic active particles with prolate shapes necessary. >>

AA addressed the following problems:

<< (i) How the clustering is affected by the swimming number Φ and the stability number Ψ? In particular, how to characterize the extent of clustering based on the three-dimensional Voronoi analysis?

(ii) How the clustering is related to the flow structures? This question has two perspectives.  From the space perspective, what are the regions that particles accumulate in? From the time perspective, how long do particles exist in an aggregated state? >>

Zehua Liu, Linfeng Jiang, Chao Sun. Accumulation and alignment of elongated gyrotactic swimmers in turbulence. arXiv:2202.04351v1 [physics.flu-dyn]. Feb 9, 2022.

https://arxiv.org/abs/2202.04351

Also

The effect of noise on the dynamics of microswimmers in externally-driven fluid flows.  

https://flashontrack.blogspot.com/2021/10/gst-effect-of-noise-on-dynamics-of.html

keywords 'turbulence' in FonT   

https://flashontrack.blogspot.com/search?q=turbulence

keywords 'turbolento' in Notes (quasi-stochastic poetry)

https://inkpi.blogspot.com/search?q=turbolento

keywords: transition, swimmers,  gyrotactic swimmers, spheroids,  prolate spheroids, stability, turbulence

# gst: flocking transitions of unfriendly species.

AA consider << two kinds of self-propelled particles, A and B, that tend to align with particles from the same species and to antialign with the other. The model shows a flocking transition (..) it has a liquid-gas phase transition and displays micro-phase-separation in the coexistence region where multiple dense liquid bands propagate in a gaseous background. >>

<< The interesting features (..) are the existence of two kinds of bands, one composed of mainly A particles and one mainly of B particles, the appearance of two dynamical states in the coexistence region: the PF (parallel flocking) state in which all bands of the two species propagate in the same direction, and the APF (antiparallel flocking) state in which the bands of species A and species B move in opposite directions. When PF and APF states exist in the low-density part of the coexistence region they perform stochastic transitions from one to the other. The system size dependence of the transition frequency and dwell times show a pronounced crossover that is determined by the ratio of the band width and the longitudinal system size. >>

AA << work paves the way for studying multispecies flocking models with heterogeneous alignment interactions. >>

Swarnajit Chatterjee, Matthieu Mangeat, et al. Flocking of two unfriendly species: The two-species Vicsek model. Phys. Rev. E 107, 024607. Feb 14, 2023

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.107.024607 

Also

keyword 'swimmers' in FonT

https://flashontrack.blogspot.com/search?q=swimmers

Keywords: gst, flocking, particles, self-propelled particles, swimmers,  microswimmers

# GST: the behavior of motile active self-propelling matter

<< From starling aberrations to self-turbulent fluids, 'active systems' encompass a wide family of phenomena in which individual objects propel themselves forward, allowing them to display intriguing collective behaviors. (..) these objects collectively spend most of their time in one of two states, between which some intriguing behaviors can emerge. (..) these collective behaviors are determined by the ability of the self-propelling objects to swim upwards against the gravitational force, and their degrees of bottom-heaviness. For lower values of these quantities, groups of swimmers will sink to the bottom of their container just like inactive dust grains; but higher values will instead collect at the top. In between these states, smaller clusters of swimmers group at the bottom, which are fed by plumes of sinking particles. Also, porous clusters of swimmers can form, which allow individual particles to escape. >>

'Bottom-heavy squirmers' adopt characteristic group behaviours. Springer. May 28, 2020.

https://phys.org/news/2020-05-bottom-heavy-squirmers-characteristic-group-behaviours.html 

Ruhle F., Stark H. Emergent collective dynamics of bottom-heavy squirmers under gravity. Eur. Phys. J. E 43, 26 (2020). doi: 10.1140/epje/ i2020-11949-8. May 25, 2020.

https://link.springer.com/article/10.1140/epje/i2020-11949-8