{"id":"d53aa893-a5cb-42ae-a7ea-79a6283b2d71","arxiv_id":"1908.00768","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Growing active nematic fluid invading a capillary exhibits three activity-controlled regimes, including collective cluster detachment above a threshold activity number.","lead":"Simulations of growing active matter entering a narrow channel show three distinct invasion modes controlled by activity: smooth coherent growth, an S-shaped wavy front, and detachment of mobile clusters. The result suggests that cells and bacteria may switch invasion strategy simply by changing their internal activity.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Load-bearing concern: the three-regime classification rests on an untested growth-source protocol; the paper asserts insensitivity to implementation details but only tests growth vs. no-growth and reservoir vs. no-reservoir.","rationale":"The reader's weakest assumption identifies the same load-bearing point: the growth implementation is the least-controlled element of the simulation. The paper's central observable is the dependence of invasion regimes on activity A, but growth is independently essential (no growth -> no regimes), and the only robustness check offered is a binary comparison in ESI A. The ESI d-collapse (Fig. S2) and 1.96 SEM error bars do support A as a control parameter for the tested channel widths, and the paper is candid about unresolved mechanism (Section 3.2.2: 'it is not possible to decide to what degree this effect is purely interfacial...'). Nevertheless, because no code or raw data is released and the growth source term is stochastic and reservoir-localized, the claim that 'details of this implementation ... are not important' is not established. The concrete test above would settle whether the three-regime classification is robust or a numerical artifact of the growth protocol. I therefore keep the reader's conditional verdict; no adjustment is needed.","tokens_in":18689,"tokens_out":6929,"duration_ms":76196,"concrete_test":"Re-run the full activity sweep A=12-28 with (i) a deterministic logistic source alpha phi (1-phi/phi_c) applied everywhere in the active phase, including inside the capillary, and (ii) the original stochastic reservoir-only source with alpha doubled and r_g halved. For each run, measure (hmax-hmin)/d, N_c, and vrms as in Figs. 2-3. If the three regimes persist and both crossover positions move by less than ~2 in A, the implementation-insensitivity assertion holds; if regime III cluster detachment disappears or the crossovers shift substantially, the three-regime classification is an artifact of the growth protocol.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that a growing active nematic in a capillary shows three regimes as a function of A=d/sqrt(KQ/zeta), with sharp crossovers near A~16 and A~20. For this to be a statement about growing active matter rather than about the particular growth algorithm, the regime structure must be robust to how mass is added. Section 2.2 introduces a stochastic, reservoir-localized growth term: at random points in the reservoir, phi is increased locally with probability r phi (1-phi/phi_c), producing dipole-like flows and a growth pressure through the isotropic part of Pi_el,1. The text asserts 'the details of this implementation or even the geometry of a reservoir are not important for the qualitative dynamics in the capillary (see ESI section A)'. But ESI A only tests two binary variations: removing growth entirely (the regimes disappear) and removing the reservoir (the first crossover is unchanged while the second crossover is 'significantly shifted to higher values of A'). Neither test varies the stochastic source parameters r, tau_g, r_g, alpha, phi_c, nor does any test place growth inside the capillary, where real biofilms and cell monolayers proliferate. Because growth is the one ingredient that makes the invasion regimes exist, a hidden sensitivity of cluster detachment or of the S-shaped interface to the growth protocol would directly invalidate the central classification. The no-reservoir shift already shows that the second crossover position is not an intrinsic function of A alone.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses two-dimensional hybrid lattice Boltzmann simulations of a two-phase active nematohydrodynamic model to study the invasion of a growing active nematic from a reservoir into a narrower capillary filled with isotropic fluid. Growth is implemented as stochastic local increases of the phase field in the reservoir with logistic saturation. Varying the dimensionless activity number A = d/sqrt(KQ/zeta) reveals three qualitatively distinct invasion regimes: a flat-interface coherent regime (I), an S-shaped interface with spontaneous flows and vortex-induced switching (II), and a regime in which active clusters detach from the main body (III). The authors characterize the two crossovers using the interface height difference (hmax - hmin)/d and the number of detached clusters Nc, connect the first crossover to the spontaneous flow transition in confined active nematics, and propose that the second crossover arises from active stresses overcoming surface tension. They further show that cluster detachment lets the active material protrude about 0.5 to 1 capillary-width deeper into the channel.","tokens_in":19003,"tokens_out":6852,"duration_ms":67870,"significance":"If the classification is robust, the paper provides a useful organizing framework for how growth, activity, and confinement combine to produce distinct modes of collective invasion, with potential relevance to biofilm expansion and collective cell migration. The strengths of the manuscript are the fully specified governing equations and parameters, well-defined observables with error bars from repeated initial conditions, and explicit falsifiable predictions (the two crossover thresholds in A and the cluster-penetration depth). The ESI adds useful controls, including no-growth and no-reservoir comparisons and a check of two channel widths. The authors are also honest about the uncertainty in the second-crossover mechanism. The central risk is the untested sensitivity of the regime structure to the details of the stochastic growth protocol, which is the ingredient that makes the regimes exist.","major_comments":[{"comment":"The claim that 'the details of this implementation or even the geometry of a reservoir are not important for the qualitative dynamics in the capillary (see ESI section A)' is not supported by the evidence presented. ESI A tests only two binary variations: removing growth entirely and removing the reservoir. It does not vary the stochastic growth parameters r, tau_g, r_g, alpha, and phi_c, nor does it test growth placed inside the capillary. Since growth is the component that makes the three invasion regimes appear, a hidden sensitivity of the crossover positions or of cluster detachment to these parameters would directly undermine the central classification. I therefore request either additional simulations varying the growth protocol (at least for a representative set of parameters and for growth distributed in the capillary) or a reformulation of the claim to state that the classification is demonstrated for the specific growth implementation used.","section":"Section 2.2 and ESI A"},{"comment":"The mechanistic explanation for the crossover from regime II to III is that active stresses overcome the stabilizing effect of surface tension, leading to pinch-off of clusters. However, the text itself admits that 'it is not possible to decide to what degree this effect is purely interfacial and whether dynamics in the bulk are important.' Because the paper explicitly claims to characterize 'the mechanical mechanisms underlying the crossovers,' a quantitative test is needed. For example, varying the surface-tension-related coefficients D or K_phi at fixed A and measuring Nc and the threshold would distinguish interfacial from bulk contributions. Without such a test, the proposed mechanism remains a plausible hypothesis rather than a demonstrated result; the existence of the regimes is still supported by the observables, but the explanatory part of the central claim is not yet established.","section":"Section 3.2.2 and Figure 4C"},{"comment":"The text describes the two transitions as 'well-defined crossovers,' but the data points are spaced by increments of order unity in A (e.g., A = 15.5, 16.7, 17.9, 19.2), so the sharpness of the transitions is not resolved. A denser sampling of A near the claimed thresholds, together with a quantitative criterion for locating the crossover (for instance, the value of A where (hmax - hmin)/d first exceeds a threshold or where Nc becomes non-zero), would strengthen the claim that the regimes are separated by well-defined crossovers rather than by gradual changes over a finite range.","section":"Section 3.1 and Figure 2"}],"minor_comments":[{"comment":"The stochastic growth algorithm is described verbally; please specify the time cadence of growth attempts (e.g., whether each lattice site in the reservoir is considered once per lattice-Boltzmann time step or once per tau_g) so that the implementation is unambiguous and reproducible.","section":"Section 2.2"},{"comment":"The statement that 'growth is an essential factor to create the phenomena reported in the main text' appears to be in tension with the note in the caption of Supplementary Figure 1 that 'clusters are also present for a system lacking growth, but it takes longer for them to appear.' Please clarify in what sense growth is essential (e.g., for the S-shaped interface and the early appearance of clusters) and reconcile these two statements.","section":"ESI A and Supplementary Figure 1"},{"comment":"The axis label in Figure 3A uses units of d over the active time scale tau_zeta, but the main text says 'rms-velocity v_rms in units of d' without specifying the time normalization until the caption; please make the axis label and text consistent.","section":"Figure 3A"},{"comment":"For the channel-width comparison, please state explicitly whether the same activity coefficients zeta were used for both d values (with A varying through d) or whether zeta was adjusted; this affects how the collapse onto A in Supplementary Figure 2 should be interpreted.","section":"ESI B and Supplementary Figure 2"},{"comment":"References [10] and [19] appear to be the same reference (Conrad and Poling-Skutvik, Annu. Rev. Chem. Biomol. Eng. 9:175–200, 2018) cited twice with slightly different formatting; please merge them into a single entry.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is a well-presented simulation study with clearly specified observables and useful controls. The main issue is that the central claim rests on an untested growth protocol; this is fixable with additional ESI simulations that vary the growth parameters and possibly the growth location. If the authors provide those robustness tests or appropriately soften the generality claim, I would support publication. I also note the inconsistency between the statement that growth is essential and the observation of clusters in the no-growth control, and the duplicate reference, both of which should be corrected."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a solid, useful numerical study, and the three-regime classification is a genuine addition to the active-matter-in-confinement literature. The main caveat is more about packaging than content: the paper claims the details of the growth implementation don't matter, but the ESI doesn't actually test that claim by varying the growth parameters.\n\nWhat's new: the combination of a growing active nematic with a reservoir-capillary geometry, and the observation that as the activity number A increases you get (I) a flat coherent front, (II) an S-shaped front with spontaneous flows and vortex-mediated switching, and (III) detached active clusters that penetrate deeper. That third regime, with cluster detachment enhancing invasion depth, is the most novel. The observables are sensible, the error bars are there, and the check across two capillary widths supports A as the control parameter. The equations and parameter set are fully specified, and the references to spontaneous-flow transitions, active turbulence, and cell-monolayer experiments are appropriate. The paper is also honest where it matters: it explicitly says the mechanism behind the second crossover is not fully disentangled between interfacial and bulk effects.\n\nWhere it's soft: the stress-test note is right. Section 2.2 states that the growth implementation 'details ... are not important for the qualitative dynamics in the capillary', with a pointer to ESI section A. But ESI A only compares growth vs no growth and reservoir vs no reservoir. It does not vary r, tau_g, rg, alpha, phi_c, nor test growth inside the capillary. The no-reservoir run shifts the second crossover to higher A, so the long-range effect is not literally negligible. That does not invalidate the qualitative picture, since the regimes still appear, but the universal claims are stronger than the tests. The fix is straightforward: a few robustness runs varying the growth source parameters, or a reworded claim. Missing code/data is a minor concern for a 2019 preprint, but it makes independent checking harder.\n\nWho should read it: people working on collective cell invasion, biofilm spreading, or active nematics in channels. It gives them a concrete phase diagram to compare against. It is not a theory breakthrough, but it doesn't need to be.\n\nRecommendation: send to peer review. The classification is worth refereeing, and the robustness gap is addressable by revision.","headline":"A competent and largely convincing numerical study mapping three invasion regimes for growing active nematics in a capillary; the main weakness is an unsupported claim about robustness to the growth implementation.","tokens_in":19472,"tokens_out":4267,"would_cite":true,"duration_ms":37564,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A growing active fluid invading a narrow capillary switches among three distinct invasion patterns—flat coherent front, S-shaped wavy front, and detaching clusters—depending only on a single dimensionless activity number.","keywords":["active nematics","active matter","collective invasion","capillary confinement","growth dynamics","spontaneous flow","topological defects","phase-field model"],"falsifier":"Run the same two-phase active-nematic equations with a different growth source term, such as spatially uniform proliferation instead of random local events, and check whether the two crossovers in (h_max − h_min)/d and cluster count N_c stay at A ≈ 16 and A ≈ 20; a shift or disappearance would show the regime classification depends on the growth implementation. Alternatively, measure interface flatness and detached cluster count in an expanding bacterial or epithelial monolayer in a channel while varying available chemical energy, and see whether the same two abrupt changes appear.","tokens_in":18519,"feed_emoji":"🧫","tokens_out":5108,"duration_ms":49901,"temperature":0.7,"pith_summary":"This paper uses computer simulations of a generic continuum model of active matter—fluid whose elongated constituents generate internal stresses—to ask how a growing colony invades a narrow fluid-filled capillary. It claims that the invasion organizes into three distinct regimes controlled by a single dimensionless number A, the ratio of capillary width to the activity-determined length scale. At low activity the front stays flat and coherent; at intermediate activity spontaneous flows bend it into an S-shape that periodically flips sides; at high activity active blobs detach and penetrate deeper into the channel. The paper matters because it suggests that biological invaders such as bacterial biofilms or cell sheets can change their mode of spread simply by changing how much mechanical stress their constituents generate, without changing geometry. It also gives concrete thresholds and mechanisms that experiments could test.","feed_headline":"Growing active matter invades capillaries in three modes","feed_subtitle":"A single activity number decides whether a flat front pushes in, an S-shaped wave advances, or clusters break off and dive deeper.","key_machinery":"The central object is a two-phase active nematohydrodynamics model: a scalar phase field φ marks active versus passive fluid, a nematic tensor Q_αβ tracks orientational order, and an active stress −ζφQ_αβ injects energy at the scale set by the activity ζ. The organizing dimensionless group is the activity number A = d/√(K_Q/ζ), the capillary width over the active length scale, which collapses data from different channel widths onto one phase map. The argument works by identifying two observables—interface deformation (h_max − h_min)/d and number of detached clusters N_c—and showing that both jump at A ≈ 16 and A ≈ 20, with the mechanisms being spontaneous flow onset and activity-versus-surface-tension pinch-off respectively.","core_discovery":"The paper claims that a growing active nematic invading a fluid-filled capillary from a reservoir passes through three qualitatively distinct invasion regimes as the dimensionless activity number A = d/√(K_Q/ζ) increases: a flat-interface, flow-free regime (A ≲ 16) where invasion is purely growth and diffusion controlled; an S-shaped-interface regime (roughly 16 ≲ A ≲ 20) driven by spontaneous flow generation that advects material, with periodic flipping of the front between capillary walls as vortices form; and a regime (A ≳ 20) where active clusters pinch off from the main body and penetrate 0.5 to 1 capillary widths deeper, even though total active material in the capillary is similar to regime II. The first crossover is traced to the well-known hydrodynamic instability to spontaneous flow in confined active nematics; the second is attributed to active stresses overcoming surface tension at the interface, aided by bulk dynamics and +1/2 topological defects.","pith_inferences":["If the three-regime classification holds generally, invasive efficiency in confined geometries could be regulated by biochemical energy supply alone: a colony could switch from coherent to cluster-shedding invasion by tuning its activity, suggesting a physical control point for slowing or aiding spread.","The ESI result that removing the reservoir shifts the second crossover to higher A implies that upstream geometry and growth pressure participate in setting cluster detachment thresholds; an experimental study varying reservoir size while holding capillary activity fixed would test this long-range influence.","A natural extension is to give the activity coefficient a curvature dependence at the interface, mimicking leader cells; one prediction of the model framework is that such a term would move the first crossover or alter the S-shape switching frequency, which could be checked in particle-based simulations.","Cluster detachment as a way to reach deeper suggests a generic physical rationale for the advantage of disseminating small groups during collective invasion, independent of specific biochemical signalling."],"forward_implications":["The activity number A, rather than activity, channel width, or elasticity separately, sets the invasion mode: the same crossovers appear for capillaries of different widths when plotted against A.","Before the first crossover, invasion is slow and nearly independent of activity because transport is purely diffusive and growth-driven, with no spontaneous flows in the capillary.","After the first crossover, activity-induced flows advect active material into the capillary and the invasion index rises approximately linearly with A.","After the second crossover, detached clusters add little to the total amount of active material in the channel but extend maximum reach by up to one capillary width.","Within regime II, the front's S-shape switches from one wall to the other when the most-forward vortex reverses its rotation, giving periodic front oscillations."],"supporting_citations":[{"why":"Supplies the two-phase active nematic model equations that the simulations solve.","marker":"[37]"},{"why":"Provides the hybrid lattice Boltzmann method used to numerically solve the model.","marker":"[38]"},{"why":"Establishes the spontaneous flow transition in active polar gels that explains the regime I to II crossover.","marker":"[49]"},{"why":"Gives the unified picture of spontaneous flow states in active nematics used to interpret the first crossover.","marker":"[50]"},{"why":"Describes vortices and disclination dynamics in confined active nematics, the basis for interpreting regime II switching.","marker":"[51]"},{"why":"Provides the vortex-lattice and mesoscale-turbulence onset results used to analyse regime II and III flows.","marker":"[52]"},{"why":"Supplies the experimental capillary-geometry collective migration modes that the setup is designed to reproduce.","marker":"[11]"},{"why":"Offers the alternative particle-based model of collective cell migration in confinement that the continuum model is contrasted with.","marker":"[36]"}],"fun_headline_variants":["Three modes for active matter invasion in capillaries","Activity sets invasion: flat, S-wave, or detached blobs","One activity number governs active matter invasion mode","From coherent front to blob breakaway in active matter","Active matter invades: flat, wavy, or by blob detachment"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The random local growth events that feed the active phase are assumed not to change the invasion regime; the paper compares growth versus no growth and reservoir versus no reservoir, but never varies how growth is implemented, so the regime boundaries could in principle be artifacts of that particular source term.","fun_headline_variants_meta":{"raw":{"variants":["Three modes for active matter invasion in capillaries","Activity sets invasion: flat, S-wave, or detached blobs","One activity number governs active matter invasion mode","From coherent front to blob breakaway in active matter","Active matter invades: flat, wavy, or by blob detachment"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000245,"raw_usage":{"total_tokens":1486,"prompt_tokens":848,"completion_tokens":638,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":464,"completion_tokens_details":{"reasoning_tokens":559}},"tokens_in":464,"tokens_out":638,"duration_ms":7274,"temperature":1.0,"reasoning_tokens":559,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T15:32:44.282546+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same two-phase active-nematic equations with a different growth source term, such as spatially uniform proliferation instead of random local events, and check whether the two crossovers in (h_max − h_min)/d and cluster count N_c stay at A ≈ 16 and A ≈ 20; a shift or disappearance would show the regime classification depends on the growth implementation. Alternatively, measure interface flatness and detached cluster count in an expanding bacterial or epithelial monolayer in a channel while varying available chemical energy, and see whether the same two abrupt changes appear.","supporting_citations":[{"cited_title":"Blow, Sumesh P","cited_arxiv_id":null,"evidence_quote":"Supplies the two-phase active nematic model equations that the simulations solve."},{"cited_title":"Marenduzzo, E","cited_arxiv_id":null,"evidence_quote":"Provides the hybrid lattice Boltzmann method used to numerically solve the model."},{"cited_title":"V oituriez, J","cited_arxiv_id":null,"evidence_quote":"Establishes the spontaneous flow transition in active polar gels that explains the regime I to II crossover."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the unified picture of spontaneous flow states in active nematics used to interpret the first crossover."},{"cited_title":"Shendruk, Kristian Thijssen, and Julia M","cited_arxiv_id":null,"evidence_quote":"Provides the vortex-lattice and mesoscale-turbulence onset results used to analyse regime II and III flows."},{"cited_title":"Kabla, Chwee Teck Lim, and Benoît Ladoux","cited_arxiv_id":null,"evidence_quote":"Supplies the experimental capillary-geometry collective migration modes that the setup is designed to reproduce."},{"cited_title":"Modeling collective cell migration in geometric confine- ment","cited_arxiv_id":null,"evidence_quote":"Offers the alternative particle-based model of collective cell migration in confinement that the continuum model is contrasted with."}],"review_version":1}