{"id":"12d6c9c9-4db6-4e07-8fe7-594f41406387","arxiv_id":"2506.13316","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"In DEM simulations of dense non-Brownian frictional suspensions, run-and-tumble activity reduces viscosity by breaking frictional contact networks and shifts shear jamming toward random close packing, with an empirical constitutive law based on suspension temperature.","lead":"Computer simulations show that adding self-propelled run-and-tumble particles to a dense frictional suspension lowers its viscosity by roughly tenfold and shifts the jamming point to higher packing fractions. The finding points to activity as a tunable way to keep industrial and biological suspensions flowing.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The contact/lubrication cutoffs control n_fc, so the activity-driven viscosity drop and jamming shift may be numerical artifacts; convergence tests are missing.","rationale":"The reader's weakest assumption is the same one I identify: the contact and lubrication truncations are load-bearing because the paper's explanatory mechanism is the reduction of frictional contacts. The concern is not that the simulations are internally inconsistent; indeed, the n_fc and η_r curves track each other, the frictionless versus frictional difference is physically sensible, and the fluidization-versus-unjamming asymmetry is a plausible qualitative result. The issue is that none of these conclusions has been shown to be independent of the numerical cutoffs that set when contacts form and how long they persist. Because active particles introduce forces and persistence times on the same scales that compete with the lubrication barrier and contact relaxation, a biased h_min or stiffness could easily change the location of F_m^a or even reverse the sign of the n_fc response. The same mechanism underlies the claimed shift of φ_J toward φ_RCP, so the central claim cannot be considered established without a numerical convergence study. This does not move the verdict: the paper remains a credible but conditional contribution pending the robustness check the reader requested.","tokens_in":9685,"tokens_out":11994,"duration_ms":136578,"concrete_test":"Run the central sweeps in Figs. 1 and 2 and Fig. 4 at fixed physical parameters but with: (i) h_min/a' = 1e-4 and 1e-2; (ii) lubrication cutoff at a'/2, a'/5, and a'/10; and (iii) k_n = k_t = 1e3, 1e4, and 1e5. For each set, recompute η_r(F_a), n_fc(F_a), and the fitted φ_J from the divergence of F_m^a. The concern is settled if the order-of-magnitude viscosity drop and the direction of the φ_J shift survive across all cutoff combinations; if the η_r(F_a) minima or n_fc(F_a) curves change by more than about 20%, or if the inferred φ_J shifts by more than the passive-active difference (about 0.05), the central claim is cutoff-controlled.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's headline mechanism is that activity lowers viscosity by reducing the mean number of frictional contacts n_fc and thereby shifts the frictional shear-jamming point toward φ_RCP. That mechanism is controlled by the numerical rules deciding when a frictional contact exists and how long it survives: lubrication is set to zero for gaps h_ij > a'/2, regularized at h_ij = 10^-3 a', and contacts use springs with stiffness k_n = k_t = 10^4 and Coulomb limiting friction. The regularization scale sets the maximum lubrication force a particle must overcome to make contact; the stiffness sets the overlap and the tangential-spring relaxation time; the a'/2 cutoff sets how much of the hydrodynamic push toward contact is included. Active particles inject random forces of magnitude F_a with persistence τ_p, so the quantities F_m^a and η_m^r that define the minimum-viscosity branch are expected to depend on these cutoffs: if F_a exceeds the lubrication barrier set by h_min, contacts break; if not, they do not. No convergence checks are reported for h_min, the lubrication cutoff, or k_n/k_t. Because the same n_fc reduction is used to explain the φ_J shift, a cutoff bias in active versus passive contact formation would make the headline viscosity reduction and the jamming shift numerical artifacts rather than robust physics. The manuscript's admitted lack of error bars and absence of released code or data make this hard to rule out from the paper alone.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports DEM simulations of dense non-Brownian suspensions containing a fraction of run-and-tumble active particles. It studies how the active force F_a, persistence time τ_p, and active fraction C_a affect the relative viscosity η_r, the mean number of frictional contacts n_fc, the force-chain microstructure, and the shear-jamming volume fraction. The main claims are that activity reduces the viscosity by up to an order of magnitude, that this reduction arises from a decrease in the number of frictional contacts and a shortening/isotropization of force chains, and that for frictional particles the shear-jamming volume fraction can be shifted toward random close packing. The paper also proposes an effective temperature T_eff ~ F_a^4 τ_p and a constitutive law μ^{1.65} Θ_s = γ F(J) that is intended to unify active and passive data.","tokens_in":10022,"tokens_out":4459,"duration_ms":44811,"significance":"If the central mechanisms hold, the work would establish a practical route to fluidize dense frictional suspensions using run-and-tumble activity and to tune the shear-jamming point, with relevance to active colloids, bacterial suspensions, and industrial dense slurries. The contrast between frictional and frictionless systems in Fig. 4 is internally consistent, and the use of n_fc as a microscopic observable is appropriate. The distinction between preventing force-chain formation and breaking an already jammed force-chain network is a valuable conceptual point. However, the quantitative claims rest on several fitted exponents and on simulation parameters that are not tested for robustness, and the paper does not report ensemble statistics. These gaps currently limit the strength of the conclusions.","major_comments":[{"comment":"No ensemble averaging or error bars are reported anywhere in the manuscript. In particular, Figs. 1, 2(b), and 3(a) present data that appear to come from a single run per parameter set, so the divergence exponents α = 2.04 and β = 2.64 and the collapses shown in Fig. 2(b) have no quantified statistical uncertainty. This is load-bearing because the central claims about the existence of a minimum in η_r and about the functional forms F_1(ϕ) and F_2(ϕ) depend on identifying minima and divergences accurately. The authors should report the number of independent runs, standard errors, or at least state why single runs are sufficient for the reported observables.","section":"Simulation details; Tuning the viscosity using active particles"},{"comment":"The contact and lubrication truncations are not tested for convergence. The lubrication force is set to zero for gaps h_ij > a'/2 and regularized at h_ij = 10^{-3} a', and contact stiffnesses are set to k_n = k_t = 10^4. Because the paper's central mechanism is that activity reduces the number of frictional contacts n_fc, and n_fc depends directly on the numerical rules for when contacts form and how long they persist, the reported viscosity reduction and the shift in ϕ_J could be artifacts of these cutoffs rather than robust physics. The authors should vary the lubrication cutoff, the regularization scale, and the contact stiffnesses for at least the key curves in Fig. 3(a) and Fig. 4(a) and show that the trends are unchanged.","section":"Simulation details; Microscopic understanding of viscosity reduction"},{"comment":"The effective temperature is first defined as T_eff ~ F_a^2 τ_p, and when this fails to collapse the data at small values, the manuscript adopts T_eff ~ F_a^4 τ_p with the explicit statement that the microscopic origin of the exponent 4 is not studied. Similarly, the constitutive law μ^{1.65} Θ_s = γ F(J) uses an exponent 1.65 that is fitted to the same active and passive data it then describes, without a quantitative collapse metric or an independent test. As presented, these are fitted descriptors rather than validated organizing laws. The paper would be materially strengthened by testing the proposed forms on parameter combinations not used in the fit and by reporting a quantitative measure of collapse quality.","section":"Tuning the viscosity using active particles; Constitutive law"},{"comment":"The claim that activity can push ϕ_J to ϕ_RCP is supported only by visual inspection of Fig. 4(a): the text says that the data for F_a = 3 and 90 'suggest' this shift, but no fitted values of ϕ_J for the active systems, no divergence analyses, and no error bars are provided. Since the shift of the jamming point toward ϕ_RCP is a headline conclusion, the authors should determine ϕ_J for each active system using the same divergence procedure as in Fig. 2(b) and report the resulting values, along with the numerical value used for ϕ_RCP.","section":"Tuning the jamming volume fraction"}],"minor_comments":[{"comment":"The figure caption does not define the symbols used in panels (f) and (g), and the meaning of the green dotted lines as 'the coordinate of the minimum' is ambiguous for panels where both F_a and τ_p vary; please clarify.","section":"Figure 1 caption"},{"comment":"In the text, 'the η_r−γ curve starts to approach that of the passive system' appears to refer to the η_r–ϕ data in Fig. 4(a); the notation should be corrected to η_r–ϕ.","section":"Tuning the jamming volume fraction"},{"comment":"The caption labels panels (c), (d), and (e) as μ(J), Θ_s(J), and μ/Θ_s(J), but the text refers to Fig. 4(d), (e), and (f) for these quantities; the panel numbering is inconsistent and should be fixed.","section":"Figure 4 caption"},{"comment":"The simulation units are not stated explicitly. The text says that values are reported in raw dimensional form, but the units of F_a and τ_p are never defined; please specify the unit system (e.g., in terms of a, k_n a, and γ̇) in the Simulation details section.","section":"Simulation details"},{"comment":"The symbol γ is used both for the shear strain in the jamming–unjamming section and for the prefactor in the constitutive law μ^{1.65} Θ_s = γ F(J); this overloading is confusing and should be resolved by renaming one of them.","section":"Constitutive law"}],"recommendation":"major_revision","confidential_remarks":"The paper presents a plausible and internally consistent simulation study, but its quantitative claims are heavily dependent on fitted exponents (α, β, the T_eff exponent 4, and the constitutive exponent 1.65) and on untested numerical truncations. No data or code release is mentioned, which makes it difficult for readers to check the collapses. I would encourage the editor to require that the authors add ensemble statistics, convergence tests for the contact/lubrication cutoffs, and an independent test of the proposed constitutive law before publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is a single-author DEM paper that claims run-and-tumble activity can reduce the viscosity of dense non-Brownian frictional suspensions by an order of magnitude and shift the shear-jamming volume fraction toward RCP. The mechanism—activity reduces the mean number of frictional contacts and makes force chains shorter and more isotropic—is supported by the n_fc curves and by the force-chain images, and the frictionless (µ_p=0) control, where neither viscosity reduction nor jamming shift occurs, is the right internal check. The jammed-versus-pre-jammed asymmetry is genuinely new to me: a smaller activity maintained from the start keeps the system fluid, while unjamming a pre-jammed state needs much stronger forcing. That asymmetry, plus the non-monotonic dependence on F_a and τ_p, is the most interesting part of the paper.\n\nWhat the paper does well: it states the model clearly, reports the raw trends honestly, and does not hide the weak points. The proposed suspension-temperature constitutive law µ^1.65 Θ_s = γF(J) is a concrete attempt to fold activity into a usable rheological framework.\n\nThe soft spots are real but mostly quantitative. There are no error bars or ensemble statistics anywhere, so the exponent values (α=2.04, β=2.64, the Teff exponent 4, the constitutive exponent 1.65) are fit parameters with no estimated uncertainty. The effective temperature is defined as F_a^2 τ_p, then changed to F_a^4 τ_p when the first form fails to collapse small-F_a data, with the text admitting no microscopic origin; that is a retrofitted description, not a prediction. The constitutive law is fitted to the same curves it then describes. None of this sinks the qualitative story, but it means the scaling laws should not be taken as established.\n\nThe stress-test note about lubrication/contact cutoffs is worth taking seriously. Lubrication is cut off at h_ij > a'/2, regularized at 10^-3 a', and contact stiffnesses are 10^4; all of these set the threshold for when a frictional contact exists, and n_fc is the load-bearing observable. The author reports no convergence tests in these parameters, and no code or data are released. That is a robustness gap, not a demonstrated artifact—the qualitative contact-breaking picture could easily survive such checks—but the paper would be much stronger if it showed the viscosity reduction and the jamming shift do not depend on h_min and k_n/k_t.\n\nVerdict: worth a serious referee. I would send it to peer review with the expectation of heavy revision, and I would ask for ensemble statistics, a parameter-convergence appendix, and a less circular treatment of Teff. The qualitative mechanism and the jam/unjam asymmetry deserve to enter the literature.","headline":"A plausible DEM study that makes a credible case for activity fluidizing dense frictional suspensions and shifting jamming, but the quantitative scaffolding needs error bars, convergence checks, and less retrofitted scaling before the exponents can be trusted.","tokens_in":10520,"tokens_out":3016,"would_cite":true,"duration_ms":29134,"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":"Run-and-tumble activity fluidizes dense frictional suspensions and shifts their jamming point toward random close packing.","keywords":["active suspensions","run-and-tumble particles","shear jamming","viscosity reduction","frictional contacts","non-Brownian suspensions","discrete element method","suspension temperature"],"falsifier":"Run the same shear protocol with a different lubrication cutoff, for example regularizing the gap at $10^{-4}a'$ instead of $10^{-3}a'$, or with contact stiffness $k_n=k_t=10^5$; if the order-of-magnitude viscosity drop and the shift of $\\phi_J^{\\mu_p}$ toward $\\phi_{\\mathrm{RCP}}$ disappear or change sign, the reported activity effect is controlled by the truncation. Alternatively, measure the relative viscosity of a dense frictional suspension ($\\phi$ near 0.55) seeded with a few percent of self-propelled Janus colloids: the claim predicts a drop of at least a factor of several compared with the passive suspension at the same $\\phi$.","tokens_in":9462,"feed_emoji":"🌀","tokens_out":8784,"duration_ms":67730,"temperature":0.7,"pith_summary":"This paper argues that adding run-and-tumble active particles to a dense, non-Brownian frictional suspension can lower its viscosity by an order of magnitude and push the shear-jamming volume fraction up to random close packing. The mechanism is microscopic: activity breaks existing frictional contacts and prevents new ones from forming, shortening and isotropizing the force-chain network. The effect is strong for frictional particles and nearly absent for frictionless ones, and a larger active force is needed to unjam an already jammed state than to keep an unjammed state flowing. The paper closes with a new constitutive law based on the suspension temperature that collapses active and passive rheology onto a single curve.","feed_headline":"Run-and-tumble activity cuts dense-suspension viscosity tenfold","feed_subtitle":"Simulations show activity breaks frictional contacts, shifting shear jamming toward random close packing.","key_machinery":"The central object is the frictional contact network, quantified by the mean number of frictional contacts per particle, $n_{fc}$. In passive dense suspensions, $n_{fc}$ grows sharply with volume fraction, producing long, anisotropic force chains whose proliferation drives the viscosity divergence and shear jamming. Run-and-tumble activity acts as local random driving: active particles push their neighbours, breaking existing contacts and preventing new ones, which lowers $n_{fc}$, shortens force chains, and isotropizes the contact network. The paper introduces an effective temperature $T_{\\mathrm{eff}} \\sim F_a^4 \\tau_p$ to collapse viscosity data at small activity (rather than the usual $F_a^2\\tau_p$), and a constitutive law built on the suspension temperature $\\Theta_s = \\eta_f \\delta u / a P$: $\\mu^{1.65}\\Theta_s = \\gamma F(J)$, where $\\mu$ is macroscopic friction, $J$ the viscous number, and $\\gamma$ distinguishes active ($\\gamma=1$) from passive ($\\gamma=2$) systems.","core_discovery":"Using discrete-element simulations of bidisperse non-Brownian spheres with contact and lubrication forces, the author shows that run-and-tumble activity—parameterized by active fraction $C_a$, active force $F_a$, and persistence time $\\tau_p$—systematically reduces the relative viscosity $\\eta_r$ of dense frictional suspensions, by more than an order of magnitude at the highest volume fractions studied. The reduction is monotonic in $C_a$ but non-monotonic in $F_a$ and $\\tau_p$: $\\eta_r$ first falls to a volume-fraction-dependent minimum $\\eta_r^m$ at optimal values $F_a^m$ and $\\tau_p^m$, then rises again because strong activity enhances diffusion. Both $F_a^m$ and $\\eta_r^m$ diverge as $\\phi$ approaches $\\phi_{\\mathrm{RCP}}$, and the shear-jamming volume fraction $\\phi_J^{\\mu_p}$ increases with activity, reaching $\\phi_{\\mathrm{RCP}}$ for full activity. Microscopically, the mean number of frictional contacts $n_{fc}$ tracks the viscosity curve, and force chains become shorter and more isotropic with a reduced velocity-correlation length. The author also finds that unjamming a pre-jammed state requires a larger active force than preventing jamming from the start, and that standard $\\mu$–$J$ rheology fails for active suspensions while the generalized law $\\mu^{1.65}\\Theta_s = \\gamma F(J)$, with $\\gamma=1$ for active and $\\gamma=2$ for passive systems, collapses the data.","pith_inferences":["The author leaves implicit that this viscosity-reduction mechanism may be generic: any local, momentum-neutral agitation that breaks frictional contacts—such as cyclic orthogonal deformation or acoustic forcing—could produce the same fluidization and jamming shift as run-and-tumble activity.","A testable extension is to map the optimal persistence time $\\tau_p^m$ onto the local structural relaxation time of the passive suspension; the data hint that activity tunes viscosity most efficiently when its reorientation time matches local rearrangement timescales.","If contact breaking is the whole story, the same tenfold viscosity drop should be observable in experiments with a small fraction of self-propelled colloids in a dense frictional suspension, which would confirm that activity is a practical rheological control knob."],"forward_implications":["Dense frictional suspensions can be fluidized without changing their composition or applying external mechanical agitation: adding a modest fraction of active particles lowers viscosity by up to an order of magnitude near jamming.","The shear-jamming volume fraction becomes tunable in situ by adjusting active fraction, force, or persistence time; at full activity it moves from $\\phi_J^{\\mu_p=1}\\approx0.58$ to $\\phi_{\\mathrm{RCP}}\\approx0.636$.","Jammed states respond differently from unjammed ones: a modest active force keeps an unjammed suspension flowing, but a jammed suspension needs a substantially larger force to break its force chains.","The standard $\\mu$–$J$ rheology does not hold for active suspensions; the suspension-temperature law $\\mu^{1.65}\\Theta_s = \\gamma F(J)$ provides a unified description for active and passive systems."],"supporting_citations":[{"why":"The passive-suspension analogue: external orthogonal deformation that reduces viscosity by breaking frictional contacts, which motivates testing activity as local driving.","marker":"[10]"},{"why":"Comparison case showing the jamming volume fraction stays unchanged for frictionless suspensions under activity.","marker":"[13]"},{"why":"Establishes the absorbing-state picture in which broken contacts proliferate under cyclic orthogonal deformation, the framework the paper extends to run-and-tumble activity.","marker":"[14]"},{"why":"Supplies the standard $\\mu$–$J$ rheology and the rate-independent regime in which the simulations are run.","marker":"[39]"},{"why":"Gives the run-and-tumble model and the effective-temperature scaling $T_{\\mathrm{eff}}\\sim F_a^2\\tau_p$ that the paper modifies to $F_a^4\\tau_p$.","marker":"[40]"},{"why":"Defines random close packing $\\phi_{\\mathrm{RCP}}$, the upper limit to which the jamming volume fraction shifts.","marker":"[41]"},{"why":"Provides the simulation conditions and friction-dependent jamming framework used for the dense non-Brownian suspensions.","marker":"[47]"},{"why":"Introduces the suspension temperature $\\Theta_s$ on which the new constitutive law is built.","marker":"[56]"}],"fun_headline_variants":["Run-and-tumble activity tunes viscosity and jamming in dense suspensions","Active particles reduce viscosity tenfold and shift jamming","Activity breaks force chains, cutting viscosity in dense suspensions","Unjamming dense suspensions needs stronger activity than jamming prevention","New constitutive law captures active non-Brownian suspension rheology"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The results depend on the simulation's choices for when lubrication forces are truncated and regularized (switched off for gaps larger than half a small-particle radius and capped at one-thousandth of that radius) and for contact stiffness; if those cutoffs bias how many frictional contacts form in active versus passive systems, the viscosity reduction and jamming shift could be model artifacts rather than robust physics.","fun_headline_variants_meta":{"raw":{"variants":["Run-and-tumble activity tunes viscosity and jamming in dense suspensions","Active particles reduce viscosity tenfold and shift jamming","Activity breaks force chains, cutting viscosity in dense suspensions","Unjamming dense suspensions needs stronger activity than jamming prevention","New constitutive law captures active non-Brownian suspension rheology"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001345,"raw_usage":{"total_tokens":5515,"prompt_tokens":1047,"completion_tokens":4468,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":663,"completion_tokens_details":{"reasoning_tokens":4385}},"tokens_in":663,"tokens_out":4468,"duration_ms":28540,"temperature":1.0,"reasoning_tokens":4385,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T20:05:40.013481+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same shear protocol with a different lubrication cutoff, for example regularizing the gap at $10^{-4}a'$ instead of $10^{-3}a'$, or with contact stiffness $k_n=k_t=10^5$; if the order-of-magnitude viscosity drop and the shift of $\\phi_J^{\\mu_p}$ toward $\\phi_{\\mathrm{RCP}}$ disappear or change sign, the reported activity effect is controlled by the truncation. Alternatively, measure the relative viscosity of a dense frictional suspension ($\\phi$ near 0.55) seeded with a few percent of self-propelled Janus colloids: the claim predicts a drop of at least a factor of several compared with the passive suspension at the same $\\phi$.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"The passive-suspension analogue: external orthogonal deformation that reduces viscosity by breaking frictional contacts, which motivates testing activity as local driving."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the absorbing-state picture in which broken contacts proliferate under cyclic orthogonal deformation, the framework the paper extends to run-and-tumble activity."},{"cited_title":"Boyer, E","cited_arxiv_id":null,"evidence_quote":"Supplies the standard $\\mu$–$J$ rheology and the rate-independent regime in which the simulations are run."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the simulation conditions and friction-dependent jamming framework used for the dense non-Brownian suspensions."},{"cited_title":"Kim and K","cited_arxiv_id":null,"evidence_quote":"Introduces the suspension temperature $\\Theta_s$ on which the new constitutive law is built."}],"review_version":1}