Pith. sign in

REVIEW 2 major objections 1 minor 63 references

Active spinners and passive colloids separate into vortices around active clusters and large active-passive bands when fluid-mediated interactions are accounted for.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗

T0 review · grok-4.3

2026-06-26 06:56 UTC pith:L25JDHPV

load-bearing objection Paper reports passive vortices and active-passive bands from intrinsic chirality plus Ewald hydrodynamics, but the summation accuracy at the scanned densities and speeds is unverified. the 2 major comments →

arxiv 2606.22903 v1 pith:L25JDHPV submitted 2026-06-22 cond-mat.soft physics.flu-dyn

Hydrodynamic Phase Separation and Morphological Evolution in Chiral Active-Passive Mixtures

Mayurakshi Deb , Rajesh Singh This is my paper
classification cond-mat.soft physics.flu-dyn
keywords hydrodynamic interactionsphase separationactive-passive mixtureschiral spinnersmorphological evolutionvorticesactive matterEwald summation
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines suspensions containing particles that actively spin and ordinary passive colloids. It shows that the fluid flows created by the spinners organize the mixture into distinct patterns that depend on how crowded the particles are and how fast they rotate. These patterns include rings of passive particles circling clumps of spinners and extended stripes where active and passive particles alternate. A reader would care because the work isolates the role of long-range fluid couplings in producing order from activity alone, without external torques or direct forces between particles.

Core claim

In mixtures of active spinners and passive colloids, many-body hydrodynamic interactions produce phase separation into active-spinner clusters surrounded by passive-particle vortices together with large-scale active-passive bands; the dynamics are tracked through the growth of characteristic length scales and the development of non-equilibrium velocity distributions in the passive population.

What carries the argument

Many-body hydrodynamic interactions computed via Ewald summation between active spinners and passive colloids.

Load-bearing premise

The Ewald summation accurately captures the hydrodynamic forces among all particles across the densities and rotation rates examined.

What would settle it

A simulation that replaces the Ewald summation with a short-range or mean-field approximation for the fluid interactions and finds neither the reported vortices nor the bands would falsify the central claim.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

Share X Bluesky LinkedIn Reddit HN

If this is right

  • Phase-separation behavior changes systematically with particle area fraction and spinner rotation speed.
  • Passive particles form stable vortex structures around separated active-spinner domains.
  • Extended bands containing both active and passive particles appear at higher densities or rotation rates.
  • Characteristic length scales of the patterns grow over time in a manner tied to the hydrodynamic coupling.
  • Passive-particle velocity statistics deviate from equilibrium distributions in the separated states.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Hydrodynamic interactions can dominate the morphology even when direct particle collisions are present.
  • The same mechanism may produce comparable banded or vortex states in other chiral active systems once long-range flow is included.
  • Varying the fluid viscosity or particle size ratio offers a direct experimental test of the reported regimes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 1 minor

Summary. The manuscript studies mixtures of active spinners and passive colloids, computing many-body hydrodynamic interactions via Ewald summation. Systematic variation of area fractions and rotational velocities reveals distinct phase-separation regimes, with reported morphologies including passive-particle vortices surrounding phase-separated active spinners and large-scale active-passive bands; these are characterized via temporal evolution of length scales and non-equilibrium velocity distributions of the passive particles.

Significance. If the morphologies prove robust, the work would demonstrate how intrinsic chirality combined with long-range hydrodynamic couplings produces self-organized structures not seen in externally torqued systems, adding to the understanding of non-equilibrium condensed matter.

major comments (2)
  1. [Methods] The central morphologies rest on many-body hydrodynamics evaluated with Ewald summation, yet the manuscript supplies no convergence tests, truncation-error bounds, or comparisons against open-boundary or alternative summation schemes across the reported range of area fractions and rotational velocities. This verification is load-bearing for the claim that the observed vortices and bands are physical rather than numerical artifacts.
  2. [Results] No error bars, run-to-run statistics, or finite-size checks are presented for the characteristic length scales or velocity distributions that underpin the phase-separation regimes and morphological claims.
minor comments (1)
  1. [Abstract] The abstract states that the morphologies are 'sensitive to the system's kinematic parameters' but does not indicate which specific combinations of area fraction and rotational velocity produce vortices versus bands.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed review and constructive feedback on our manuscript. We address each major comment below and outline the revisions we will make to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: [Methods] The central morphologies rest on many-body hydrodynamics evaluated with Ewald summation, yet the manuscript supplies no convergence tests, truncation-error bounds, or comparisons against open-boundary or alternative summation schemes across the reported range of area fractions and rotational velocities. This verification is load-bearing for the claim that the observed vortices and bands are physical rather than numerical artifacts.

    Authors: We agree that explicit verification of the Ewald summation is important for establishing the robustness of the reported morphologies. The original manuscript does not contain such tests. In the revised version we will add a new appendix (or subsection) that reports convergence with respect to Ewald cutoff parameters, provides truncation-error estimates, and includes limited comparisons against open-boundary calculations for representative values of area fraction and rotational velocity. revision: yes

  2. Referee: [Results] No error bars, run-to-run statistics, or finite-size checks are presented for the characteristic length scales or velocity distributions that underpin the phase-separation regimes and morphological claims.

    Authors: We acknowledge that the manuscript presents the length-scale evolution and velocity distributions without accompanying error bars or statistical measures. In the revision we will re-analyze the existing simulation data to compute run-to-run standard deviations and will add error bars to the relevant figures. We will also include a brief discussion of finite-size effects based on the system sizes already simulated; if additional runs are required for a fuller finite-size study, these will be performed and reported. revision: yes

Circularity Check

0 steps flagged

Simulation outputs from hydrodynamic solver; no derivation chain reduces to fitted inputs or self-citations

full rationale

This is a computational study that reports morphologies (vortices, bands) as direct outputs of particle simulations incorporating Ewald-summed hydrodynamic interactions. No equations, parameters, or predictions are fitted to data and then re-presented as independent results. The abstract and described methods contain no self-citations that bear the central claims, no ansatzes smuggled via prior work, and no renaming of known results as new derivations. The morphologies emerge from the solver rather than by construction from any input definitions or fits. This is the expected non-circular outcome for a pure simulation paper whose claims rest on numerical integration rather than analytic reduction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review yields minimal ledger entries; the central claim rests on standard hydrodynamic methods and particle models whose details are not supplied.

axioms (1)
  • domain assumption Ewald summation correctly computes long-range hydrodynamic interactions in periodic domains for the particle densities studied.
    Invoked when the abstract states that many-body hydrodynamic interactions are computed via Ewald summation.

pith-pipeline@v0.9.1-grok · 5692 in / 1183 out tokens · 34911 ms · 2026-06-26T06:56:15.303156+00:00 · methodology

0 comments
Cite this review

Pith. "Pith review of Hydrodynamic Phase Separation and Morphological Evolution in Chiral Active-Passive Mixtures." pith.science (2026). https://pith.science/paper/L25JDHPV

@misc{pith2026260622903,
  author       = {Pith},
  title        = {Pith review of: Hydrodynamic Phase Separation and Morphological Evolution in Chiral Active-Passive Mixtures},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/L25JDHPV}},
  note         = {Machine review of arXiv:2606.22903}
}
Share X Bluesky LinkedIn Reddit HN
read the original abstract

The collective behavior of passive particles within chiral active matter has emerged as a significant area of soft matter research. However, most existing studies focus on systems where chirality is imposed by external torques rather than intrinsic activity. In this work, we study emergent dynamics in a suspension of active spinners and passive colloids by computing many-body hydrodynamic interactions via Ewald summation. By systematically exploring a broad range of area fractions and rotational velocities, we identify distinct phase-separation regimes sensitive to the system's kinematic parameters. Specifically, we report the emergence of unique structural morphologies, including the formation of passive particle vortices surrounding phase-separated active spinners and the development of large-scale active-passive bands. We characterize the underlying dynamics by analyzing the temporal evolution of characteristic length scales and the non-equilibrium velocity distributions of the passive particles. Our findings provide new insights into the role of long-range hydrodynamic couplings in governing the self-organization of non-equilibrium condensed matter.

Figures

Figures reproduced from arXiv: 2606.22903 by Mayurakshi Deb, Rajesh Singh.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Schematic representation of the system: spinning active particles are shown in green with curved arrows, while [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Top panel shows phase diagram in the plane of di [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Coarsening kinetics during phase separation in a sys [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. PDF (probability distribution function) for the [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Snapshots (top row) and vector plots of displacements (bottom row) at distinct time points of active particles (green) [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Role of number asymmetry in temporal dynamics of [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Time evolution of length scale [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

63 extracted references · 2 canonical work pages

  1. [1]

    M. t. Vrugt, B. Liebchen, and M. E. Cates, What ex- actly is’ active matter’?, arXiv preprint arXiv:2507.21621 (2025)

    work page arXiv 2025

  2. [2]

    M. C. Marchetti, J. F. Joanny, S. Ramaswamy, T. B. Liverpool, J. Prost, M. Rao, and R. A. Simha, Hydrody- namics of soft active matter, Rev. Mod. Phys.85, 1143 (2013)

    2013

  3. [3]

    M. J. Bowick, N. Fakhri, M. C. Marchetti, and S. Ra- maswamy, Symmetry, thermodynamics, and topology in active matter, Phys. Rev. X12, 010501 (2022)

    2022

  4. [4]

    M. E. Cates and J. Tailleur, Motility-induced phase sep- aration, Annu. Rev. Condens. Mat. Phys.6, 219 (2015)

    2015

  5. [5]

    M. E. Cates and C. Nardini, Active phase separation: new phenomenology from non-equilibrium physics, Re- ports on Progress in Physics88, 056601 (2025)

    2025

  6. [6]

    Ramaswamy, The Mechanics and Statistics of Active Matter, Annu

    S. Ramaswamy, The Mechanics and Statistics of Active Matter, Annu. Rev. Condens. Mat. Phys.1, 323 (2010)

    2010

  7. [7]

    Di Leonardo, L

    R. Di Leonardo, L. Angelani, D. Dell’Arciprete, G. Ruocco, V. Iebba, S. Schippa, M. P. Conte, F. Mecarini, F. De Angelis, and E. Di Fabrizio, Bacterial ratchet motors, Proceedings of the National Academy of Sciences107, 9541 (2010)

    2010

  8. [8]

    L¨ owen, Chirality in microswimmer motion: From cir- cle swimmers to active turbulence, The European Phys- 9 ical Journal Special Topics225, 2319 (2016)

    H. L¨ owen, Chirality in microswimmer motion: From cir- cle swimmers to active turbulence, The European Phys- 9 ical Journal Special Topics225, 2319 (2016)

    2016

  9. [9]

    Lauga,The fluid dynamics of cell motility, Vol

    E. Lauga,The fluid dynamics of cell motility, Vol. 62 (Cambridge University Press, 2020)

    2020

  10. [10]

    T. H. Tan, A. Mietke, J. Li, Y. Chen, H. Higinbotham, P. J. Foster, S. Gokhale, J. Dunkel, and N. Fakhri, Odd dynamics of living chiral crystals, Nature607, 287 (2022)

    2022

  11. [11]

    A. P. Petroff, X.-L. Wu, and A. Libchaber, Fast-moving bacteria self-organize into active two-dimensional crys- tals of rotating cells, Phys. Rev. Lett.114, 158102 (2015)

    2015

  12. [12]

    M. Han, M. Fruchart, C. Scheibner, S. Vaikuntanathan, J. J. De Pablo, and V. Vitelli, Fluctuating hydrodynam- ics of chiral active fluids, Nature Physics17, 1260 (2021)

    2021

  13. [13]

    Singh and R

    R. Singh and R. Adhikari, Universal hydrodynamic mech- anisms for crystallization in active colloidal suspensions, Phys. Rev. Lett.117, 228002 (2016)

    2016

  14. [14]

    Angelani, C

    L. Angelani, C. Maggi, A. Bernardini, M. L. an Rizzo, and R. Di Leonardo, Effective interactions between col- loidal particles suspended in a bath of swimming cells, Phys. Rev. Lett.107, 138302 (2011)

    2011

  15. [15]

    Stenhammar, R

    J. Stenhammar, R. Wittkowski, D. Marenduzzo, and M. E. Cates, Activity-induced phase separation and self- assembly in mixtures of active and passive particles, Phys. Rev. Lett.114, 018301 (2015)

    2015

  16. [16]

    R. C. Krafnick and A. E. Garc´ ıa, Impact of hydrodynam- ics on effective interactions in suspensions of active and passive matter, Phys. Rev. E91, 022308 (2015)

    2015

  17. [17]

    K. Yeo, E. Lushi, and P. M. Vlahovska, Dynamics of inert spheres in active suspensions of micro-rotors, Soft matter 12, 5645 (2016)

    2016

  18. [18]

    Smrek and K

    J. Smrek and K. Kremer, Small activity differences drive phase separation in active-passive polymer mixtures, Phys. Rev. Lett.118, 098002 (2017)

    2017

  19. [19]

    Dolai, A

    P. Dolai, A. Simha, and S. Mishra, Phase separation in binary mixtures of active and passive particles, Soft Mat- ter14, 6137 (2018)

    2018

  20. [20]

    Reichhardt and C

    C. Reichhardt and C. Reichhardt, Reversibility, pattern formation, and edge transport in active chiral and passive disk mixtures, J. Chem. Phys.150(2019)

    2019

  21. [21]

    P. Liu, S. Ye, F. Ye, K. Chen, and M. Yang, Constraint dependence of active depletion forces on passive particles, Phys. Rev. Lett.124, 158001 (2020)

    2020

  22. [22]

    S. M. Mousavi, G. Gompper, and R. G. Winkler, Active bath-induced localization and collapse of passive semi- flexible polymers, J. Chem. Phys.155(2021)

    2021

  23. [23]

    I. P. Madden, L. Wang, J. Simmchen, and E. Luijten, Hy- drodynamically controlled self-organization in mixtures of active and passive colloids, Small18, 2107023 (2022)

    2022

  24. [24]

    Gokhale, J

    S. Gokhale, J. Li, A. Solon, J. Gore, and N. Fakhri, Dy- namic clustering of passive colloids in dense suspensions of motile bacteria, Phys. Rev. E105, 054605 (2022)

    2022

  25. [25]

    Grober, I

    D. Grober, I. Palaia, M. C. U¸ car, E. Hannezo, A. ˇSari´ c, and J. Palacci, Unconventional colloidal aggregation in chiral bacterial baths, Nature Physics19, 1680 (2023)

    2023

  26. [26]

    Kushwaha, V

    P. Kushwaha, V. Semwal, S. Maity, S. Mishra, and V. Chikkadi, Phase separation of passive particles in ac- tive liquids, Phys. Rev. E108, 034603 (2023)

    2023

  27. [27]

    Yuan and H

    J. Yuan and H. Tanaka, Colloid phase separation dy- namics driven by chiral turbulent flows, Physical Review Research6, 023186 (2024)

    2024

  28. [28]

    R. F.-Q. Garc´ ıa, E. Chac´ on, P. Tarazona, and C. Vale- riani, Dynamics and rupture of doped motility induced phase separation, Soft Matter (2025)

    2025

  29. [29]

    Serna, C

    H. Serna, C. M. Barriuso G., I. Pagonabarraga, M. Polin, and C. Valeriani, Sorting of binary active–passive mix- tures in designed microchannels, Soft Matter21, 8781 (2025)

    2025

  30. [30]

    Jhajhria, S

    M. Jhajhria, S. K. Das, and S. Thakur, Kinetics of phase transition in nonreciprocal mixtures of passive and chemophoretically active particles, J. Chem. Phys.162 (2025)

    2025

  31. [31]

    Bhattacharyya and J

    S. Bhattacharyya and J. M. Yeomans, Active sorting to boundaries in active nematic–passive isotropic fluid mix- tures, Soft Matter 10.1039/D5SM00636H (2025)

    work page doi:10.1039/d5sm00636h 2025

  32. [32]

    Chamolly and T

    A. Chamolly and T. Ishikawa, Self-organised structures in mixed active–passive suspensions due to hydrody- namic interactions, Journal of Fluid Mechanics1030, A22 (2026)

    2026

  33. [33]

    N. H. Nguyen, D. Klotsa, M. Engel, and S. C. Glotzer, Emergent collective phenomena in a mixture of hard shapes through active rotation, Phys. Rev. Lett.112, 075701 (2014)

    2014

  34. [34]

    K. Yeo, E. Lushi, and P. M. Vlahovska, Collective dy- namics in a binary mixture of hydrodynamically coupled microrotors, Phys. Rev. Lett.114, 188301 (2015)

    2015

  35. [35]

    Massana-Cid, D

    H. Massana-Cid, D. Levis, R. J. H. Hern´ andez, I. Pago- nabarraga, and P. Tierno, Arrested phase separation in chiral fluids of colloidal spinners, Phys. Rev. Research3, L042021 (2021)

    2021

  36. [36]

    Hrishikesh, K

    B. Hrishikesh, K. Takae, E. Mani, and H. Tanaka, Phase separation of rotor mixtures without domain coarsening driven by two-dimensional turbulence, Communications Physics5, 337 (2022)

    2022

  37. [37]

    Lushi and P

    E. Lushi and P. M. Vlahovska, Periodic and chaotic orbits of plane-confined micro-rotors in creeping flows, Journal of Nonlinear Science25, 1111 (2015)

    2015

  38. [38]

    Ghose and R

    S. Ghose and R. Adhikari, Irreducible representations of oscillatory and swirling flows in active soft matter, Phys. Rev. Lett.112, 118102 (2014)

    2014

  39. [39]

    Ishikawa, T

    T. Ishikawa, T. Pedley, K. Drescher, and R. E. Goldstein, Stability of dancing volvox, Journal of Fluid Mechanics 903, A11 (2020)

    2020

  40. [40]

    Y. Fily, A. Baskaran, and M. C. Marchetti, Cooperative self-propulsion of active and passive rotors, Soft Matter 8, 3002 (2012)

    2012

  41. [41]

    J. L. Anderson, Colloid transport by interfacial forces, Annu. Rev. Fluid Mech.21, 61 (1989)

    1989

  42. [42]

    S. J. Ebbens and J. R. Howse, In pursuit of propulsion at the nanoscale, Soft Matter6, 726 (2010)

    2010

  43. [43]

    M. J. Lighthill, On the squirming motion of nearly spher- ical deformable bodies through liquids at very small Reynolds numbers, Commun. Pure. Appl. Math.5, 109 (1952)

    1952

  44. [44]

    J. R. Blake, A spherical envelope approach to ciliary propulsion, J. Fluid Mech.46, 199 (1971)

    1971

  45. [45]

    Brennen and H

    C. Brennen and H. Winet, Fluid mechanics of propul- sion by cilia and flagella, Annu. Rev. Fluid Mech.9, 339 (1977)

    1977

  46. [46]

    Singh, S

    R. Singh, S. Ghose, and R. Adhikari, Many-body micro- hydrodynamics of colloidal particles with active bound- ary layers, J. Stat. Mech2015, P06017 (2015)

    2015

  47. [47]

    Singh and R

    R. Singh and R. Adhikari, Generalized Stokes laws for active colloids and their applications, J. Phys. Commun. 2, 025025 (2018)

    2018

  48. [48]

    G. Turk, R. Singh, and R. Adhikari, Stokes traction on an active particle, Phys. Rev. E106, 014601 (2022)

    2022

  49. [49]

    G. Turk, R. Adhikari, and R. Singh, Fluctuating hydro- dynamics of an autophoretic particle near a permeable 10 interface, J. Fluid Mech.998, A34 (2024)

    2024

  50. [50]

    G. Turk, R. Singh, and H. A. Stone, Autophoretic skat- ing along permeable surfaces, Journal of Fluid Mechanics 1019, A57 (2025)

    2025

  51. [51]

    Deb and R

    M. Deb and R. Singh, Ewald summing irreducible com- ponents of flow around active particles, J. Chem. Phys. 163(2025)

    2025

  52. [52]

    N´ emeth, T

    B. N´ emeth, T. Kobayashi, and R. Adhikari, Nonrecipro- cal constitutive laws for oriented active solids, New Jour- nal of Physics28, 034401 (2026)

    2026

  53. [53]

    O. S. Pak and E. Lauga, Generalized squirming motion of a sphere, J. Eng. Math.88, 1 (2014)

    2014

  54. [54]

    Drescher, K

    K. Drescher, K. C. Leptos, I. Tuval, T. Ishikawa, T. J. Pedley, and R. E. Goldstein, Dancing volvox: Hydrody- namic bound states of swimming algae, Phys. Rev. Lett. 102, 168101 (2009)

    2009

  55. [55]

    R. E. Goldstein, Green algae as model organisms for bi- ological fluid dynamics, Ann. Rev. Fluid Mech.47, 343 (2015)

    2015

  56. [56]

    C. W. J. Beenakker, Ewald sum of the Rotne–Prager tensor, J. Chem. Phys.85, 1581 (1986)

    1986

  57. [57]

    V. M. Kendon, M. E. Cates, I. Pagonabarraga, J.-C. De- splat, and P. Bladon, Inertial effects in three-dimensional spinodal decomposition of a symmetric binary fluid mix- ture: a lattice boltzmann study, J. Fluid Mech.440, 147 (2001)

    2001

  58. [58]

    P. C. Hohenberg and B. I. Halperin, Theory of dynamic critical phenomena, Rev. of Mod. Phys.49, 435 (1977)

    1977

  59. [59]

    A. J. Bray, Theory of phase-ordering kinetics, Adv. Phys. 43, 357 (1994)

    1994

  60. [60]

    Sierou and J

    A. Sierou and J. F. Brady, Shear-induced self-diffusion in non-colloidal suspensions, Journal of Fluid Mechanics 506, 285–314 (2004)

    2004

  61. [61]

    Pozrikidis,Boundary Integral and Singularity Meth- ods for Linearized Viscous Flow(Cambridge University Press, 1992)

    C. Pozrikidis,Boundary Integral and Singularity Meth- ods for Linearized Viscous Flow(Cambridge University Press, 1992)

    1992

  62. [62]

    Singh and R

    R. Singh and R. Adhikari, Pystokes: phoresis and stoke- sian hydrodynamics in python, J. Open Source Software 5, 2318 (2020)

    2020

  63. [63]

    Nijboer and F

    B. Nijboer and F. De Wette, On the calculation of lattice sums, Physica23, 309 (1957)

    1957