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REVIEW 2 major objections 5 minor 41 references

Influence of active breathing on rheology and jamming of amorphous solids: insights from microscopic and mesoscale analysis

T0 review · 2 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read Periodic breathing of particles can fluidize a jammed amorphous solid once the oscillation amplitude exceeds a critical value.

desk verdict Solid qualitative case for activity-induced fluidization via breathing, but the headline threshold and exponent need stronger low-shear-rate convergence checks before I'd trust them. read the letter →

arxiv 2505.14520 v1 pith:57LQD7M7 submitted 2025-05-20 cond-mat.soft

classification cond-mat.soft
keywords activematterjammingyieldstressamorphoussolidselasto-plasticmodelparticlesizeoscillationsfluidizationshearrheology
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that a purely internal activity—periodic size oscillations, called 'breathing,' of soft particles—can fluidize a dense amorphous solid that would otherwise stay jammed. Using molecular dynamics simulations of 16,000 particles and a complementary tensorial elasto-plastic mesoscale model, the authors show that below a critical oscillation amplitude the material retains a finite yield stress, while above it the yield stress vanishes and the unsheared system flows as a Newtonian fluid. The two levels of description agree qualitatively, pointing to a mechanism of local strain injection coupled to long-range elastic stress redistribution. If correct, the result makes active size fluctuations an internal control parameter for tuning rigidity in disordered materials, with implications for biological tissues and synthetic active matter.

What carries the argument

The central object is the dimensionless amplitude a of sinusoidal diameter oscillations, with period T = 820τ0, applied to each particle with a phase choice that keeps the packing fraction constant. The carrying mechanism is the combination of local strain injection—each breathing particle acts as a dilating or contracting elastic inclusion—with long-range stress redistribution. In the mesoscale active elasto-plastic model (AEPM), the total stress is the sum of external, active, and plastic contributions; the active contribution is computed by convolving the active deformation field with an elastic propagator F derived from the linear response to an isotropic inclusion, while plastic events relax stress through the Eshelby kernel G. The agreement between microscopic and mesoscale results identifies this elastic propagator response as the ingredient that carries the fluidization.

What would settle it

Extend the shear-rate-controlled simulations to rates below $10^{-7}$, or run long enough to verify convergence at the current lowest rates; if the apparent yield stress continues to decrease and a Newtonian plateau appears for amplitudes below 0.0506, the reported threshold and yield-stress picture would need revision.

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Extended reading notes

Core claim

The central discovery is an activity-induced unjamming transition driven by particle size oscillations. In the microscopic model, the number of plastic events per breathing cycle stays low and nearly constant until a threshold amplitude, then rises sharply, while the von Mises stress passes through a cusp at the same amplitude. Under imposed shear, flow curves retain Herschel-Bulkley (yield-stress) form for low amplitudes, and the fitted yield stress decreases continuously with amplitude, extrapolating to zero at a* = 0.0506 with an exponent of about 0.39. Above the threshold, flow curves develop a constant viscosity at low shear rates, the signature of Newtonian flow. The mesoscale elasto-plastic model reproduces this softening and the yield-stress-to-Newtonian crossover by coupling local active dilations and contractions to stress redistribution through an elastic propagator.

Load-bearing premise

The claimed yield-stress regime below threshold rests on Herschel-Bulkley fits to flow curves measured only down to shear rates near $10^{-7}$, with no demonstrated convergence of the steady state at the lowest rates.

Editorial extensions

If this is right

  • At low breathing amplitudes the jammed solid keeps a finite yield stress, and the yield stress falls continuously as amplitude increases until rigidity disappears at a* = 0.0506.
  • Above the critical amplitude, the shear-rate-dependent viscosity develops a constant plateau at low rates, meaning the unsheared system is a Newtonian fluid.
  • The fluidization is generated entirely by internal activity, with no external forcing or thermal fluctuations.
  • The mesoscale model matches the microscopic trends, indicating that only local strain injection plus long-range elastic coupling are needed to capture the transition.
  • Creep tests under constant stress confirm the picture: at low activity strain saturates, while at higher activity it flows steadily.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the mechanism is generic, experimental systems with controllable size oscillations—such as responsive colloids or cells—should show the same yield-stress drop and Newtonian plateau in oscillatory rheology.
  • The fitted exponent of about 0.39 for the yield stress vanishing near the threshold suggests a possible scaling law that could be tested across different particle interactions, softness, or packing fractions.
  • The same mesoscale framework could be applied to other internal activators such as swelling, contraction, or local contractility, which would link it to tissue remodeling and active gel theories.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 5 minor

Summary. This manuscript studies dense two-dimensional assemblies of soft particles whose diameters oscillate periodically in time ('active breathing'), using large-scale Langevin dynamics simulations (N = 16,000; packing fraction 0.94) combined with a mesoscale tensorial elasto-plastic model in which local dilations are coupled to long-range elastic stress redistribution. The main claims are: (i) for breathing amplitudes below a critical value a* ≈ 0.0506 the system is a yield-stress solid whose yield stress decreases with amplitude and vanishes as (a* − a)^ζ with ζ ≈ 0.39; (ii) above a* the low-shear-rate response becomes Newtonian, with a constant-viscosity plateau; and (iii) the mesoscale model reproduces the same qualitative softening and fluidization, identifying internally generated stresses as the fluidization mechanism. Supporting evidence includes a sharp rise in plastic events per breathing cycle near a ≈ 0.05, a cusp in the von Mises stress, a collapse of σ − σ_y versus shear rate at low amplitudes, and constant-stress creep curves consistent with reduced mechanical resistance.

Significance. The question is timely: whether purely internal, isotropic size oscillations can continuously soften and fluidize a jammed amorphous solid, and how this manifests in rheology. The paper has clear strengths: a well-defined microscopic model; a mesoscale model built from a derived linear-elastic propagator for dilating inclusions (Appendix B), rather than fitted to the MD flow curves; error-barred event statistics in Fig. 3; and a falsifiable structural prediction (collapse of σ − σ_y with γ^{1/2} for a < a*). The qualitative transition is supported by multiple independent observables (events per cycle, von Mises stress, flow-curve shape). However, the headline quantitative outputs (a* and ζ) and the Newtonian-plateau identification rest on Herschel-Bulkley fits to data at shear rates as low as 10^-7 without demonstrated steady state or reported uncertainties, so the precise exponents are not yet established. The mesoscale agreement is qualitative and depends on parameters that are not reported. With the requested convergence checks, error bars, and parameter reporting, the paper would make a solid contribution to the active-matter and jamming literature.

major comments (2)
  1. [Sec. III B 1, Fig. 4] The central quantitative results—the vanishing yield stress with a* = 0.0506 and ζ ≈ 0.39, and the identification of a Newtonian regime for a ≥ 0.054—are obtained from flow curves extending down to shear rates near 10^-7, but the manuscript gives no evidence that these lowest-rate points are in steady state and provides no error bars on σ_y in Fig. 4(c). Since T = 820 τ_0, a shear rate of 10^-7 accumulates a strain of only about 8 × 10^-5 per breathing cycle; for a < a*, where plastic events are rare (Fig. 3), the run time required to reach a statistically stationary stress is not established, and an under-sampled or transient low-rate point would bias the HB extrapolation of σ_y. That bias would propagate through the power-law fit (a* − a)^ζ, and the situation is delicate because the a = 0.050 point lies within about 1% of the fitted a* = 0.0506. The same caveat applies to the data collapse and the γ^{1/2} claim in Fig. 4(d). The creep measurements in Appendix A, at one applied stress per amplitude and for amplitudes up to only a = 0.048, neither bracket σ_y nor probe the near-threshold region, so they cannot corroborate the extrapolation. Please report the averaging protocol (number of cycles and independent runs), add error bars or confidence intervals to σ_y(a), provide convergence checks (e.g., block averages) at the lowest shear rates, and test the sensitivity of a* and ζ to the fit range and to the inclusion or exclusion of the slowest points.
  2. [Sec. II B, Fig. 5] The mesoscale results in Fig. 5 and the statement in Sec. III B 2 that the model 'captures this behavior' and 'validates the modeling approach' cannot be properly assessed as presented: the manuscript does not report the yield-threshold distribution (form, mean, width), the relaxation and elastic-recovery timescales τ and τ_el, the active-strain amplitude and its mapping to the MD amplitude a, the system size, or the number of disorder realizations. The paper itself acknowledges that the assumption of zero stress at an active site 'is not very realistic' (Sec. II B). Given the number of free parameters, the qualitative agreement in Fig. 5 is weak evidence unless the parameter values are specified and the trend is shown to be robust to reasonable parameter choices. Please include the full parameter list (reproducing the relevant values rather than only citing Ref. [31]) and a minimal robustness check of the softening-to-fluidization trend.
minor comments (5)
  1. [Sec. III B 1] The quantity σ_c in δσ_xy = σ_xy − σ_c (Fig. 4(d)) is never defined; it should be stated that σ_c is the Herschel-Bulkley yield stress σ_y.
  2. [Sec. II B, Eqs. (4)-(5)] The sentence concerning the zero-wavevector components, '−σ_xy(0,0) = σ_xy(0,0) = 0', is garbled and should be rewritten. In addition, the kernel 2 q_x q_y / q^2 has a direction-dependent limit as q → 0, so setting the active stress at q = 0 to zero is a specific regularization of the uniform mode that should be stated and justified.
  3. [Appendix B, Eq. (B1)] In Eq. (B1) the second unit vector is printed as e_x; it should be e_y.
  4. [Sec. IV] The concluding paragraph asserts that at low amplitudes plastic events 'remain synchronized with the breathing cycle' and at higher amplitudes 'span multiple cycles,' but no phase-resolved or multi-cycle analysis is presented in Sec. III; either add the supporting analysis or soften these claims.
  5. [References] Ref. [25] cites the arXiv preprint of the Bonn et al. yield-stress review; the published version (Rev. Mod. Phys. 89, 035005 (2017)) should be cited.

Circularity Check

0 steps flagged · score 1.0 of 10

No circular derivation: the fluidization threshold and rheology are outputs of MD simulation, the mesoscale model is independently constructed from linear elasticity, and the cited prior threshold is an external benchmark.

full rationale

The paper's central claims are not circular. The microscopic model prescribes only the pair potential (Eq. 1), the breathing rule d_i(t)=d_i0(1+a cos(ωt+ψ_i)) (Eq. 2), and athermal Langevin dynamics (Eq. 3). The plastic-event counts, von Mises stress, flow curves, and yield stresses are all measured outputs of the simulation, not inputs. The critical amplitude a*=0.0506 is obtained by fitting the simulated σ_y(a) to (a*−a)^ζ (Sec. III B 1, Fig. 4(c)), and the independent observation of a Newtonian viscosity plateau at large a corroborates the fluidization without being fed into the fit. The comparison with a_c=0.0498 uses Ref. [23] (Tjhung and Berthier), which is external to the present authors and is used as a benchmark, not as a premise. The mesoscale AEPM is built from the linear-elastic response to dilating inclusions derived in Appendix B, plus standard Eshelby and elasto-plastic ingredients (Refs. [31,32]); no MD flow-curve data are injected into the EPM, and the agreement is explicitly qualitative. Self-citations (Refs. [30], [31], [32]) concern numerical methods and model framework and are not load-bearing for the fluidization claim. The Herschel-Bulkley extrapolation at shear rates near 1e-7 raises a legitimate steady-state/error-bar concern, but that is a question of statistical convergence and fit robustness, not circular identification of inputs with outputs.

Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

The simulation is a direct numerical experiment, so its main claim is not derived from an external theory. The listed free parameters are either mesoscale-model constants that are never specified or fit parameters in the phenomenological Herschel-Bulkley threshold analysis. The axioms are standard model choices plus the incompressibility and point-inclusion assumptions in the EPM. No new physical particles, forces, or conserved quantities are introduced; the active stress field is a modeling construct built from standard elasticity.

free parameters (5)
  • EPM yield threshold distribution (mean and width)
    Needed in Eq. 10 for the local yielding rule; mean and width are never reported in the paper.
  • EPM relaxation timescale tau and elastic recovery time tau_el
    Govern the plastic dynamics in Eq. 10 and the recovery rule; values are not given.
  • EPM active-strain amplitude and forcing schedule
    The active deformation field gamma_act is imposed but its amplitude and spatial pattern are not quantified in the text.
  • Herschel-Bulkley parameters (sigma_y, A, beta) per amplitude a = sigma_y values shown in Fig. 4(c)
    Fitted to each flow curve in Fig. 4(a); the paper does not report A or beta values or fit uncertainties.
  • Rigidity threshold a* and exponent zeta = a* = 0.0506, zeta = 0.39
    Obtained by fitting sigma_y(a) with (a* - a)^zeta; the functional form is assumed and only a few data points are used.
assumptions (5)
  • domain assumption Uniformly distributed breathing phases psi_i = 2 pi i/N keep the instantaneous packing fraction constant (Eq. 2).
    The model imposes a global phase distribution that suppresses density fluctuations; the authors do not test correlated phases.
  • domain assumption Athermal Langevin dynamics with harmonic repulsive interactions (Eqs. 1 and 3) captures the mechanical response of dense amorphous solids.
    Standard model choice from prior literature (Refs. [23,28]); the paper does not validate it against experiments.
  • domain assumption A contact change between stroboscopic frames is a faithful definition of a plastic event.
    The authors verify agreement with mean-squared-displacement and potential-energy maps in Fig. 2, but the equivalence is only demonstrated for a few amplitudes.
  • ad hoc to paper The mesoscale EPM assumes an incompressible medium and uses point-like active inclusions with the stress at the inclusion center set to zero.
    The authors explicitly note the origin-value assumption is 'not very realistic' and defer more realistic local stress rules to future work (Sec. II B).
  • standard math Linear elasticity solutions (Eshelby kernel and the dilation-inclusion response) apply to the mesoscale plastic and active stress propagation.
    Uses standard continuum mechanics from earlier elasto-plastic model literature (Ref. [32] and Appendix B).

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Cite this review

Pith. "Pith review of Influence of active breathing on rheology and jamming of amorphous solids: insights from microscopic and mesoscale analysis." pith.science (2026). https://pith.science/paper/57LQD7M7

@misc{pith2026250514520,
  author       = {Pith},
  title        = {Pith review of: Influence of active breathing on rheology and jamming of amorphous solids: insights from microscopic and mesoscale analysis},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/57LQD7M7}},
  note         = {Machine review of arXiv:2505.14520}
}
read the original abstract

We study the flow behavior and unjamming transition in dense assemblies of actively deforming particles that periodically change size, a process that we refer to as breathing. Using extensive molecular dynamics simulations and a complementary mesoscale elasto-plastic model, we explore how this internal activity influences plasticity and rheology. At low amplitudes of breathing, the system remains jammed and displays localized, reversible rearrangements. As the amplitude of the breathing increases beyond a critical threshold, the system undergoes an activity-induced fluidization marked by a surge in plastic events and a drop in yield stress. The flow curve analysis reveals a transition from yield-stress behavior to Newtonian flow at high activity. The mesoscale model captures these trends and provides insight into the role of stress redistribution due to local active deformations. Our findings highlight the potential of internal active driving to tune the mechanical state of amorphous materials without external forcing.

Figures

Figures reproduced from arXiv: 2505.14520 by the authors.

Figure 1
Figure 1. FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3: (Top) [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p007_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7 [PITH_FULL_IMAGE:figures/full_fig_p008_7.png]

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Reviewed August 7, 2026 · model on record in the stance chip above.