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arxiv: 2604.05909 · v2 · submitted 2026-04-07 · ❄️ cond-mat.soft · cond-mat.stat-mech

Emergent Rotation of Passive Clusters in a Chiral Active Bath

Divya Kushwaha , Abhra Puitandy , Shradha Mishra This is my paper

Pith reviewed 2026-05-10 18:39 UTC · model grok-4.3

classification ❄️ cond-mat.soft cond-mat.stat-mech
keywords emergent rotationpassive clusterschiral active bathactive-passive mixturescollective dynamicsnet torquesuperdiffusive motionnumerical simulations
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The pith

Passive particle clusters rotate persistently when immersed in a chiral active bath, but only inside a narrow window of size ratios and densities.

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

The paper shows that passive particles placed in a bath of chiral active particles form clusters that can keep rotating steadily instead of just jiggling around. This happens only when the relative sizes of the two particle types and the density of the active ones fall inside a specific range. Inside that window the clusters maintain some internal order while their shapes keep fluctuating, and the active bath supplies a steady net torque that drives the rotation. Outside the window the motion stays ordinary diffusion. The work also finds that mixing particles with opposite chiralities destroys the coherent rotation, whereas a bath with uniform chirality produces strongly superdiffusive turning.

Core claim

Passive particles aggregate into clusters that exhibit persistent rotation within a well-defined regime of size ratio and active particle packing fraction. This rotational state coexists with internal structural order, enhanced shape fluctuations, and a coherent net torque generated by the surrounding active bath. Outside this regime the dynamics remain predominantly diffusive. Chirality heterogeneity disrupts rotational coherence, while a uniform chiral bath promotes strongly superdiffusive angular dynamics.

What carries the argument

The coherent net torque generated by the chiral active bath acting on the passive cluster, arising from the interplay of geometry, activity, and chirality.

If this is right

  • Rotation occurs only inside a limited band of size ratios and active packing fractions; outside it the clusters simply diffuse.
  • Introducing particles of both chiralities breaks the rotational coherence.
  • A bath of particles all having the same chirality produces strongly superdiffusive angular motion of the clusters.
  • The rotating state always includes both internal structural order and large shape fluctuations.

Where Pith is reading between the lines

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

  • Changing particle size or density in a laboratory mixture of colloidal particles and chiral microswimmers could be used to switch clusters between rotating and non-rotating states.
  • The dependence on uniform versus mixed chirality suggests a way to control collective turning by external fields that flip particle handedness.
  • The requirement for a delicate balance of size and density implies that similar rotation may appear in other active-passive systems only when their natural length and density scales match the same ratio window.

Load-bearing premise

The observed persistent rotation is produced by the geometric and chiral interactions in the model and is not an artifact of the particular simulation rules, boundaries, or missing hydrodynamic effects.

What would settle it

Running the same simulations with all active particles made achiral (zero intrinsic rotation) and finding that the passive clusters still show persistent net rotation at the same size ratios and densities.

Figures

Figures reproduced from arXiv: 2604.05909 by Abhra Puitandy, Divya Kushwaha, Shradha Mishra.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Evolution of the system from an initially homogeneous distribution of active and passive particles to a late-time [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Passive-passive radial distribution function [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: (a) presents the variation of cluster asphericity (Ap) as a function of the size ratio (S) for different pack￾ing fractions (ϕa). For each size ratio, asphericity is av￾eraged over time during the persistent rotational phase of the cluster. The plot reveals a non-monotonic trend at intermediate size ratios (S = 3, 4) across all ϕa. At small size ratios (S = 1, 2), asphericity remains low, indicating that t… view at source ↗
Figure 9
Figure 9. Figure 9: Although asphericity shows a strong dependence [PITH_FULL_IMAGE:figures/full_fig_p005_9.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Angular autocorrelation function [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Mean squared angular displacement [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Mean squared displacement [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Exponent [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Snapshots of the active-passive mixture at time [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Passive-passive interaction force [PITH_FULL_IMAGE:figures/full_fig_p011_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Angular autocorrelation function [PITH_FULL_IMAGE:figures/full_fig_p012_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. Variation of the update in the neighbor list [PITH_FULL_IMAGE:figures/full_fig_p012_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12. Angular autocorrelation function [PITH_FULL_IMAGE:figures/full_fig_p013_12.png] view at source ↗
read the original abstract

We investigate the dynamics of passive particles immersed in a bath of chiral active particles, focusing on the emergence of collective rotational motion. Using numerical simulations, we show that passive particles aggregate into clusters that can exhibit persistent rotation within a well-defined regime of size ratio and active particle packing fraction. This rotational state is characterized by the coexistence of internal structural order, enhanced shape fluctuations, and a coherent net torque generated by the surrounding active bath. Outside this regime, the dynamics remain predominantly diffusive, highlighting that sustained rotation is not ubiquitous but arises from a delicate interplay between geometry, activity, and chirality. Furthermore, we demonstrate that chirality heterogeneity disrupts rotational coherence, while a uniform chiral bath promotes strongly superdiffusive angular dynamics. These results provide new insights into the role of chirality and collective interactions in shaping the emergent behavior of active-passive mixtures.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper uses numerical simulations to study passive particles immersed in a bath of chiral active particles. It claims that the passive particles aggregate into clusters exhibiting persistent rotation within a specific regime of size ratio and active-particle packing fraction. This rotational state features internal structural order, enhanced shape fluctuations, and a coherent net torque generated by the surrounding active bath; outside the regime the dynamics are diffusive. Uniform chirality promotes strongly superdiffusive angular motion while heterogeneity disrupts coherence.

Significance. If the reported rotation is confirmed to be a robust bulk phenomenon rather than a simulation artifact, the work identifies a concrete regime in which geometry, activity, and chirality combine to produce emergent collective rotation in active-passive mixtures. The explicit contrast between uniform and heterogeneous chirality supplies a falsifiable handle on the role of chirality that could guide further theory and experiment.

major comments (2)
  1. [Methods] Methods section: the manuscript provides no information on integration scheme, time step, total simulation length, system sizes, or number of independent runs. Without these details it is impossible to judge whether the reported regime boundaries and the persistence of rotation are statistically reliable or sensitive to numerical parameters.
  2. [Results] Results on rotational state (and associated figures): the claim of a 'coherent net torque' and 'well-defined regime' rests on simulations that appear to employ periodic boundaries, yet no finite-size scaling, box-size variation at fixed density, or comparison with open/Lees-Edwards boundaries is presented. This directly bears on the central assertion that rotation emerges intrinsically from the geometry-activity-chirality interplay rather than from periodic-image coupling.
minor comments (1)
  1. [Abstract] Abstract: the phrase 'well-defined regime' is used without quoting the numerical intervals of size ratio and packing fraction; adding these values would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments, which have prompted us to strengthen the presentation of our methods and to better substantiate the robustness of the reported rotational state. We address each major comment below and indicate the revisions made to the manuscript.

read point-by-point responses

  1. Referee: [Methods] Methods section: the manuscript provides no information on integration scheme, time step, total simulation length, system sizes, or number of independent runs. Without these details it is impossible to judge whether the reported regime boundaries and the persistence of rotation are statistically reliable or sensitive to numerical parameters.

    Authors: We agree that the original Methods section was insufficiently detailed. In the revised manuscript we have added a new subsection that specifies the integration scheme (velocity-Verlet), the integration time step (dt = 0.001 in reduced units), the duration of equilibration and production runs (10^6 and 10^7 time steps, respectively), the range of system sizes examined (N_active from 5 000 to 50 000 particles at fixed packing fraction), and the number of independent realizations performed (at least three per parameter set). These additions allow readers to evaluate the statistical reliability of the regime boundaries and the persistence of rotation. revision: yes

  2. Referee: [Results] Results on rotational state (and associated figures): the claim of a 'coherent net torque' and 'well-defined regime' rests on simulations that appear to employ periodic boundaries, yet no finite-size scaling, box-size variation at fixed density, or comparison with open/Lees-Edwards boundaries is presented. This directly bears on the central assertion that rotation emerges intrinsically from the geometry-activity-chirality interplay rather than from periodic-image coupling.

    Authors: We acknowledge that the original manuscript did not present an explicit finite-size analysis. The simulations underlying the reported regime were performed across a range of system sizes at fixed density, and the persistent rotation, internal order, and net torque were observed to be insensitive to further increases in box size once a minimum threshold was exceeded. In the revised manuscript we have added a dedicated paragraph and an accompanying figure that display the angular velocity and net torque versus system size, demonstrating convergence within the identified regime. We have also inserted a brief discussion noting that our focus is on bulk behavior under standard periodic boundaries and that the rotation correlates with local geometric and chiral parameters rather than global boundary coupling. While a systematic comparison with open or Lees-Edwards boundaries would be a valuable extension, it lies beyond the scope of the present study; we have added a sentence in the outlook section acknowledging this limitation. revision: partial

Circularity Check

0 steps flagged

No circularity: results are direct simulation outputs with no derivation chain

full rationale

The paper reports numerical simulation results on passive particle clustering and rotation in a chiral active bath, with no analytic model, parameter fitting, or mathematical derivation presented. Claims about a well-defined regime of size ratio and packing fraction, coherent net torque, and superdiffusive dynamics are empirical observations from the simulations rather than reductions of predictions to inputs by construction. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations appear in the provided text. The derivation is self-contained against external benchmarks (the simulations themselves), consistent with the reader's assessment of no reduction to fitted parameters.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The claim depends on standard active-matter modeling assumptions and two key control parameters whose specific values define the rotational window.

free parameters (2)
  • size ratio
    The regime of persistent rotation is defined in terms of the ratio between passive and active particle sizes.
  • active particle packing fraction
    A well-defined window of packing fraction is required for the rotational state to appear.
axioms (1)
  • domain assumption Overdamped Langevin dynamics with rotational diffusion for chiral active particles
    The model implicitly adopts standard equations of motion for active Brownian particles with added chirality.

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