Unifying hydrodynamic theory for motility-regulated active matter: from single particles to interacting polymers
Pith reviewed 2026-05-10 16:54 UTC · model grok-4.3
The pith
The autocorrelation tensor of orientations unifies the large-scale hydrodynamics of motility-regulated active systems from particles to polymers.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We derive a closed hydrodynamics for scalar active matter with spatially-regulated motility, under general hypotheses for the microscopic dynamics of the particles' orientations. We show that, at large scales, the contribution of the latter is entirely captured by the autocorrelation tensor of the orientations. This allows us to establish a macroscopic equivalence within a broad class of motility-regulated active systems, from single particles to active polymers. Our formalism allows us to reveal a new form of motility-induced phase separation for quorum-sensing active polymers, which we term anti-MIPS, where dense phases exhibit enhanced activity relative to dilute regions. Our theory shows
What carries the argument
The orientation autocorrelation tensor, which encodes all microscopic reorientation details and closes the hydrodynamic equations for density and polarization at large scales.
If this is right
- A single hydrodynamic theory describes both single particles and active polymers with spatially regulated motility.
- Quorum-sensing polymers undergo anti-MIPS in which dense regions become more active than dilute ones.
- Multiple distinct transition pathways to phase separation exist for agents that possess internal structure.
- The equations close without retaining the full microscopic orientation distribution.
Where Pith is reading between the lines
- Matching the orientation autocorrelation tensor between different experimental realizations could demonstrate macroscopic equivalence even when the agents differ in size or connectivity.
- Anti-MIPS may allow polymers to accumulate in regions of higher activity, suggesting a route to self-concentration that is unavailable to point particles.
- The same reduction technique could be applied to other regulation mechanisms such as alignment or external gradients.
Load-bearing premise
The microscopic orientation dynamics must obey general hypotheses that let their entire large-scale contribution be summarized by the autocorrelation tensor alone.
What would settle it
Two motility-regulated systems that share identical orientation autocorrelation tensors yet produce measurably different large-scale density patterns or phase boundaries would disprove the closure.
Figures
read the original abstract
Understanding how microscopic motility shapes emergent collective behaviors is a challenging task in active matter, especially when self-propulsion is regulated by external cues or via quorum-sensing interactions. To address this problem, we derive a closed hydrodynamics for scalar active matter with spatially-regulated motility, under general hypotheses for the microscopic dynamics of the particles' orientations. We show that, at large scales, the contribution of the latter is entirely captured by the autocorrelation tensor of the orientations. This allows us to establish a macroscopic equivalence within a broad class of motility-regulated active systems, from single particles to active polymers. Our formalism allows us to reveal a new form of motility-induced phase separation for quorum-sensing active polymers, which we term anti-MIPS, where dense phases exhibit enhanced activity relative to dilute regions. Our theory shows that anti-MIPS generically arises for motility-regulated agents with internal structure, uncovering the existence of several distinct transition pathways.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript derives a closed hydrodynamic description for scalar active matter with spatially regulated motility. Under general hypotheses on the microscopic orientation dynamics, it claims that all orientation contributions reduce exactly to the orientation autocorrelation tensor at large scales. This reduction is used to establish a macroscopic equivalence across a broad class of systems, from single particles to interacting active polymers. The theory is then applied to quorum-sensing polymers, where it predicts a novel 'anti-MIPS' instability in which dense phases exhibit higher activity than dilute regions, along with multiple transition pathways.
Significance. If the central reduction to the autocorrelation tensor is rigorously justified, the work would supply a unifying hydrodynamic framework for motility-regulated active matter that encompasses both point particles and extended objects. The identification of anti-MIPS as a generic consequence of internal structure offers a new, falsifiable prediction that could guide experiments on active filaments or bacterial chains. The absence of free parameters in the closure and the explicit mapping from microscopic hypotheses to macroscopic equations are notable strengths.
major comments (2)
- [§3.2] §3.2 (Closure for interacting polymers): The claim that chain connectivity and position-orientation cross-correlations are fully absorbed into the single-particle orientation autocorrelation tensor is not demonstrated explicitly. The derivation for free particles does not automatically extend to polymers; kinematic constraints along the chain can generate additional hydrodynamic terms that survive coarse-graining. Without an explicit calculation showing these terms vanish or are re-expressed by the tensor, the asserted equivalence between particles and polymers remains unproven.
- [Eq. (27)] Eq. (27) and surrounding text (anti-MIPS dispersion relation): The linear stability analysis for quorum-sensing polymers assumes the same closure as the single-particle case. If the polymer-specific correlations identified above are present, they would modify the effective motility regulation term and could alter the sign of the instability or the location of the transition. A side-by-side comparison of the dispersion relations with and without connectivity corrections is required.
minor comments (2)
- [Abstract] Abstract: The phrase 'anti-MIPS' is introduced without a one-sentence definition; adding a brief parenthetical description would improve accessibility for readers unfamiliar with the standard MIPS literature.
- [Notation] Notation: The autocorrelation tensor is denoted differently in the general derivation and in the polymer section; consistent symbols and an explicit definition in a single location would reduce confusion.
Simulated Author's Rebuttal
We thank the referee for their thorough review and constructive feedback on our manuscript. We address each of the major comments below, providing clarifications and indicating the revisions we plan to implement.
read point-by-point responses
-
Referee: [§3.2] §3.2 (Closure for interacting polymers): The claim that chain connectivity and position-orientation cross-correlations are fully absorbed into the single-particle orientation autocorrelation tensor is not demonstrated explicitly. The derivation for free particles does not automatically extend to polymers; kinematic constraints along the chain can generate additional hydrodynamic terms that survive coarse-graining. Without an explicit calculation showing these terms vanish or are re-expressed by the tensor, the asserted equivalence between particles and polymers remains unproven.
Authors: We appreciate the referee pointing out the need for a more explicit demonstration in the polymer case. The general hypotheses on orientation dynamics in Section 2 are designed to encompass systems with internal structure, including polymers, where the autocorrelation tensor is computed from the constrained dynamics of connected segments. This ensures that chain connectivity effects are incorporated into the tensor rather than generating independent hydrodynamic terms. To strengthen the presentation, we will revise §3.2 to include a step-by-step extension of the derivation to polymers and add an appendix with the explicit calculation showing the absorption of cross-correlations. revision: yes
-
Referee: [Eq. (27)] Eq. (27) and surrounding text (anti-MIPS dispersion relation): The linear stability analysis for quorum-sensing polymers assumes the same closure as the single-particle case. If the polymer-specific correlations identified above are present, they would modify the effective motility regulation term and could alter the sign of the instability or the location of the transition. A side-by-side comparison of the dispersion relations with and without connectivity corrections is required.
Authors: We agree that a direct comparison would enhance the robustness of our claims. However, because the closure relation is derived under general hypotheses that already account for polymer connectivity via the orientation autocorrelation tensor, the hydrodynamic equations and thus the dispersion relation in Eq. (27) are identical for both single particles and polymers. In the revised manuscript, we will add a paragraph comparing the dispersion relations explicitly, confirming that no additional terms arise from connectivity and that the anti-MIPS instability persists unchanged. revision: yes
Circularity Check
Derivation self-contained under general hypotheses; no reduction to inputs by construction
full rationale
The paper derives closed hydrodynamics by positing general hypotheses on microscopic orientation dynamics and showing via explicit coarse-graining that their large-scale effects reduce to the orientation autocorrelation tensor, yielding equivalence across particles and polymers. This reduction is presented as a derived consequence rather than an input definition or fitted parameter; the tensor is obtained from the microscopic level independently of the target hydrodynamic equations. No load-bearing self-citations, ansatzes smuggled via prior work, or renaming of known results appear in the provided derivation outline. The central unification follows directly from the stated hypotheses without the output being forced by redefinition of inputs.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption General hypotheses for the microscopic dynamics of the particles' orientations
Reference graph
Works this paper leans on
-
[1]
E. O. Budrene and H. C. Berg, Complex patterns formed by motile cells of escherichia coli, Nature349, 630 (1991)
work page 1991
-
[2]
M. J. Schnitzer, Theory of continuum random walks and application to chemotaxis, Phys. Rev. E48, 2553 (1993)
work page 1993
-
[3]
H. C. Berg,E. coli in Motion(Springer, 2004)
work page 2004
- [4]
-
[5]
M. B. Miller and B. L. Bassler, Quorum sensing in bacte- ria, Annu. Rev. Microbiol.55, 165 (2001)
work page 2001
-
[6]
R. Daniels, J. Vanderleyden, and J. Michiels, Quorum sensing and swarming migration in bacteria, FEMS Mi- crobiol. Rev28, 261 (2004)
work page 2004
-
[7]
J. Tailleur and M. E. Cates, Statistical mechanics of interacting run-and-tumble bacteria, Phys. Rev. Lett.100, 218103 (2008)
work page 2008
-
[8]
M. E. Cates and J. Tailleur, Motility-induced phase sepa- ration, Annu. Rev. Condens. Matter Phys.6, 219 (2015)
work page 2015
-
[9]
A. P. Solon, J. Stenhammar, M. E. Cates, Y. Kafri, and J. Tailleur, Generalized thermodynamics of phase equi- libria in scalar active matter, Phys. Rev. E97, 020602 (2018)
work page 2018
-
[10]
H. Zhao, A. Kosmrlj, and S. S. Datta, Chemotactic motility-induced phase separation, Phys. Rev. Lett.131, 118301 (2023)
work page 2023
-
[11]
E. F. Keller and L. A. Segel, Initiation of slime mold aggregation viewed as an instability, J. Theor. Biol.26, 399 (1970). 6
work page 1970
-
[12]
P.-H. Chavanis, Critical mass of bacterial populations and critical temperature of self-gravitating brownian particles in two dimensions, Phys. A384, 392 (2007)
work page 2007
-
[13]
O.PohlandH.Stark,Dynamicclusteringandchemotactic collapse of self-phoretic active particles, Phys. Rev. Lett. 112, 238303 (2014)
work page 2014
-
[14]
S. Saha, R. Golestanian, and S. Ramaswamy, Clusters, asters, and collective oscillations in chemotactic colloids, Phys. Rev. E89, 062316 (2014)
work page 2014
-
[15]
J. O’Byrne and J. Tailleur, Lamellar to micellar phases and beyond: when tactic active systems admit free energy functionals, Phys. Rev. Lett.125, 208003 (2020)
work page 2020
-
[16]
Z. You, A. Baskaran, and M. C. Marchetti, Nonreciprocity as a generic route to traveling states, Proc. Natl. Acad. Sci. U.S.A.117, 19767 (2020)
work page 2020
-
[17]
S. Saha, J. Agudo-Canalejo, and R. Golestanian, Scalar active mixtures: The nonreciprocal cahn-hilliard model, Phys. Rev. X10, 041009 (2020)
work page 2020
-
[18]
A. Dinelli, J. O’Byrne, A. Curatolo, Y. Zhao, P. Sollich, and J. Tailleur, Non-reciprocity across scales in active mixtures, Nat. Commun.14, 7035 (2023)
work page 2023
-
[19]
V. Ouazan-Reboul, J. Agudo-Canalejo, and R. Golesta- nian, Self-organization of primitive metabolic cycles due to non-reciprocal interactions, Nat. Commun.14, 4496 (2023)
work page 2023
-
[20]
Y. Duan, J. Agudo-Canalejo, R. Golestanian, and B. Mahault, Dynamical pattern formation without self- attraction in quorum-sensing active matter: the interplay between nonreciprocity and motility, Phys. Rev. Lett. 131, 148301 (2023)
work page 2023
-
[21]
G. Pisegna, S. Saha, and R. Golestanian, Emergent polar order in nonpolar mixtures with nonreciprocal interac- tions, Proc. Natl. Acad. Sci. U.S.A.121, e2407705121 (2024)
work page 2024
-
[22]
J. Palacci, S. Sacanna, A. P. Steinberg, D. J. Pine, and P. M. Chaikin, Living crystals of light-activated colloidal surfers, Science339, 936 (2013)
work page 2013
-
[23]
T. Bäuerle, A. Fischer, T. Speck, and C. Bechinger, Self- organization of active particles by quorum sensing rules, Nat. Commun.9, 3232 (2018)
work page 2018
-
[24]
F. A. Lavergne, H. Wendehenne, T. Bäuerle, and C. Bechinger, Group formation and cohesion of active particles with visual perception–dependent motility, Sci- ence364, 70 (2019)
work page 2019
-
[25]
M. A. Fernandez-Rodriguez, F. Grillo, L. Alvarez, M. Rathlef, I. Buttinoni, G. Volpe, and L. Isa, Feedback- controlled active brownian colloids with space-dependent rotational dynamics, Nat. Commun.11, 4223 (2020)
work page 2020
-
[26]
C. Liu, X. Fu, L. Liu, X. Ren, C. K. Chau, S. Li, L. Xiang, H. Zeng, G. Chen, L.-H. Tang,et al., Sequential establish- ment of stripe patterns in an expanding cell population, Science334, 238 (2011)
work page 2011
-
[27]
J. Arlt, V. A. Martinez, A. Dawson, T. Pilizota, and W. C. Poon, Painting with light-powered bacteria, Nat. Commun.9, 768 (2018)
work page 2018
-
[28]
G. Frangipane, D. Dell’Arciprete, S. Petracchini, C. Maggi, F. Saglimbeni, S. Bianchi, G. Vizsnyiczai, M. L. Bernardini, and R. Di Leonardo, Dynamic density shaping of photokinetic e. coli, Elife7, e36608 (2018)
work page 2018
-
[29]
A. Curatolo, N. Zhou, Y. Zhao, C. Liu, A. Daerr, J. Tailleur, and J. Huang, Cooperative pattern formation in multi-component bacterial systems through reciprocal motility regulation, Nat. Phys.16, 1152 (2020)
work page 2020
-
[30]
R. F. Fox, Uniform convergence to an effective fokker- planck equation for weakly colored noise, Phys. Rev. A 34, 4525 (1986)
work page 1986
-
[31]
R. F. Fox, Functional-calculus approach to stochastic differential equations, Phys. Rev. A33, 467 (1986)
work page 1986
-
[32]
M. E. Cates and J. Tailleur, When are active brownian particles and run-and-tumble particles equivalent? conse- quences for motility-induced phase separation, EPL101, 20010 (2013)
work page 2013
-
[33]
R. Wittmann, C. Maggi, A. Sharma, A. Scacchi, J. M. Brader, and U. M. B. Marconi, Effective equilibrium states in the colored-noise model for active matter i. pairwise forces in the fox and unified colored noise approximations, J. Stat. Mech.2017, 113207 (2017)
work page 2017
-
[34]
A. Dinelli, J. O’Byrne, and J. Tailleur, Fluctuating hy- drodynamics of active particles interacting via taxis and quorum sensing: static and dynamics, J. Phys. A: Math. Theor.57, 395002 (2024)
work page 2024
-
[35]
S. Bureković, F. De Luca, M. E. Cates, and C. Nardini, Active cahn–hilliard theory for non-equilibrium phase separation: quantitative macroscopic predictions and a microscopic derivation, arXiv preprint arXiv:2601.16539 (2026)
-
[36]
C. Kurzthaler, Y. Zhao, N. Zhou, J. Schwarz-Linek, C. De- vailly, J. Arlt, J.-D. Huang, W. C. Poon, T. Franosch, J. Tailleur,et al., Characterization and control of the run-and-tumble dynamics of escherichia coli, Phys. Rev. Lett.132, 038302 (2024)
work page 2024
-
[37]
R. Golestanian, T. Liverpool, and A. Ajdari, Designing phoretic micro-and nano-swimmers, New J. Phys.9, 126 (2007)
work page 2007
- [38]
-
[39]
I. Theurkauff, C. Cottin-Bizonne, J. Palacci, C. Ybert, and L. Bocquet, Dynamic clustering in active colloidal suspensions with chemical signaling, Phys. Rev. Lett.108, 268303 (2012)
work page 2012
-
[40]
G. Szamel, Self-propelled particle in an external potential: Existence of an effective temperature, Phys. Rev. E90, 012111 (2014)
work page 2014
- [41]
-
[42]
R. E. Isele-Holder, J. Elgeti, and G. Gompper, Self- propelled worm-like filaments: spontaneous spiral for- mation, structure, and dynamics, Soft Matter11, 7181 (2015)
work page 2015
-
[43]
A. Ghosh and N. S. Gov, Dynamics of active semiflexible polymers, Biophys. J.107, 1065 (2014)
work page 2014
- [44]
-
[45]
R. G. Winkler, J. Elgeti, and G. Gompper, Active poly- mers—emergent conformational and dynamical properties: A brief review, J. Phys. Soc. Jpn.86, 101014 (2017)
work page 2017
- [46]
-
[47]
C. Abaurrea Velasco, M. Abkenar, G. Gompper, and T. Auth, Collective behavior of self-propelled rods with quorum sensing, Phys. Rev. E98, 022605 (2018)
work page 2018
-
[48]
R. G. Winkler and G. Gompper, The physics of active polymers and filaments, J. Chem. Phys.153, 040901 7 (2020)
work page 2020
-
[49]
U. Pfreundt, J. Słomka, G. Schneider, A. Sengupta, F. Carrara, V. Fernandez, M. Ackermann, and R. Stocker, Controlled motility in the cyanobacterium trichodesmium regulates aggregate architecture, Science380, 830 (2023)
work page 2023
-
[50]
M. Dedenon, C. Blanch-Mercader, K. Kruse, and J. Elgeti, The importance of being discrete: fluctuations, defects, and density-orientation coupling in agent-based active nematics, New Journal of Physics28, 024401 (2026)
work page 2026
-
[51]
H. D. Vuijk, H. Merlitz, M. Lang, A. Sharma, and J.- U. Sommer, Chemotaxis of cargo-carrying self-propelled particles, Phys. Rev. Lett.126, 208102 (2021)
work page 2021
-
[52]
P. L. Muzzeddu, H. D. Vuijk, H. Löwen, J.-U. Sommer, and A. Sharma, Active chiral molecules in activity gradi- ents, J. Chem. Phys.157, 134902 (2022)
work page 2022
-
[53]
P. L. Muzzeddu, É. Roldán, A. Gambassi, and A. Sharma, Taxis of cargo-carrying microswimmers in traveling activ- ity waves, EPL142, 67001 (2023)
work page 2023
-
[54]
P. L. Muzzeddu, A. Gambassi, J.-U. Sommer, and A. Sharma, Migration and separation of polymers in nonuniform active baths, Phys. Rev. Lett.133, 118102 (2024)
work page 2024
-
[55]
B. Valecha, H. Vahid, P. L. Muzzeddu, J.-U. Sommer, and A. Sharma, Active transport of cargo-carrying and inter- connected chiral particles, Soft Matter21, 3384 (2025)
work page 2025
-
[56]
A. Dinelli and P. L. Muzzeddu, Multiscale perturbative approach to active matter with motility regulation, Com- panion paper (2026)
work page 2026
- [57]
-
[58]
C. W. Gardineret al.,Handbook of stochastic methods, Vol. 3 (springer Berlin, 2004)
work page 2004
-
[59]
G. Pavliotis and A. Stuart,Multiscale methods: averaging and homogenization(Springer Science & Business Media, 2008)
work page 2008
-
[60]
M. S. Green, Markoff random processes and the statistical mechanics of time-dependent phenomena. ii. irreversible processes in fluids, J. Chem. Phys.22, 398 (1954)
work page 1954
-
[61]
A. Sharma and J. M. Brader, Communication: Green- kubo approach to the average swim speed in active brow- nian systems, J. Chem. Phys.145, 161101 (2016)
work page 2016
-
[62]
S. Dal Cengio, D. Levis, and I. Pagonabarraga, Linear re- sponse theory and green-kubo relations for active matter, Phys. Rev. Lett.123, 238003 (2019)
work page 2019
- [63]
-
[64]
J. O’Byrne, Y. Kafri, J. Tailleur, and F. van Wijland, Time irreversibility in active matter, from micro to macro, Nat. Rev. Phys.4, 167 (2022)
work page 2022
-
[65]
J. O’Byrne and M. E. Cates, Geometric theory of (ex- tended) time-reversal symmetries in stochastic processes: I. finite dimension, J. Stat. Mech.2024, 113207 (2024)
work page 2024
-
[66]
J. O’Byrne and M. Cates, Geometric theory of (extended) time-reversal symmetries in stochastic processes: Ii. field theory, J. Stat. Mech.2025, 053204 (2025)
work page 2025
-
[67]
Y. Duan, J. Agudo-Canalejo, R. Golestanian, and B. Ma- hault, Phase coexistence in nonreciprocal quorum-sensing active matter, Phys. Rev. Res.7, 013234 (2025)
work page 2025
-
[68]
J. Metzger, S. Ro, and J. Tailleur, Revisiting the ratchet principle: When hidden symmetries prevent steady cur- rents, arXiv preprint arXiv:2412.07851 (2024)
-
[69]
J. Metzger, S. Ro, and J. Tailleur, Exceptions to the ratchet principle in active and passive stochastic dynamics, arXiv preprint arXiv:2503.11902 (2025)
-
[70]
B. B. Mandelbrot and J. W. Van Ness, Fractional brow- nian motions, fractional noises and applications, SIAM review10, 422 (1968)
work page 1968
-
[71]
E. Kalz, A. Sharma, and R. Metzler, Field theory of active chiral hard disks: a first-principles approach to steric interactions, J. Phys. A: Math. Theor.57, 265002 (2024)
work page 2024
-
[72]
A. R. Sprenger, L. Caprini, H. Löwen, and R. Wittmann, Dynamics of active particles with translational and ro- tational inertia, J. Phys. Condens. Matter35, 305101 (2023)
work page 2023
-
[73]
J. R. Gomez-Solano and F. J. Sevilla, Active particles with fractional rotational brownian motion, J. Stat. Mech.: Theory Exp.2020(6), 063213
work page 2020
-
[74]
S. Ravichandir, B. Valecha, P. L. Muzzeddu, J.-U. Som- mer, and A. Sharma, Transport of partially active poly- mers in chemical gradients, Soft Matter21, 1835 (2025)
work page 2025
-
[75]
T. Lefranc, A. Dinelli, C. Fernández-Rico, R. P. Dullens, J. Tailleur, and D. Bartolo, Synthetic quorum sensing and absorbing phase transitions in colloidal active matter, Phys. Rev. X15, 031050 (2025)
work page 2025
-
[76]
Y. Fily and M. C. Marchetti, Athermal phase separation of self-propelled particles with no alignment, Phys. Rev. Lett.108, 235702 (2012)
work page 2012
-
[77]
P. C. Hohenberg and B. I. Halperin, Theory of dynamic critical phenomena, Rev. Mod. Phys.49, 435 (1977)
work page 1977
-
[78]
P. M. Chaikin, T. C. Lubensky, and T. A. Witten,Prin- ciples of condensed matter physics, Vol. 10 (Cambridge university press Cambridge, 1995)
work page 1995
-
[79]
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 mo- tors, Proc. Natl. Acad. Sci. U.S.A.107, 9541 (2010)
work page 2010
-
[80]
N. Pellicciotta, O. S. Bagal, V. C. Sosa, G. Frangipane, G. Vizsnyiczai, and R. D. Leonardo, Light controlled biohybrid microbots, Adv. Funct. Mater.33, 2214801 (2023)
work page 2023
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.