# gst: behavior of microswimmers in a vortex with translational and rotational noise

AA << propose a theoretical model to investigate the dynamics of elongated microswimmers in elementary vortices, namely active particles in two- and three-dimensional rotlets. A deterministic model first reveals the existence of bounded orbits near the centre of the vortex and unbounded orbits elsewhere. (AA) further discover a conserved quantity of motion that allows (..) to map the phase space according to the type of the orbit (bounded vs unbounded). (They) next introduce translational and rotational noise into the system. Using a Fokker--Planck formalism, (AA) quantify the quality of trapping near the centre of the vortex by examining the probability of escape and the mean time of escape from the region of deterministically bounded orbits. (AA) finally show how to use these findings to formulate a prediction for the radius of the depletion zone, which compares favourably with the experiments of Sokolov and Aranson (2016). >>

Ivan Tanasijevic, Eric Lauga. Microswimmers in vortices: Dynamics and trapping. arXiv: 2211.05866v1 [physics.bio-ph].  Nov 10, 2022. 

https://arxiv.org/abs/2211.05866

Also

'microswimmers' in FonT 

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

Keywords: gst, behav, translation,  rotation, trapping, noise, swimmer, swimming,  microswimmers, fluid dynamics, vortex, vortices, vortexes, vorticity

# 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: 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: ️apropos of spontaneous active matter, the active droploids.

<< Active matter comprises self-driven units, such as bacteria and synthetic microswimmers, that can spontaneously form complex patterns and assemble into functional microdevices. These processes are possible thanks to the out-of-equilibrium nature of active-matter systems, fueled by a one-way free-energy flow from the environment into the system. Here, (AA) take the next step in the evolution of active matter by realizing a two-way coupling between active particles and their environment, where active particles act back on the environment giving rise to the formation of superstructures. >>️

<< These structures hinge on mutually coupled structure formation processes of the colloids, which form an engine, and the surrounding solvent, which phase separates in regions of high colloidal density and encapsulates the engine within a droplet shell.  >>

️

Jens Grauer, Falko Schmidt, et al. Active droploids. arXiv:2109.10677v1 [cond-mat.soft]. Sep 22, 2021.

https://arxiv.org/abs/2109.10677

Also

keyword 'drop' | 'droplet' in FonT

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

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

keyword 'goccia' in Notes (quasi-stochastic poetry): 

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

keywords: gst, drops, droplets, colloids, active matter, active droploids, self-assembly, solitons.

# 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: "diffusing wave paradox", synthetic microswimmers can mimic the behavior of Amoeba

<< Amoeba are unusual creatures that form when a dispersed population of cells spontaneously comes together and reorganizes itself into a multicellular macroscopic organism. To do this, a few leader cells emit chemical pulses that cause the other individual cells to move in the direction opposite to that of the traveling pulses (the "diffusing wave paradox," ), leading to the formation of dense clusters. >>

<< In experiments, the researchers used spherical particles that are half-coated by a carbon cap and placed in a viscous liquid. When illuminated by light, the particles propel themselves forward with the cap in front. >>

<< At low pulse speeds, the particles have enough time to reorient themselves, if needed, so that their caps are facing in the same direction as that of the traveling pulses. This orientation ensures that the particles travel in the same direction as the pulses. >>

<< At high pulse speeds, on the other hand, the pulses come too quickly for the particles to reorient themselves before the next one comes. This is because the speed of the particles' rotation is limited by the friction of the viscous liquid. So if the particles' caps are initially facing the oncoming pulses, the particles will move counter to the direction of the traveling pulses, resembling the behavior of amoeba in the diffusing wave paradox. >>

Lisa Zyga. Diffusing wave paradox may be used to design micro-robotics. Jun 12, 2019.

https://m.phys.org/news/2019-06-diffusing-paradox-micro-robotics.html

Celia Lozano, Clemens Bechinger. Diffusing wave paradox of phototactic particles in traveling light pulses. Nature Communications 10, Article number: 2495.  Jun 7, 2019.

https://www.nature.com/articles/s41467-019-10535-z