{"id":"09746d3f-5371-4c56-85ac-7c57c0907d4e","arxiv_id":"2509.01523","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"low","formal_verification":"none","parameter_count":5,"one_line_summary":"Extensile active fluid is pushed to planar-anchored walls and away from homeotropic-anchored walls; contractile activity reverses both.","lead":"A simulation study of a mixture of an active nematic fluid and a passive fluid confined in a box shows that which species collects at the walls depends on how the nematic molecules are anchored at the boundary. The result offers a mechanical, non-thermal explanation for why some cell or bacterial populations sort to the edges of a colony, and it can be tested in microtubule-kinesin active nematic experiments.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The sorting mechanism is demonstrated only for a paranematic bulk (S0=0, Eq. 16); if equilibrium bulk nematic order is nonzero, the required ∇S gradient at the wall is weakened and the predicted sign, especially for contractile activity, is untested.","rationale":"The reader's weakest-assumption capture is exactly where the argument is most exposed. The abstract and Sec. VI present a mechanism that requires a boundary-induced gradient in the nematic order parameter S, and that gradient is only guaranteed when the equilibrium bulk has zero order. The paper is transparent about S0=0, and within that model the simulations consistently support the claim. But the central claim's reach beyond the paranematic case is untested, and the force argument in Sec. III explicitly leans on the contrast between an ordered boundary layer and a disordered bulk. For contractile activity the bulk is quiescent in this model, so the S0>0 case is especially consequential: with a uniformly ordered bulk the active force from ∇(Sφ) may be absent or reversed. This is a genuine correctness risk for the generality of the claim, not merely a disagreement with community practice. I agree with the reader's identification and with the high-confidence acceptance for the paper as written, because the claim is explicitly a model result and the limitation is acknowledged. I do not change the verdict, but I specify a concrete numerical test that would convert the caveat into either support or a restriction of the claim.","tokens_in":12833,"tokens_out":21516,"duration_ms":260373,"concrete_test":"Repeat the square-box protocol of Fig. 1(c) with S0 = 0.1, 0.3, and 0.5 (keeping C, K, λ, γ, and the activity range) for both extensile and contractile activity and for planar and homeotropic anchoring; measure the time-averaged boundary concentration φ_edge and the steady-state S(r) profile. If the planar>homeotropic ordering for extensile and its reversal for contractile survive with comparable magnitude, S0=0 is not load-bearing. If the effect weakens, vanishes, or reverses, the sorting claim must be restricted to a paranematic bulk and the abstract's generality revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central mechanism in Sec. III is that boundary anchoring creates a gradient in the magnitude S of nematic order between the wall and the bulk, and the active stress Eq. (14) then exerts a force proportional to ζ∇(Sφ) that pushes the active component toward or away from the wall. This gradient exists only because the model chooses S0=0 in Eq. (16): the bulk free-energy minimum is isotropic, so the wall is the sole source of order. No simulation with S0>0 is reported. If S0 is nonzero, the equilibrium bulk is already ordered; the wall-induced S variation is attenuated (or its sign can depend on whether the wall orders more or less strongly than the active-steady bulk), and the ∇(Sφ) force may no longer set the sorting direction. For contractile activity, the bulk is quiescent (Sec. IV notes λζ<0 prevents shear-induced ordering), so with S0>0 the bulk would have uniform equilibrium S and the predicted depletion/enhancement from order gradients could be absent or reversed. Since the abstract advertises tests in microtubule-kinesin networks and cell colonies, where bulk nematic order is common, the headline claim is not established beyond the paranematic limit. This is a modeling choice, not an inconsistency, but it is the load-bearing assumption for the proposed mechanism.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents lattice-Boltzmann simulations of a two-fluid model in which an active nematic component is mixed with a passive isotropic fluid and confined in square, channel, and circular geometries. The central finding is that an extensile active fluid accumulates at a boundary with planar anchoring and is depleted at a homeotropic boundary, with the opposite behavior for contractile activity. The authors attribute sorting to active forces generated by gradients in nematic order and concentration, and they show that the same mechanism works when boundary anchoring arises spontaneously from active flows. A circular-confinement analysis separates contributions from gradients in the magnitude of nematic order and gradients in director orientation, and a final demonstration shows an active droplet spreading into a rotating boundary ring.","tokens_in":13170,"tokens_out":6471,"duration_ms":81111,"significance":"If the proposed mechanism holds, it provides a minimal nonequilibrium route to boundary sorting that does not rely on differential adhesion, and it yields a clear, falsifiable design rule: the sign of activity and the anchoring angle determine whether the active component accumulates or depletes at the boundary. The paper has notable strengths: the result is measured from simulation rather than obtained by fitting; it uses multiple confinement protocols (free-slip and no-slip boxes, channels, circles); it reports time-averaged concentration profiles with standard deviations; it includes parameter sweeps in activity and radius; and the simulation code is publicly available on GitHub. The analytical decomposition in Sec. V uses standard active-stress expressions, and the supplementary movies and figures support the interpretation. The main caveat, discussed below, is that the model fixes the equilibrium nematic order to zero, making the bulk paranematic; the generality of the sorting mechanism outside this limit is not established.","major_comments":[{"comment":"The model sets S0=0, so the bulk equilibrium state is isotropic. The sorting mechanism described in Sec. III relies on a gradient of nematic order S between the boundary and the bulk; with S0>0 this gradient is attenuated, and its sign can depend on whether the wall orders more or less strongly than the bulk. The predicted sorting direction, especially for contractile activity, is therefore untested outside the paranematic limit. Indeed, Sec. IV explicitly uses S0=0 to argue that contractile activity produces no active flows when λ>0. Because the abstract and discussion generalize to microtubule-kinesin networks and cell colonies, where bulk nematic order is common, the central claim is not established beyond the paranematic regime. I request either additional simulations with S0>0 or a clear qualification that the results apply to the paranematic limit of the model.","section":"Sec. II, Eq. (16); Sec. IV"}],"minor_comments":[{"comment":"The text says 'If a>0 and b>0' the two fluids are homogeneously mixed, but the simulation parameters list b=0. Clarify that b=0 (a purely quadratic Landau term) is also used and still corresponds to mixing.","section":"Sec. II, after Eq. (16)"},{"comment":"The horizontal axis is labeled only 'activity'. Please indicate explicitly that positive values are extensile and negative values are contractile, and state the range of ζ plotted.","section":"Fig. 1(c) and Fig. 3(c)"},{"comment":"Minor typo: 'as shown in in Fig. 2(a)' contains a duplicated 'in'.","section":"Sec. III, text before Fig. 2"},{"comment":"The derivation assumes a constant tilting angle Θ0. The text should note that this is an approximation and that near the boundary Θ varies, as acknowledged later when forder changes sign.","section":"Eqs. (19)-(20)"},{"comment":"The method for calculating the concentration enhancement near the wall is described in words, but the annulus width (or the criterion for choosing the peak) is not quantified. Please provide the numerical value used.","section":"Sec. V, Fig. 4"},{"comment":"The flow arrows in the schematics are illustrative, but the text refers to 'flows due to an extensile active nematic' and 'contractile' without explaining the dipole direction. Adding a sentence defining the dipole force direction would help the reader.","section":"Sec. III, Fig. 2(b)-(c)"}],"recommendation":"major_revision","confidential_remarks":"The S0=0 assumption is the main risk to the generality of the headline claim. The authors should at least report a few test simulations with S0>0 or reframe the paper's claims to the paranematic regime. I do not recommend rejection: the simulations are carefully executed, the mechanism is clearly explained, and the code is available for verification. The requested change is within the scope of a revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear [colleague],\n\nThis paper is a straightforward, honest extension of the authors' earlier two-fluid model, and it does what it claims: it shows that in a confined active nematic/passive isotropic mixture, the sign of activity and the anchoring direction determine whether the active component accumulates at or leaves the boundary. The result is new relative to their bulk work, and the mechanism--active forces from gradients of nematic order between an anchored wall and a disordered bulk--is clearly laid out and supported by force decomposition in the circular geometry. The simulations are thorough: free-slip and no-slip boxes, channels, circles, parameter sweeps, and time-averaged profiles with error bars. Code is on GitHub. That is real evidence.\n\nThe soft spot is the one the stress-test flags: the model sets S0=0, so the bulk is a paranematic. The sorting mechanism is built on the contrast between wall-induced order and disordered bulk. If you give the bulk intrinsic order (S0>0), that gradient is attenuated, and for contractile activity the predicted depletion could easily change sign. The paper does not test this. The abstract mentions microtubule-kinesin networks and cell colonies, where bulk nematic order is often present, so the gap is not merely formal--it is directly relevant to the claimed experimental relevance. I don't see this as a fatal flaw, because the paper is explicit about the S0=0 choice and the claim is framed within the model. But it means the headline result is a model prediction about a specific regime, not a robust general law. A single simulation with S0>0 would have helped a lot.\n\nThe other caveats are minor: statistical reporting is light (number of snapshots, convergence), and the analytical explanation in Sec. III is a scaling argument rather than a derivation. Neither undermines the main result.\n\nWho is this for? Active-matter theorists and experimentalists working on confined active nematics. It's a reasonable contribution, not a breakthrough. I'd send it to a serious referee; the paranematic limitation is worth a careful review, and the authors should be pushed to discuss or simulate S0>0.\n\nRecommend: accept with revision, after the S0 robustness question is addressed.","headline":"Solid simulation study showing anchoring-dependent boundary sorting in active-passive mixtures; the mechanism is convincing within the paranematic model, but the S0=0 assumption leaves a real gap for realistic bulk-nematic systems.","tokens_in":13638,"tokens_out":2629,"would_cite":true,"duration_ms":29006,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":["76A15"],"pacs":[],"model":"deepseek-v4-flash","headline":"Active nematic sorting at walls is controlled by anchoring, not wetting","keywords":["active nematic","two-fluid model","boundary sorting","active anchoring","nematic order gradients","active stress","cell sorting","confinement"],"falsifier":"Run the same two-fluid simulations with S0 raised to a finite equilibrium bulk order (e.g. S0 = 0.5) in a square box with planar and homeotropic anchoring: the paper's mechanism predicts the boundary enrichment and depletion for extensile activity should weaken or reverse; if the concentration profile is unchanged, the ∇(ϕS) force is not the controlling factor. Experimentally, in a microtubule-kinesin extensile nematic with a fluorescently labeled active component in a chamber with planar surface anchoring, measure the wall concentration: the paper predicts a stationary enrichment above bulk.","tokens_in":12745,"feed_emoji":"🌀","tokens_out":5838,"duration_ms":69902,"temperature":0.7,"pith_summary":"This paper sets out to show that a confined mixture of an active nematic fluid and a passive isotropic fluid sorts spontaneously at a wall, with no thermodynamic wetting needed. The direction of sorting is controlled by two signs: the sign of activity (extensile vs contractile) and the anchoring of the nematic director at the boundary (planar vs homeotropic). Extensile activity accumulates at a planar-anchored wall and is depleted at a homeotropic-anchored wall; contractile activity reverses both. The mechanism is an active force generated by gradients in nematic order between the ordered boundary layer and the disordered bulk. Because the same sorting occurs when anchoring arises spontaneously from active flows, the result suggests a generic route by which active stresses can position cell populations at tissue boundaries.","feed_headline":"Extensile fluid gathers at planar walls; contractile does the opposite","feed_subtitle":"Order gradients create active forces that push one component to the boundary, no wetting or adhesion required.","key_machinery":"The central object is the active stress Π_act = -ζϕQ, where ζ is the activity (positive extensile, negative contractile), ϕ is the active-fluid concentration, and Q is the nematic tensor. Its divergence creates body forces on the combined fluid; retaining spatial variations of concentration and nematic order gives a force ∝ ζ∇(ϕS)·(2nn-I) from order and concentration gradients and a force ∝ ζSϕ∇·(nn) from director curvature. The model is a two-fluid formulation in which a center-of-mass fluid and a relative flow are separated, with strong drag between components, plus Beris-Edwards dynamics for the nematic tensor.","core_discovery":"In a two-fluid model of an active nematic mixed with a passive isotropic fluid and confined by walls, the paper claims that the active component sorts to the boundary purely through active stresses. With imposed anchoring, an extensile active fluid enriches the boundary layer when anchoring is planar and depletes it when anchoring is homeotropic; a contractile active fluid does the reverse. The same boundary accumulation occurs without imposed anchoring, because active flows spontaneously create planar active anchoring. The driving force is the divergence of the active stress, which produces forces from gradients of the product of concentration and nematic order, with direction set by the an","pith_inferences":["If wall anchoring can be patterned, a single active species should be steerable: regions with planar anchoring attract extensile material, homeotropic patches repel it, so confinement geometry alone could route active components without chemical patterning.","The contractile case may map onto actomyosin cell aggregates, where contractile stresses and homeotropic-like alignment at tissue boundaries would drive inward sorting, providing a mechanical complement to differential adhesion in explaining interior placement.","Because activity sign is set by motor protein directionality, toggling motor action should reverse the wall concentration in the same chamber, a directly testable prediction of the stress-gradient mechanism."],"forward_implications":["With imposed planar (homeotropic) anchoring, extensile activity enriches (depletes) the boundary layer, and contractile activity does the opposite.","The same boundary sorting appears when anchoring is not imposed but generated by active flows, so a bare confining wall is enough for extensile material to accumulate.","The active force normal to the wall, ζ∇(ϕS)(2(m·n)^2 - 1), directly ties anchoring angle and activity sign to the direction of sorting.","In circular confinement, gradients in the magnitude of nematic order and gradients in director orientation contribute roughly equally to the radial force pushing active material outward.","If the active species starts as a thermodynamically phase-separated droplet, activity drives it into a spontaneously rotating boundary ring, showing active forces can override equilibrium placement."],"supporting_citations":[{"why":"Supplies the two-fluid model of active nematic–passive isotropic mixtures on which all simulations are based.","marker":"[46]"},{"why":"Establishes the free energy and numerical scheme used here, including the choice S0 = 0 for the bulk nematic order.","marker":"[48]"},{"why":"Gives the active anchoring mechanism and the normal and tangential active force expressions used to explain boundary sorting.","marker":"[40]"},{"why":"Defines the active stress form and the active turbulence instability that shapes the bulk flows in the simulations.","marker":"[35]"},{"why":"Predicts azimuthal circulating flow in circular confinement, the state used to isolate director-curvature gradients.","marker":"[58]"},{"why":"Demonstrates experimentally the confined circulating flow state in extensile nematics that the circular simulations reproduce.","marker":"[38]"}],"fun_headline_variants":["Planar walls attract extensile fluid; homeotropic repels it","Extensile hugs planar walls; contractile does the reverse","Nematic order gradients push active fluids to boundaries","Active sorting: extensile planar-rich, contractile homeotropic-rich"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The model sets the equilibrium nematic order of the bulk to zero, so the sorting relies on a strong contrast between an anchored, ordered boundary and a disordered bulk; if the bulk were already nematic, the order gradient that drives the effect would shrink and the predicted directions could change.","fun_headline_variants_meta":{"raw":{"variants":["Planar walls attract extensile fluid; homeotropic repels it","Extensile hugs planar walls; contractile does the reverse","Nematic order gradients push active fluids to boundaries","Active sorting: extensile planar-rich, contractile homeotropic-rich"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000835,"raw_usage":{"total_tokens":3416,"prompt_tokens":616,"completion_tokens":2800,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":360,"completion_tokens_details":{"reasoning_tokens":2742}},"tokens_in":360,"tokens_out":2800,"duration_ms":24519,"temperature":1.0,"reasoning_tokens":2742,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T12:28:45.663145+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same two-fluid simulations with S0 raised to a finite equilibrium bulk order (e.g. S0 = 0.5) in a square box with planar and homeotropic anchoring: the paper's mechanism predicts the boundary enrichment and depletion for extensile activity should weaken or reverse; if the concentration profile is unchanged, the ∇(ϕS) force is not the controlling factor. Experimentally, in a microtubule-kinesin extensile nematic with a fluorescently labeled active component in a chamber with planar surface anchoring, measure the wall concentration: the paper predicts a stationary enrichment above bulk.","supporting_citations":[{"cited_title":"Bhattacharyya and J","cited_arxiv_id":null,"evidence_quote":"Supplies the two-fluid model of active nematic–passive isotropic mixtures on which all simulations are based."},{"cited_title":"Bhattacharyya and J","cited_arxiv_id":null,"evidence_quote":"Establishes the free energy and numerical scheme used here, including the choice S0 = 0 for the bulk nematic order."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Defines the active stress form and the active turbulence instability that shapes the bulk flows in the simulations."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Predicts azimuthal circulating flow in circular confinement, the state used to isolate director-curvature gradients."},{"cited_title":"Opathalage, M","cited_arxiv_id":null,"evidence_quote":"Demonstrates experimentally the confined circulating flow state in extensile nematics that the circular simulations reproduce."}],"review_version":1}