REVIEW 4 major objections 4 minor 48 references
Phase separation in a mixture of proliferating and motile active matter
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read In a dense binary mixture of growing and motile cells, the motile cells condense into a single dense cluster when their self-propulsion is weak relative to homeostatic pressure, a transition set by the ratio $M/F_{\max}$ and opposite in…
desk verdict Genuinely new phenomenological observation of growth-induced condensation in a repulsive binary active mixture, but the causal pair-attraction story is not yet established and 'new type of phase transition' overstates the current evidence. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central mechanism is the effective interaction between motile particles mediated by the proliferating bath. A growing particle's division and pressure-induced removal creates a local turnover imbalance (net surplus of births in a ring around a tracer, net surplus of removals outside it), which advects bath particles and produces a short-range attraction between two tracer particles at distances below about two cell widths. The paper quantifies this as an effective potential $V(\delta) = -(2D_t/\mu)\log P_s(\delta)$ obtained from the steady-state pair distance distribution of tracers via the Fokker-Planck equation, and models single-particle bath effects as an active Brownian particle with fitted translational diffusion, self-propulsion velocity, and persistence time. In the reduced model, this potential plus the ABP parameters reproduces the condensation transition.
What would settle it
Measure the pair interaction between motile particles inside a dense cluster of the full two-component model: if the inferred attraction weakens, vanishes, or reverses at cluster densities, the pairwise-medium mechanism is not what drives condensation. A cleaner test: fix the homeostatic pressure, sweep the self-propulsion force across the $M/F_{\max}$ threshold, and check whether the fraction of motile particles in clusters drops sharply at the predicted value.
Extended reading notes
Core claim
The central claim is that a dense binary mixture of growing and motile particles with exclusively repulsive contact forces spontaneously phase separates: the motile particles condense into a single dense cluster at high homeostatic pressure and weak self-propulsion, and mix uniformly when motility is strong. The transition is governed by the ratio $M/F_{\max}$ of the self-propulsion force to the axial force threshold that sets homeostatic pressure, and the phase-separated state is the opposite of MIPS because here self-propulsion acts to dissolve, not create, clusters. The condensation is caused by interactions between motile particles that are mediated by the growing phase: single tracers locally bias the turnover of the growing bath, and pairs of tracers experience a short-range effective attraction, which the authors encode in an effective potential. A single-component model of active Brownian particles with this effective attraction qualitatively reproduces the transition.
Load-bearing premise
The explanation assumes that the effective attraction measured for two isolated tracer particles in the growing bath is the same interaction that acts within a dense many-body cluster, even though larger objects perturb the bath more strongly and particles inside clusters are shielded from bath noise.
Editorial extensions
If this is right
- In dense cellular mixtures with only steric repulsion, proliferating cells can drive the segregation of a weakly motile subpopulation into compact clusters, without any attractive biochemical signaling.
- The phase boundary is set by $M/F_{\max}$: increasing self-propulsion relative to homeostatic pressure dissolves clusters, so tuning either growth-induced pressure or cell motility controls clustering.
- The condensation is not MIPS: in single-component active matter, faster self-propulsion typically promotes phase separation, whereas here it suppresses it.
- The phenomenon can be captured by a single-component active Brownian particle model with an effective short-range attraction, so the essential physics is a motility-independent effective interaction from the growing medium.
- The results suggest biofilms and tumors, which contain both growing and motile populations, should be interpreted as mixed active matter systems whose spatial organization can be controlled by mechanical homeostasis.
Reading between the lines
- The effective attraction between tracers likely is a nonequilibrium, fluctuation-induced (Casimir-like) force generated by the mechanical noise of the growing bath; if so, its magnitude and range should depend on the bath's turnover statistics and could be tuned by changing division rate or removal threshold.
- Because larger clusters perturb the bath more strongly (as the paper's Supplemental Fig. S4 indicates), the pairwise, environment-independent interaction measured for isolated tracers will underestimate cluster growth; the coarsening speed and cluster stability may depend on cluster size in a way not captured by the single-component model.
- A direct testable prediction: in an experimental or simulated system where growth pressure is held fixed, increasing the swimming speed of the motile subpopulation across the $M/F_{\max}$ threshold should sharply reduce the fraction of cells in clusters; this could be probed in bacterial mixtures with adjustable flagellar activity.
- The model suggests that in tumors, cells undergoing EMT (motile) might be spatially sorted by the proliferating bulk through purely mechanical means, which could influence invasion patterns; testing this would require measuring homeostatic pressure and motile cell speed in tissue spheroids.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript studies a dense binary mixture of two types of spherocylindrical particles: proliferating particles that grow, divide, and are removed when their compression force exceeds a threshold Fmax, and non-growing motile particles that self-propel with force M. In two-component simulations with purely repulsive contact forces, the authors observe that motile particles condense into a dense cluster for large Fmax and small M, and they report a data collapse when the abscissa is rescaled to M/Fmax. To explain this, they measure tracer dynamics in the growing bath, fit the tracer mean-squared displacement to an effective active Brownian particle (ABP) model, infer an effective pair potential from the steady-state distance distribution of two tracers, and then simulate a single-component ABP model with this attraction. The effective model qualitatively reproduces the condensation-to-mixed transition. The authors conclude that growth-induced effective interactions cause condensation and describe the result as a new type of phase transition, in contrast to motility-induced phase separation.
Significance. If the direct two-component observation is robust, this is an intriguing and original phenomenon: a dense binary mixture with only repulsive interactions phase-separates because the proliferating phase mediates an effective attraction between motile particles, and stronger self-propulsion destroys the cluster, the opposite of MIPS. The direct simulations are self-contained and clearly presented, the model is minimal, and the code availability statement is a strength. The quantitative mechanism claim, however, rests on an effective pair interaction measured from isolated tracers and transplanted into a single-component model without independent validation, and the phase-transition characterization lacks error bars and finite-size scaling. The central qualitative observation is plausible, but the mechanistic and 'new phase transition' claims need additional support before they can be accepted.
major comments (4)
- [§Model, Eq. (2), Fig. 3c] The effective pair potential V(δ) is measured from two isolated tracer particles in the growing bath and then used as an environment-independent interaction in the one-component ABP model. The Discussion explicitly concedes that larger non-growing objects induce much longer-ranged perturbations of the growing bath (SM Fig. S4) and that motile particles inside clusters become shielded from bath noise; these are exactly the many-body effects that would change the interaction at cluster-relevant densities. The manuscript therefore does not establish that the dilute-pair attraction is the mechanism causing dense-cluster condensation. The authors should either test whether the pair potential survives at cluster-relevant densities and ranges, for example by measuring forces near pre-formed clusters of varying size, or explicitly reframe the single-component model as a minimal effective description rather than evidence for the causal mechanism.
- [§Results, Fig. 1d and Fig. 4b] The condensed fraction is plotted without error bars, and no system-size scaling or time-convergence analysis is provided. Since the paper states that single clusters do not evaporate once formed (SM Fig. S3), the observed transition from 'full condensation' to 'mixed' could be influenced by the finite simulation time and by the finite box size of 80×80 rather than reflecting a genuine phase transition in the thermodynamic limit. The authors should add error bars from independent runs, test at least two or three larger system sizes, and report the time dependence of the condensed fraction to separate kinetic arrest from steady-state coexistence.
- [§Effective model, Figs. 3b and 4b] The single-component ABP model is parameterized from the same two-component data that it is then compared against: MSD fits provide Dt, v, and trot, and the pair-distance distribution provides the potential well and barrier. Its qualitative reproduction of the transition is therefore not an independent confirmation of the mechanism. The authors should provide a falsifiable prediction of the effective model that can be tested in the full two-component simulations, such as the scaling of cluster size with M/Fmax, the density profile at the interface, or the coarsening exponent, and verify that prediction quantitatively.
- [Abstract and Discussion] The claim of 'a new type of phase transition' goes beyond what is demonstrated. No order parameter analysis, finite-size scaling, or comparison with existing condensation transitions (e.g., diffusivity-edge condensation) is presented, and the mechanism itself remains unresolved as noted above. The authors should either provide the missing phase-transition characterization or soften the claim to a condensation transition in a binary mixture with purely repulsive interactions, contrasting with MIPS.
minor comments (4)
- [Discussion] The phrase 'single-component APB model' appears to be a typo and should read 'ABP model'.
- [§Model, Eq. (2)] The mobility μ that appears in Eq. (2) is not defined in the main text; please define it and state its relation to Dt.
- [§Model, Eq. (3)] The notation '0 < r ≤ r2 (1 + sqrt(V2/ΔV))' is confusing because r2 has not been defined as a separate length scale; please clarify the domain and the role of the barrier height V2.
- [§Results, Fig. 1d] The clustering criterion of 'more than 300 particles' should be justified or accompanied by a sensitivity analysis, since the measured condensed fraction may depend strongly on this threshold.
Circularity Check
No significant circularity: the two-component phase separation is directly simulated, and the effective single-component model predicts the transition from micro-scale fitted parameters rather than being fitted to the condensation itself.
full rationale
The full two-component phase separation (Fig. 1) is a direct, self-contained simulation result: the model is described fully in the text ('All particles have the same width... Growing cells elongate linearly in time... Total cell density is kept approximately constant by removing growing cells when their axial compression force exceeds a threshold Fmax') and the code is publicly available (InPartS/InPartSBiome), so this observation rests on no fitted input and cannot be circular. The effective single-component model is parameterized exclusively from micro-scale measurements: the single-tracer MSD fitted to Eq. (1) provides Dt, v, trot ('Fitting this function to the numerically measured mean squared displacement allows us to obtain estimates for the effective ABP parameters'), and the pair potential comes from the two-tracer steady-state distance distribution via Eq. (2), V(delta) = -(2Dt/mu) log Ps(delta), with Eq. (3) fitted to that pair-level V(r). None of these fits use the cluster-fraction curves (Fig. 1d versus Fig. 4b); the many-body phase separation is predicted by running the parameterized ABP model, and the paper reports the transition appears 'at slightly different values of M than in the two-component model', a quantitative mismatch confirming the transition was not forced by construction. The claim that condensation is 'caused by interactions between motile particles induced by the growing phase' is supported by an independently measured two-tracer attraction (Fig. 2e) transplanted into the effective model, and the effective model reproduces the non-trivial direction of the effect (condensation at low self-propulsion, opposite to MIPS), so the recapitulation carries real content. Self-citations ([37] model framework, [42,43] ballistic short-time motion, [9] diffusivity-edge condensation) are not load-bearing: the model is described in the paper itself, the code is public, and those citations support minor or speculative points. Finally, the Discussion's admission that the ABP model omits longer-ranged bath perturbations near large clusters (SM Fig. S4) and noise shielding inside clusters is an honest limitation of the mechanism attribution, but this is a transferability/correctness risk, not circularity: the pair potential is measured in few-body configurations that do not contain the condensation, and it is not defined in terms of, or fitted to, the transition it is used to explain.
Assumptions & free parameters
free parameters (5)
- Dt (translational diffusion) =
varies with Fmax
- trot (persistence time) =
varies with Fmax
- v (effective self-propulsion velocity) =
varies with M and Fmax
- Delta-V (potential well depth) =
not reported
- V2 (barrier height) =
not reported
assumptions (4)
- domain assumption The steady-state pair-distance distribution P_s(delta) of two tracers in the growing bath is related to an effective interaction potential by the equilibrium relation V(delta) = -(2 Dt / mu) log P_s(delta).
- domain assumption Effective interactions between motile particles are pairwise, isotropic, and independent of the local environment, including cluster size and density.
- ad hoc to paper The effective single-particle dynamics can be represented by an ABP with parameters Dt, v, trot, and with Dt and trot independent of M.
- domain assumption Pressure-induced removal at axial force threshold Fmax maintains homeostasis and is a valid proxy for biological density control.
Cite this review
Pith. "Pith review of Phase separation in a mixture of proliferating and motile active matter." pith.science (2026). https://pith.science/paper/PLSSJWDV
@misc{pith2026250605288,
author = {Pith},
title = {Pith review of: Phase separation in a mixture of proliferating and motile active matter},
year = {2026},
howpublished = {\url{https://pith.science/paper/PLSSJWDV}},
note = {Machine review of arXiv:2506.05288}
}
read the original abstract
Proliferation and motility are ubiquitous drivers of activity in biological systems. Here, we study a dense binary mixture of motile and proliferating particles with exclusively repulsive interactions, where homeostasis in the proliferating subpopulation is maintained by pressure-induced removal. Using computer simulations, we show that phase separation emerges naturally in this system at high density and weak enough self-propulsion. We show that condensation is caused by interactions between motile particles induced by the growing phase, and recapitulate this behavior in an effective model of only motile particles with attractive interactions. Our results establish a new type of phase transition and pave a way to reinterpret the physics of dense cellular populations, such as bacterial colonies or tumors, as systems of mixed active matter.
Figures
Reference graph
Works this paper leans on
-
[1]
H. H. Wensink, J. Dunkel, S. Heidenreich, K. Drescher, R. E. Goldstein, H. L¨ owen, and J. M. Yeomans, Pro- ceedings of the National Academy of Sciences109, 14308–14313 (2012)
work page 2012
-
[2]
R. Golestanian, inActive Matter and Nonequilibrium Statistical Physics: Lecture Notes of the Les Houches Summer School: Volume 112, September 2018(Oxford University Press, 2022)
work page 2018
-
[3]
Gompper, R
G. Gompper, R. G. Winkler, T. Speck, A. Solon, C. Nar- dini, F. Peruani, H. L¨ owen, R. Golestanian, U. B. Kaupp, L. Alvarez, T. Kiørboe, E. Lauga, W. C. K. Poon, A. DeSimone, S. Mui˜ nos-Landin, A. Fischer, N. A. S¨ oker, F. Cichos, R. Kapral, P. Gaspard, M. Ripoll, F. Sagues, A. Doostmohammadi, J. M. Yeomans, I. S. Aranson, C. Bechinger, H. Stark, C. K....
2020
-
[4]
M. C. Marchetti, J. F. Joanny, S. Ramaswamy, T. B. Liverpool, J. Prost, M. Rao, and R. A. Simha, Reviews of Modern Physics85, 1143–1189 (2013)
work page 2013
-
[5]
M. E. Cates and J. Tailleur, Annual Review of Condensed Matter Physics6, 219–244 (2015)
work page 2015
-
[8]
J¨ ulicher, S
F. J¨ ulicher, S. W. Grill, and G. Salbreux, Reports on Progress in Physics81, 076601 (2018)
2018
- [9]
- [10]
Show all 48 references
-
[11]
Gr´ egoire and H
G. Gr´ egoire and H. Chat´ e, Physical Review Letters92, 10.1103/physrevlett.92.025702 (2004)
2004 doi
-
[12]
Toner and Y
J. Toner and Y. Tu, Physical Review Letters75, 4326–4329 (1995)
1995
-
[13]
Theurkauff, C
I. Theurkauff, C. Cottin-Bizonne, J. Palacci, C. Ybert, and L. Bocquet, Physical Review Letters108, 10.1103/physrevlett.108.268303 (2012)
2012 doi
-
[14]
Buttinoni, J
I. Buttinoni, J. Bialk´ e, F. K¨ ummel, H. L¨ owen, C. Bechinger, and T. Speck, Physical Review Letters 110, 10.1103/physrevlett.110.238301 (2013)
2013 doi
-
[15]
Hallatschek, S
O. Hallatschek, S. S. Datta, K. Drescher, J. Dunkel, J. El- geti, B. Waclaw, and N. S. Wingreen, Nature Reviews Physics , 1 (2023), publisher: Nature Publishing Group
2023
-
[16]
Ranft, M
J. Ranft, M. Basan, J. Elgeti, J.-F. Joanny, J. Prost, and F. J¨ ulicher, Proceedings of the National Academy of Sciences107, 20863–20868 (2010)
2010
-
[17]
F. D. C. Farrell, O. Hallatschek, D. Marenduzzo, and B. Waclaw, Physical Review Letters111, 10.1103/Phys- RevLett.111.168101 (2013)
2013 doi
-
[18]
M. A. A. Grant, B. Wac law, R. J. Allen, and P. Cicuta, Journal of The Royal Society Interface11, 20140400 (2014)
2014
-
[19]
Dell’Arciprete, M
D. Dell’Arciprete, M. L. Blow, A. T. Brown, F. D. C. Farrell, J. S. Lintuvuori, A. F. McVey, D. Maren- duzzo, and W. C. K. Poon, Nature Communications9, 10.1038/s41467-018-06370-3 (2018)
2018 doi
-
[20]
Y. I. Yaman, E. Demir, R. Vetter, and A. Kocabas, Nature Communications10, 10.1038/s41467-019-10311-z (2019)
2019 doi
-
[21]
Zhang, J
Q. Zhang, J. Li, J. Nijjer, H. Lu, M. Kothari, R. Alert, T. Cohen, and J. Yan, Proceedings of the National Academy of Sciences118, 10.1073/pnas.2107107118 (2021)
2021 doi
-
[22]
Y. G. Pollack, P. Bittihn, and R. Golestanian, New Jour- nal of Physics24, 073003 (2022)
2022
-
[23]
Isensee, L
J. Isensee, L. Hupe, R. Golestanian, and P. Bit- tihn, Journal of The Royal Society Interface19, 10.1098/rsif.2022.0512 (2022)
2022
-
[24]
Steinberg, A
N. Steinberg, A. Keren-Paz, Q. Hou, S. Doron, K. Yanuka-Golub, T. Olender, R. Hadar, G. Rosenberg, R. Jain, J. C´ amara-Almir´ on, D. Romero, S. van Teeffelen, and I. Kolodkin- Gal, Science Signaling13, eaaw8905 (2020), https://www.science.org/doi/pdf/10.1126/scisignal.aaw8905
2020 doi
-
[25]
Ravel, M
G. Ravel, M. Bergmann, A. Trubuil, J. Deschamps, R. Briandet, and S. Labarthe, eLife11, e76513 (2022)
2022
-
[26]
J. P. Thiery, Nature Reviews Cancer2, 442 (2002)
2002
-
[27]
Stenhammar, R
J. Stenhammar, R. Wittkowski, D. Marenduzzo, and M. E. Cates, Phys. Rev. Lett.114, 018301 (2015)
2015
-
[28]
Wysocki, R
A. Wysocki, R. G. Winkler, and G. Gompper, New Jour- nal of Physics18, 123030 (2016)
2016
-
[29]
Dolai, A
P. Dolai, A. Simha, and S. Mishra, Soft Matter14, 6137 (2018)
2018
-
[30]
S. R. McCandlish, A. Baskaran, and M. F. Hagan, Soft Matter8, 2527 (2012)
2012
-
[31]
S. Saha, J. Agudo-Canalejo, and R. Golestanian, Physical Review X10, 10.1103/physrevx.10.041009 (2020)
2020 doi
-
[32]
Ouazan-Reboul, J
V. Ouazan-Reboul, J. Agudo-Canalejo, and R. Golesta- nian, Nature Communications14, 4496 (2023)
2023
-
[33]
Dinelli, J
A. Dinelli, J. O’Byrne, A. Curatolo, Y. Zhao, P. Sollich, and J. Tailleur, Nature Communications14, 7035 (2023)
2023
-
[34]
Ghosh, J
P. Ghosh, J. Mondal, E. Ben-Jacob, and H. Levine, Proceedings of the National Academy of Sciences112, 201504948 (2015)
2015
-
[35]
Kolb and D
T. Kolb and D. Klotsa, Soft Matter16, 1967 (2020), publisher: The Royal Society of Chemistry
2020
-
[36]
Q. J. S. Braat, C. Storm, and L. M. C. Janssen, Physical Review E110, 10.1103/physreve.110.064401 (2024)
2024 doi
-
[37]
Hupe*, Y
L. Hupe*, Y. G. Pollack*, J. Isensee, A. Amiri, R. Golestanian, and P. Bittihn, A minimal model of smoothly dividing disk-shaped cells (2024), arXiv:2409.01959 [cond-mat]
2024 arXiv
-
[38]
See supplemental material for details on the numerical model, the clustering analysis, and the fitting procedure for the effective potential, as well as supporting evidence for the coarsening dynamics and bath dynamics near larger objects
-
[39]
To keep the analysis consistent with subsequent velocity calculations, we take the spatial location to be defined by the corresponding particles’ center positions
-
[40]
J. R. Howse, R. A. L. Jones, A. J. Ryan, T. Gough, R. Vafabakhsh, and R. Golestanian, Phys. Rev. Lett.99, 048102 (2007)
2007
-
[41]
Bechinger, R
C. Bechinger, R. Di Leonardo, H. L¨ owen, C. Reichhardt, G. Volpe, and G. Volpe, Reviews of Modern Physics88, 045006 (2016)
2016
-
[42]
Sunkel, L
T. Sunkel, L. Hupe, and P. Bittihn, Communications Physics8, 1 (2025)
2025
-
[43]
S. R. Lish, L. Hupe, R. Golestanian, and P. Bit- tihn, Isovolumetric dividing active matter (2024), arXiv:2409.20481 [cond-mat]
2024 arXiv
-
[44]
Kardar and R
M. Kardar and R. Golestanian, Rev. Mod. Phys.71, 1233 (1999)
1999
-
[45]
Mahault and R
B. Mahault and R. Golestanian, New Journal of Physics 22, 063045 (2020)
2020
-
[46]
Waclaw, J
B. Waclaw, J. Sopik, W. Janke, and H. Meyer-Ortmanns, Journal of Statistical Mechanics: Theory and Experi- ment2009, P10021 (2009)
2009
-
[47]
Waclaw and M
B. Waclaw and M. R. Evans, Physical Review Letters 108, 070601 (2012)
2012
-
[48]
Bezanson, A
J. Bezanson, A. Edelman, S. Karpinski, and V. Shah, SIAM Rev.59, 65 (2017)
2017
-
[49]
L. Hupe, J. Isensee, and P. Bittihn, InPartS: Interacting Particle Simulations in Julia, InPartS.org (2022)
2022
-
[50]
Danisch and J
S. Danisch and J. Krumbiegel, Journal of Open Source Software6, 3349 (2021)
2021
Reviewed August 7, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.