# gst: from fluttering to drifting: inertialess sedimentation of an achiral particle.

<< ️There has been much recent interest in the chiral motion of achiral particles that sediment in a viscous fluid in a regime where inertial effects can be neglected. This occurs in a broad range of applications such as those involving biological objects like algae, ultrathin graphene flakes, or colloidal suspensions. It is known that particles with two planes of symmetry can be categorized as “settlers,” “drifters,” or “flutterers,” where the latter sediment along chiral trajectories despite their achiral shapes. >>

<< ️Previous work investigated the sedimentation of circular disks bent into a U-shape and identified them as “flutterers.” In this work (AA) analyze the change in behavior of such particles when (They) break one of their symmetries by pinching the disks isometrically along their axis, a shape change that can arise during the sedimentation of thin elastic sheets. The “fluttering” behavior is found to be robust to such shape changes, with the trajectories now evolving toward helical paths. However, the behavior changes when the degree of pinching becomes too strong, at which point the particles become “drifters” which sediment steadily without rotation. >>

<< ️(AA) establish criteria for the transition between the two types of behavior and confirm (Their) predictions in experiments. Finally, (AA) discuss the implications of (Their) observations for the dispersion of dilute suspensions made of such particles. >>

Christian Vaquero-Stainer, Tymoteusz Miara, Anne Juel, et al. From fluttering to drifting: Inertialess sedimentation of an achiral particle. Phys. Rev. Fluids 11, 034102. March 10, 2026.

https://journals.aps.org/prfluids/abstract/10.1103/scj8-6lpf

Also: particle, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, behaviors, particles, achiral particles, settlers, drifters, flutterers.

# gst: settling dynamics of an oloid, experiments and simulations.

<< ️This (AA) study presents a combined experimental and computational investigation of an oloid shaped particle settling in a quiescent fluid. The oloid, a unique convex shape with anisotropic geometry, provides a distinctive model for exploring how a particle's shape and orientation affect its settling dynamics. >>

<< ️(AA) results indicate two distinct falling modes for the oloid, separated by Galileo number. The stable mode is characterised by a preferential orientation, with a rotation around the vertical axis, whereas the tumbling mode has randomly distributed orientation and rotation statistics. (They) characterise the falling velocity, orientation, and rotation dynamics of the oloids over a range of Galileo numbers. Additionally, the influence of the initial orientation is revealed to determine the rotation dynamics at low Galileo numbers. >>

Mees M. Flapper, Giulia Piumini, Roberto Verzicco, et al. Settling dynamics of an oloid: experiments and simulations. arXiv: 2511.05137v1 [physics.flu-dyn]. Nov 7, 2025.

https://arxiv.org/abs/2511.05137

Also: particle, transition, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, particle, transitions, oloids, oloid shaped particles, multiple falling regimes, falling mode, tumbling mode.

# gst: stochastic surfing turbulent vorticity.

<< The chaotic dynamics of small-scale vorticity plays a key role in understanding and controlling turbulence, with direct implications for energy transfer, mixing, and coherent structure evolution. >>

Here AA << use a combination of experiments, theory and simulations to show that small magnetic particles of different densities, exploring flow regions of distinct vorticity statistics, can act as effective probes for measuring and forcing turbulence at its smallest scale. The interplay between the magnetic torque, from an externally controllable magnetic field, and hydrodynamic stresses, from small-scale turbulent vorticity, reveals an extremely rich phenomenology. >>

Notably, AA << present the first observation of stochastic resonance for particles in turbulence: turbulent fluctuations, effectively acting as noise, counterintuitively enhance the particle rotational response to external forcing. (They) identify a pronounced resonant peak in particle rotational phase-lag when the applied magnetic field matches the characteristic intensity of small-scale vortices. >>

<< Furthermore, (They) uncover a novel symmetry-breaking mechanism: an oscillating magnetic field with zero-mean angular velocity remarkably induces net particle rotation in turbulence with zero-mean vorticity, as turbulent fluctuations aid the particle in "surfing" the magnetic field. >>

Ziqi Wang, Xander M. de Wit, et al. Stochastic surfing turbulent vorticity. arXiv: 2504.08346v1 [physics.flu-dyn]. Apr 11, 2025. 

https://arxiv.org/abs/2504.08346

Also: vortex, turbulence, noise, transition, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, vortices, turbulence, turbulent fluctuations, small-scale turbulent vorticity, stochastic resonance, noise, transitions 

# gst: switching from active Brownian motion to stationary rotation of Janus particles in a viscoelastic fluid.

<< Swimming micro-objects exist in viscoelastic fluids. Elucidating the effect of viscoelasticity on the motion of these objects is important for understanding their behavior. >>

AA << examined the motion of Janus particles self-propelled by induced charge electrophoresis over a wide range of speeds in semidilute polymer solutions. In (Their) system, the motion of Janus particles changed from active Brownian motion to stationary rotation as the speed increased. The torque for stationary rotation originates from the difference between the direction of self-propulsion and that of the time-delayed restoring force from the polymer solution, which has been reported in another self-propelled particle system. The switch from active Brownian motion to stationary rotation at different polymer concentrations can be explained by the Weisenberg number, which is defined as the ratio of the relaxation time of the polymer network to the travel time of the Janus particle to its size. >>

Keita Saito, Ryunosuke Kawano, et al. Self-propelled motion of induced-charge electrophoretic Janus particles in viscoelastic fluids. Phys. Rev. E 111, 045409. Apr 10, 2025.

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.111.045409

️

Also: Janus, transition, particle, in FonT:

Janus https://flashontrack.blogspot.com/search?q=janus 

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

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

Keywords: gst, Janus, transitions, particles, self-propelled particles

# gst: multiparticle dispersion in rotating-stratified turbulent flows (when stratification increases turbulent fluctuations may not be weaker)

AA << study the relative movement of groups of two (pairs) and four (tetrahedra) Lagrangian particles using direct numerical simulations of the stably stratified Boussinsesq equations, with Brunt-Väisälä frequency 𝑁 and Coriolis parameter 𝑓. >>

<< In all cases considered, (AA) demonstrate that the relative particle motion differs depending on whether dispersion is considered forward or backward in time, although the asymmetry becomes less pronounced when stratification and rotation increase. On the other hand, the strong fluctuations in the dispersion between two particles become more extreme when 𝑁 and 𝑓  increase. (They) also find evidence for the formation of shear layers, which become more pronounced as 𝑁 and 𝑓  become larger. Finally, (They) show that the irreversibility on the dispersion of a set of particles initially forming a regular tetrahedron becomes weaker when the influence of stratification and rotation increases, a property that (They) relate to that of the perceived rate-of-strain tensor. >>️

<< Unexpectedly, (AA) observe that the higher moments of particle separation, in particular the normalized fourth-order central moment of the separation (the kurtosis Kr) is an increasing function of stratification and rotation. This is surprising, as when stratification increases the turbulent fluctuations are expected to be weaker, (..) >>️

Sebastian Gallon, Fabio Feraco, et al. Multiparticle dispersion in rotating-stratified turbulent flows. Phys. Rev. Fluids 10, 034605. Mar 17, 2025. 

https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.10.034605

Also: particle, turbulence, disorder & fluctuations, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, particle, turbulence, disorder, fluctuations

# gst: transition to inverse cascade in turbulent rotating convection in absence of the large-scale vortex.

<< Turbulent convection under strong rotation can develop an inverse cascade of kinetic energy from smaller to larger scales. In the absence of an effective dissipation mechanism at the large scales, this leads to the pile-up of kinetic energy at the largest available scale, yielding a system-wide large-scale vortex (LSV). Earlier works have shown that the transition into this state is abrupt and discontinuous. >>

Here, AA << study the transition to the inverse cascade in the case where the inverse energy flux is dissipated before it reaches the system scale, suppressing the LSV formation. (They) demonstrate how this can be achieved in direct numerical simulations by using an adapted form of hypoviscosity on the horizontal manifold. (They) find that in the absence of the LSV, the transition to the inverse cascade becomes continuous. This shows that it is the interaction between the LSV and the background turbulence that is responsible for the observed discontinuity. >>

AA << furthermore show that the inverse cascade in absence of the LSV has a more local signature compared to the case with LSV. >>️

Xander M. de Wit. Transition to inverse cascade in turbulent rotating convection in absence of the large-scale vortex. arXiv: 2502.16275v1 [physics.flu-dyn]. Feb 22, 2025. 

https://arxiv.org/abs/2502.16275

Also: turbulence, dissipation, transition, vortex, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, turbulence, dissipation, transitions, vortices

# life: spontaneous emergence of run-and-tumble-like dynamics in coupled self-propelled bots.

<< Drawing inspiration from the motility behaviour of microorganisms, we introduce a highly tunable, robotic system self-actuating into the run-and-tumble (RT)-like motion. It comprises two disk-shaped, centimeter-scale programmable robots individually programmed to perform overdamped active Brownian (AB) motion and connected by a rigid rod. The rod is attached to pivot points located on off-centered, mirror-symmetric points on each robot, allowing for its free rotation at both ends. >>

AA << show that the collective dynamics of this system execute RT-like motion with characteristic sharp tumble events and exponentially distributed run times, similar to those observed in microorganisms. (They) further quantify emerging dynamics in terms of tumbling frequency and tune it over a wide range of experimental parameters. >>

AA << also develop a theoretical model that reproduces our experimental results and elucidates the underlying physical mechanisms governing the rich phase behavior of RT motion. >>

 ️

Somnath Paramanick, Umashankar Pardhi, et al. Spontaneous emergence of run-and-tumble-like dynamics in coupled self-propelled robots: experiment and theory. arXiv: 2502.01257v1 [cond-mat.soft]. Feb 3, 2025.

https://arxiv.org/abs/2502.01257

Also: behav, evolution, bot, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, life, behavior, evolution, bots, self-propelled bots, run-and-tumble motion 

# gst: phase transitions in anisotropic turbulence.

<<  

Turbulence is a widely observed state of fluid flows, characterized by complex, nonlinear interactions between motions across a broad spectrum of length and time scales. While turbulence is ubiquitous, from teacups to planetary atmospheres, oceans and stars, its manifestations can vary considerably between different physical systems. For instance, three-dimensional (3D) turbulent flows display a forward energy cascade from large to small scales, while in two-dimensional (2D) turbulence, energy cascades from small to large scales. In a given physical system, a transition between such disparate regimes of turbulence can occur when a control parameter reaches a critical value. The behavior of flows close to such transition points, which separate qualitatively distinct phases of turbulence, has been found to be unexpectedly rich. Here, (AA) survey recent findings on such transitions in highly anisotropic turbulent fluid flows, including turbulence in thin layers and under the influence of rapid rotation. (They) also review recent work on transitions induced by turbulent fluctuations, such as random reversals and transitions between large-scale vortices and jets, among others. The relevance of these results and their ramifications for future investigations are discussed.

>>️

Adrian van Kan. Phase Transitions in Anisotropic Turbulence. arXiv: 2408.02844v1 [physics.flu-dyn]. Aug 5, 2024. 

https://arxiv.org/abs/2408.02844

Also:  turbulence, vortex, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, turbulence, vortex

# gst: emergent chirality in active rotation even with spontaneous chiral symmetry breaking.

<< Collective cell dynamics play a crucial role in many developmental and physiological contexts. While two-dimensional (2D) cell migration has been widely studied, how three-dimensional (3D) geometry and topology interplay with collective cell behavior to determine dynamics and functions remains an open question. >>️

<< Using murine pancreas-derived organoids as a model system, (AA) find that epithelial spheres exhibit persistent rotation, rotational axis drift, and rotation arrest. Using a 3D vertex model, (they) demonstrate how the combined action of traction force and polarity alignment can account for these distinct rotational dynamics near a solid to flow transition. Furthermore, (their) analysis shows that the spherical tissue rotates as an active solid occasionally switching to a flowing state and exhibits spontaneous chiral symmetry breaking. >>️

Tzer Han Tan, Aboutaleb Amiri, et al. Emergent chirality in active solid rotation of pancreas spheres. PRX Life 2, 033006. Aug 8, 2024.

https://journals.aps.org/prxlife/abstract/10.1103/PRXLife.2.033006

AA << say their work shows how symmetry-breaking processes in living active matter can be induced by the interplay of geometry, topology, and collective dynamics. >>️

Charles Day. Emergent Chirality in Active Rotation. Physics 17, s102. Aug 8, 2024. 

https://physics.aps.org/synopsis-for/10.1103/PRXLife.2.033006 

Also: chiral, transition, in https://www.inkgmr.net/kwrds.html 

Keywords: gst, transition, chiral, chiral symmetry breaking 

# gst: breakup of Janus droplet in a bifurcating microchannel

<< Droplet breakup is frequently observed in natural and industrial processes (..)  Although valuable insights on the breakup mechanisms of single-phase droplets in microchannels have been provided over the past decades, the breakup physics of complex emulsions is still poorly understood. >>️

<< Spatially asymmetric Janus microdroplets, distinct from single-phase or double emulsion droplets possessing one uniform interface with the ambient phase, are anticipated to show unique breakup behaviors, which has not been explored.  >>️

AA << conduct both microfluidic experiments and three-dimensional lattice Boltzmann simulations to investigate the dynamic breakup of ionic liquid (IL)-water Janus droplets in an assembled 3D-printed microchannel with a bifurcation. >>️

<< Three different flow regimes are identified: (i) division into two daughter Janus droplets, (ii) breakup into a single-phase droplet and a smaller Janus droplet, and (iii) nonbreakup.  >>️

AA << find that the strong constraint effect of the main channel and large Ca_av (average capillary numbers) values are essential to the symmetrical breakup of Janus droplets. The tunnel between the mother droplet and the wall of the main channel, which allows the lateral shift of the Janus droplet, and moderate flow rates facilitate the breakup of the IL single-phase portion of Janus droplets.  >>

<< Through 90° rotation of the splitting microchannel, (AA) elucidate the distinctions in Janus droplet behaviors under two baffle orientations. Potential impacts of the oblique flow characteristic of [bmim]⁢Fe⁢Cl4-water Janus droplets on the droplet breakup are discussed. >>️

️

Hao Wang, Shiteng Wang, et al. Dynamic breakup of Janus droplet in a bifurcating microchannel. Phys. Rev. Fluids 9, 064203. Jun 11, 2024. 

https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.9.064203 

Also: 'Janus' in FonT https://flashontrack.blogspot.com/search?q=janus

Also: 'drop', 'droplet', 'droploid', in https://www.inkgmr.net/kwrds.html 

Keywords: gst, Janus, drop, droplet, droploid

# gst: apropos of modulational instabilities, the case of vortex-ring quantum droplets in a radially-periodic potential.

FIG. 11: (Color online) Typical examples of stable nested patterns with soliton and vortex QDs (quantum droplets)  which were created in adjacent radial troughs. In panels (a1-b4) the pattern was created from the initial dynamical states with parameters (N,S,On) = (46,0,2) and (N,S,On) = (35,1,1) in the outer and inner troughs, respectively. In panels (c1-d4) the input was taken with parameter sets (N,S,On) = (120,1,3) and (N,S,On) = (46,0,2) in the outer and inner troughs.

AA << establish stability and characteristics of two-dimensional (2D) vortex ring-shaped quantum droplets (QDs) formed by binary Bose-Einstein condensates. >>️

<< another noteworthy option is to construct a two-ring complex in which one vortex-ring component is subject to the MI  (modulational instability), hence it is replaced by an azimuthal soliton (or maybe several solitons), (..), while the vortex component trapped in another potential trough avoids the azimuthal MI and remains essentially axisymmetric. >>️

<< Examples of such heterogeneous robust states, produced by simulations of Eq. (3), are displayed in Fig. 11. Panels 11(a1-b4) show a complex in which the MI takes place in the outer circular trough, producing an azimuthal soliton which performs rotary motion, while the inner vortex ring is  modulationally stable. An opposite example is produced in Figs. 11(c1-d4), where the outer vortex ring remains stable against azimuthal perturbations, while the MI creates a soliton exhibiting the rotary motion in the embedded (inner) circular trough. The rotation direction of the soliton is driven by the vorticity sign of the underlying QD (quantum droplet). It is relevant to mention that the multi-ring potential considered here holds different vortex-ring or azimuthal-soliton states nearly isolating them from each other. (..) An additional problem, which is left for subsequent analysis, is interplay between adjacent radial modes in the case when the separation between the adjacent rings is essentially smaller. >>️

Bin Liu, Yi xi Chen, et al. Vortex-ring quantum droplets in a radially-periodic potential. arXiv: 2212.05838v1 [nlin.PS]. Dec 12, 2022.

https://arxiv.org/abs/2212.05838

Also

keyword 'drop' | 'droplet' | 'droploids' in FonT

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

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

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

keyword 'goccia' in Notes 

(quasi-stochastic poetry): 

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

keyword 'instability' | 'instabilities' in FonT

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

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

keyword 'instabile' in Notes 

(quasi-stochastic poetry)

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

Keywords: gst, drop, droplet, vortex, vortices, vortexes, vorticity, instability,  modulational instabilities

# 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: a screw mechanism to separate realistic racemic mixtures by local vorticity

AA developed << an explanatory "screw"  model for predicting the shear-flow separation of enantiomers >>

<< Enantiomers of chiral molecules are non-superposable mirror images with the same structural formula. In achiral environments, enantiomers have identical physical and chemical properties, and this prevents separation by classical methods. >>

<< A screw is a simple chiral object that couples rotation around one axis with displacement along that axis. This displacement is perpendicular to the plane of rotation and is characterized by the screw’s pitch (or lead). Here (it is defined) pitch (P) as the perpendicular distance advanced by a screw in a 2π-revolution. In a lab-fixed frame, left-and right-handed screws have pitches with the same magnitude, but with flipped signs, rotating in opposite directions to do the same task. This concept can be easily extended to chiral molecules by considering the translation-rotation coupling tensor (..). In a medium which induces rotation of molecules (as in a vortex flow), chiral molecules will translate in opposite directions. As a consequence, this asymmetry may enable the resolution of the enantiomers. >>

Thus  a << competition model and continuum drift diffusion equations are developed to predict separation of realistic racemic mixtures. (..) Additionally, (AA) find that certain achiral objects can also exhibit a non-zero molecular pitch. >>

<< Although it has been used << shear flow as the source to rotate the chiral molecules and achieve separation, it may be possible to use external forces to rotate the enantiomers >>️

️

Duraes A, Gezelter JD. Separation of Enantiomers through Local Vorticity: A Screw Model Mechanism. ChemRxiv. Cambridge: Cambridge Open Engage; Vers 1. doi: 10.33774/ chemrxiv-2021-196zw. Oct 04, 2021. 

https://chemrxiv.org/engage/chemrxiv/article-details/61575a0d46030a1245c52107  

keywords: gst, vortices, separation, enantiomeric separation, racemic mixture, screw mechanism, translation-rotation coupling, transitions.

# gst: apropos of 'strange' transitions to self-assemble into an egg; the coordinated elastic behavior to swirl in a vortex (D. melanogaster)

 << At the end of its first week of development, a striking change occurs in a fruit-fly egg cell. The cell’s internal fluid motion transitions from a disordered mix of small-scale flows to a single vortex that encompasses the entire cell. >> 

 << Given the fluid’s incompressibility, those forces can give rise to what the researchers call a  "swirling" instability, and the flow switches to a cell-spanning rotation. The threshold of molecular motor activity for the transition depends on the buckling of individual microtubules, which are treated as elastic rods. >> 

A Vortex in an Egg Cell. Physics 14, s1. Jan 13, 2021.  

https://physics.aps.org/articles/v14/s1

David B. Stein, Gabriele De Canio, et al. Swirling Instability of the Microtubule Cytoskeleton. Phys. Rev. Lett. 126, 028103.  doi: 10.1103/ PhysRevLett.126.028103. Jan 13, 2021.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.028103  

# gst: apropos of waves perturbed by weak turbulences

<< Wave phenomena are ubiquitous at all scales throughout the Universe, and where there are waves, there’s the potential for wave turbulence. Wave turbulence is the physical state that arises in a continuous medium when a large number of wave modes interact with each other randomly. >>

<< A specific case of wave turbulence in which weakly nonlinear waves propagate in an unbounded space is described by weak turbulence theory. Capturing the dynamics of this regime will be helpful to accurately model large fluid systems. >>

<< In a rotating fluid, the predicted wave spectrum for interacting weakly nonlinear inertial waves is perturbed by the formation of geostrophic modes—columnar vortices aligned with the rotation axis (..) (AA) successfully suppress geostrophic modes with the addition of two honeycomb-patterned plates, allowing them to confirm the predictions of weak turbulence theory. >>

Sergey Nazarenko. Verifying Weak Turbulence Theory.  Physics 13, 194. Dec 14, 2020. 

https://physics.aps.org/articles/v13/194

Eduardo Monsalve, Maxime Brunet, et al. Quantitative Experimental Observation of Weak Inertial-Wave Turbulence. Phys. Rev. Lett. 125, 254502. Dec 14, 2020.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.254502

# physics: SQUID, a probe at the boundary between 'quantum' and 'classic' worlds

<< A new device that relies on flowing clouds of ultracold atoms promises potential tests of the intersection between the weirdness of the quantum world and the familiarity of the macroscopic world we experience every day. The atomtronic Superconducting QUantum Interference Device (SQUID) is also potentially useful for ultrasensitive rotation measurements and as a component in quantum computers. >>

AA << created the device by trapping cold atoms in a sheet of laser light. A second laser intersecting the sheet "painted" patterns that guided the atoms into two semicircles separated by small gaps known as Josephson Junctions. When the SQUID is rotated and the Josephson Junctions are moved toward each other, the populations of atoms in the semicircles change as a result of quantum mechanical interference of currents through Josephson Junctions. By counting the atoms in each section of the semicircle, the researchers can very precisely determine the rate the system is rotating.>>

James Riordon. Atomtronic device could probe boundary between quantum, everyday worlds. Los Alamos National Laboratory. July 17, 2020.

https://phys.org/news/2020-07-atomtronic-device-probe-boundary-quantum.html

C. Ryu, E. C. Samson, M. G. Boshier. Quantum interference of currents in an atomtronic SQUID. Nat Commun 11, 3338. doi: 10.1038/ s41467-020-17185-6. Jul 3, 2020.

https://www.nature.com/articles/s41467-020-17185-6

# gst: apropos of strange fluctuations in oscillation and translation

<< The dynamical processes of the inner core rely significantly on the viscous strength of iron. Since plastic deformation of iron may produce crystallographic preferred orientations  (CPO), creep is commonly considered to be a potential source contributing to the seismic anisotropy observed in the inner core. The viscosity of the inner core also influences the rotational dynamics of the Earth. >>

AA << show  that  dislocation  creep  is  a key  mechanism driving  deformation  of  hcp (hexagonal  close  packed) iron  at  inner  core  conditions. The  associated  viscosity  agrees well  with  the  estimates  from  geophysical  observations  supporting  that  the  inner  core  is  significantly less  viscous  than  the  Earth’s  mantle. Such  low  viscosity  rules  out  inner  core  translation,  with  melting on one side  and  solidification  on  the  opposite,  but  allows  for  the  occurrence  of  the  seismically  observed fluctuations  in  inner  core  differential  rotation. >>

Sebastian Ritterbex & Taku Tsuchiya. Viscosity of hcp iron at Earth’s inner core conditions from density functional theory. Sci Rep 10, 6311 (2020). doi: 10.1038/s41598-020-63166-6. Apr 14, 2020.

https://www.nature.com/articles/s41598-020-63166-6

Is the Earth's inner core oscillating and translating anomalously? Ehime University.  Apr 14, 2020.

https://phys.org/news/2020-04-earth-core-oscillating-anomalously.html

# gst: apropos of swirling granular media, a geometric frustration; will these entities turn left or right?

<< Granular material in a swirled container exhibits a curious transition as the number of particles is increased: At low densities, the particle cluster rotates in the same direction as the swirling motion of the container, while at high densities it rotates in the opposite direction.  >>

AA << show that the transition to counterrotation is friction dependent. At high particle densities, frictional effects result in geometric frustration, which prevents particles from cooperatively rolling and spinning. Consequently, the particle cluster rolls like a rigid body with no-slip conditions on the container wall, which necessarily counterrotates around its own axis. Numerical simulations verify that both wall-disk friction and disk-disk friction are critical for inducing counterrotation. >>

Lisa M. Lee, John Paul Ryan, et al. Geometric frustration induces the transition between rotation and counterrotation in swirled granular media. Phys. Rev. E 100, 012903. July 8, 2019.     https://journals.aps.org/pre/abstract/10.1103/PhysRevE.100.012903  

Leah Burrows. Solving the pancake problem. Harvard University. Aug 30, 2019.    https://m.phys.org/news/2019-08-pancake-problem.html  

# 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

# gst: periodic oscillations of flexible knots

AA << study the dynamics of knotted deformable closed chains sedimenting in a viscous fluid. (..) trefoil and other torus knots often attain a remarkably regular horizontal toroidal structure while sedimenting, with a number of intertwined loops, oscillating periodically around each other. (..) this motion (..) is accompanied by a very slow rotation around the vertical symmetry axis. (..) this oscillating mode of the dynamics can spontaneously form even when starting from a qualitatively different initial configuration. (..) the oscillating modes are usually present as transients or final stages of the evolution, depending on chain aspect ratio and flexibility, and the number of loops. >>

Magdalena Gruziel, Krishnan Thyagarajan, et al. Periodic Motion of Sedimenting Flexible Knots. Phys. Rev. Lett. 121, 127801 Sep 18,  2018. doi: 10.1103/PhysRevLett.121.127801.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.127801

The periodic motion of flexible knots, and the connection to DNA. University of Warsaw. Sep 26, 2018.

https://m.phys.org/news/2018-09-periodic-motion-flexible-dna.html