{"id":"02a43bb3-3579-4fcc-b53d-ba40aefb8e3c","arxiv_id":"2510.24535","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Motile E. coli in sessile droplets reduce droplet-surface adhesion and increase contact-line mobility via active, spatially coordinated swimming that opposes evaporation-driven capillary flow.","lead":"Live E. coli bacteria inside water droplets make the droplets stick less firmly to surfaces than droplets with dead bacteria, despite having lower surface tension. The reason appears to be the bacteria's active swimming, which creates internal flows that help the droplet's edge slip free and slide off more easily.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Live-vs-dead comparison is confounded by IPA-induced aggregation; the central claim that motility alone lowers adhesion needs a clean non-motile control.","rationale":"The reader's weakest_assumption and mine coincide: the dead-cell control is confounded by IPA-induced aggregation. This is more load-bearing than the substrate mismatch or PIV pseudoreplication because every element of the causal story—live droplets resist capillary flow, depin more easily, and slide sooner—is ultimately anchored to a comparison in which the 'no motility' arm differs in particle size distribution and surface state, not just motility. A non-motile mutant control would settle it directly. The proxy-particle sliding tests reduce concern about surface tension but do not close the adhesion gap, and they were not run on the adhesion substrate. I therefore keep the CONDITIONAL verdict; no adjustment beyond the reader's is needed.","tokens_in":27036,"tokens_out":5512,"duration_ms":55847,"concrete_test":"Repeat the cantilever adhesion measurements (Fig. 3d) at 10^7 CFU/mL using (i) an isogenic non-motile E. coli mutant (e.g., ΔmotAB or ΔfliC) in the same live culture medium, (ii) wild-type cells killed by heat (e.g., 60°C, 30 min) instead of IPA, and (iii) IPA-killed cells filtered to remove aggregates (e.g., 5-μm or 0.45-μm syringe filter) and characterized by DLS/microscopy. If the non-motile mutant and heat-killed cells show F_adhesion close to the IPA-dead value while wild-type shows ~9 μN, the motility attribution survives; if any of these clean non-motile controls yields low adhesion comparable to live wild-type, the live-vs-dead gap is a killing artifact rather than a motility effect.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim—that flagellar motility alone reduces droplet-surface adhesion—rests entirely on the live-vs-dead contrast, but the dead arm is not a clean non-motile control. Dead cells were generated by suspension in 70% isopropyl alcohol, a treatment that lyses cells; the authors' own DLS (Fig. S4) reports a high polydispersity index for the dead sample and notes possible aggregates from lysis products. Larger aggregates are known to increase contact-line pinning (cf. refs. 45–46, 50), and the dead droplets show both higher adhesion (~39 μN vs ~9 μN at 10^7 CFU/mL) and a monotonically increasing adhesion with concentration—exactly the inert-pinning signature. Because aggregate size, surface chemistry, and concentration all change simultaneously between live and dead suspensions, the adhesion difference cannot be uniquely attributed to the absence of swimming. The proxy sliding experiments (Fig. S7) show that 1-μm polystyrene beads cannot reproduce the live droplet mobility, but these were performed on PMMA, not on the NeverWet adhesion substrate, and the proxy particles do not mimic the aggregated dead-cell morphology. The limitation is acknowledged in Study Scope and Limitations but not experimentally controlled; it is the load-bearing gap in the causal argument.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript investigates how flagellar motility of E. coli within sessile droplets affects droplet-surface adhesion and wetting. Using a cantilever-deflection method on superhydrophobic NeverWet surfaces, the authors report that live-bacteria-laden droplets exhibit lower adhesion than dead-bacteria-laden droplets, and that adhesion decreases with increasing live-bacteria concentration while increasing (nonsignificantly) for dead ones. Micro-PIV on glass shows that live bacteria have lower net velocity toward the contact line and stronger spatial correlations than dead bacteria, which is interpreted as rheotactic resistance and collective motion. Sliding experiments on PMMA show more frequent stick-slip and earlier depinning for live droplets, which inert-particle and surface-tension-matched proxy suspensions do not reproduce. The authors conclude that bacterial motility alone reorganizes internal flow and weakens contact-line pinning, thereby reducing droplet adhesion.","tokens_in":27342,"tokens_out":7333,"duration_ms":65147,"significance":"If the causal claim survives, this would be a novel and useful result for self-cleaning and antifouling surface design, combining a direct droplet-scale adhesion measurement with internal flow visualization. The paper's strengths include the cantilever-based adhesion quantification, the inclusion of falsifiable proxy experiments (surface-tension-matched fluids and inert particles), and the careful statistical treatment of PIV velocity data. The central live-vs-dead comparison, however, is not a clean test of motility because the inactivation protocol likely introduces aggregates, and the mechanistic chain is assembled from measurements on three different substrates. The paper's own limitation section acknowledges the aggregate issue but does not resolve it, leaving the magnitude of the motility contribution uncertain.","major_comments":[{"comment":"The live-vs-dead contrast is the sole experimental basis for attributing reduced adhesion to motility, but the dead control is confounded by aggregation. Dead cells were produced by suspension in 70% IPA, which can lyse cells; the authors' own DLS (Fig. S4) reports a high polydispersity index for the dead sample and notes possible aggregates from lysis products. Since the authors themselves argue, citing refs. 45-46 and 50, that inert microparticles increase contact-line pinning and adhesion, larger aggregates in the dead sample could independently raise adhesion. The dead samples do show the expected inert-particle signature: higher adhesion (about 39 µN vs about 9 µN at 10^7 CFU/mL) and a monotonically increasing adhesion with concentration, although the latter is not statistically significant. Without a clean non-motile control, such as heat-killed cells with confirmed absence of aggregates or a motile-deficient strain, the adhesion difference cannot be uniquely attributed to the absence of swimming. This issue is acknowledged in the Study Scope and Limitations but is not experimentally controlled; it is load-bearing for the central claim.","section":"Study Scope and Limitations; Fig. S4; Fig. 3(d)"},{"comment":"The mechanistic inference is assembled across mismatched substrates. Adhesion is measured on superhydrophobic NeverWet surfaces, PIV is performed on clean glass, and sliding experiments on PMMA. Since contact-line pinning and droplet retention depend strongly on substrate roughness, chemistry, and wettability, the claim that the observed internal flow organization promotes depinning on the NeverWet surface is an extrapolation. In particular, the proxy sliding experiments (Fig. S7 and Supporting Videos S6-S7) demonstrate only that surface tension and 1-µm polystyrene beads cannot reproduce the live-droplet sliding behavior on PMMA; they do not test the adhesion scenario on NeverWet, and the proxy particles do not mimic the aggregated morphology of the dead cells. A bridging experiment, for example PIV on the actual adhesion substrate or adhesion measurement on the PIV substrate, is needed to support the causal chain.","section":"Microscopic visualization of internal motion dynamics; Sliding Behavior Explains Near Contact Line Dynamics"},{"comment":"The passive surface-tension channel is not excluded on the adhesion substrate. Live droplets at 10^7 CFU/mL have surface tension 65.82 +/- 1.6 mN/m versus about 72 mN/m for water and dead droplets, and Fig. S5 shows a further time-dependent decrease to about 57.5 mN/m during the measurement window. The proxy suspensions used to rule out surface-tension effects (Fig. S7) were characterized only in sliding experiments on PMMA, not in cantilever adhesion on NeverWet. On a superhydrophobic surface, the capillary adhesion force can scale directly with surface tension, so the lower adhesion of live droplets may be in part a passive consequence of lower surface tension rather than of motility-induced depinning. The manuscript should either measure adhesion of a surface-tension-matched passive suspension on NeverWet or provide a quantitative argument showing that the observed roughly 30 µN reduction cannot be explained by the surface tension difference.","section":"Direct quantification of surface adhesion using cantilever method; Fig. S5"}],"minor_comments":[{"comment":"The figure containing the sliding data is referenced in the text as 'Figure 8' but is captioned 'Figure 6'; the in-text references to 'Figure 6c' and 'Figure 6a' in the same section should be corrected to the actual figure number and panels.","section":"Sliding Behavior Explains Near Contact Line Dynamics of Bacterial Droplets"},{"comment":"The sentence 'It leads to inaccurate identification of the baseline of the droplet, thereby causing the propagation of substantial systematic errors' is missing a clear antecedent for 'It'; consider rephrasing to identify the source of the error, such as optical noise or baseline misidentification.","section":"Dynamic Wetting Characterization via Contact Angle Goniometry"},{"comment":"The statistical description 'two-way ANOVA (Tukey) test' should be clarified as 'two-way ANOVA followed by Tukey's post hoc test'.","section":"Materials and Methods - Wetting & adhesion measurements"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the journal's scope, and the experimental effort is substantial. The main obstacle is the cleanliness of the non-motile control: the IPA-killed cells are likely aggregated, and the aggregate effect is not isolated. I would advise the editor to request a revision with a clean non-motile control (e.g., heat-killed cells or a motile-deficient strain) or a substantially toned-down causal claim, together with an attempt to bridge the substrate mismatch between the adhesion, PIV, and sliding experiments."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a solid experimental study with a clear quantitative result—live E. coli droplets adhere less to a superhydrophobic surface than IPA-killed ones—but the dead control is not clean enough to prove that flagellar motility is the cause. Worth sending to a good referee, but it needs another round of controls.\n\nWhat's new and good: The cantilever force data are direct, and the live-vs-dead difference is large (~9 vs ~39 μN) and statistically well-supported. The concentration dependence in live droplets (adhesion drops as concentration increases) is a real internal trend. The PIV work is careful: they compare directionality, compute spatial correlations, and the dead-bacteria pathlines reproduce passive capillary transport. The sliding experiments on PMMA with surface-tension-matched EG-water and with polystyrene beads are a nice falsification check—neither passive low-surface-tension liquid nor inert particles replicate the stick-slip mobility of live droplets. That is legitimate evidence that something about the live cells matters.\n\nWhere it's soft: The live-vs-dead comparison is the load-bearing wall, and it has a crack the authors themselves identify. Killing with 70% IPA lyses cells, and their own DLS shows high polydispersity and possible aggregates in the dead sample. Larger particles are known to pin contact lines. So part of the 30 μN gap could be aggregate-induced pinning, not the absence of swimming. The interpretation also moves across substrates: adhesion on NeverWet, PIV on glass, sliding on PMMA. The internal flow on a hydrophilic glass slide is not necessarily the same as that near a superhydrophobic contact line. The PIV statistics use different inter-frame times and frame counts for live vs dead; the correlation function is smoothed and may inflate coherence. And because live droplets have lower surface tension, the adhesion decrease with concentration might partly be a surface-tension effect; the sliding proxy addresses this on PMMA but not on NeverWet.\n\nNone of this destroys the paper. The experiments are reproducible, the measurements are direct, and the authors are upfront about the limitations. But the strong claim—'motility alone reorganizes internal flow and weakens pinning'—needs a cleaner non-motile control: a flagellar motor mutant, or CCCP-paralyzed cells, or heat killing with careful aggregate screening. With that control, and ideally adhesion measurements on at least one additional substrate, this would be a strong paper.\n\nBottom line: who is this for? People working on active droplets, bacteria-surface interactions, and self-cleaning coatings will want to read it. I'd cite it if I worked in that area, and I'd send it to a serious referee expecting a major revision. The right referee time is on the control experiments, not on the measurements.","headline":"A quantitative but not yet watertight case that bacterial motility reduces droplet adhesion; the dead control is the weak link.","tokens_in":27769,"tokens_out":2996,"would_cite":true,"duration_ms":29615,"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":"Live, swimming E. coli lower droplet adhesion to surfaces, and more bacteria lower it further, by opposing capillary flow and promoting contact-line depinning.","keywords":["bacteria","motility","surface adhesion","active migration","flagella","collective behavior","capillary flow","dynamic wetting"],"falsifier":"A decisive test would measure cantilever adhesion for viable non-motile mutants at the same concentration and on the same superhydrophobic surface: the paper's claim predicts that droplets with mutant cells stick near the dead-cell value (about 39 µN at $10^7$ CFU/mL) rather than the live-cell value (about 9 µN). If the mutant droplets also show low adhesion, the reduction is not caused by swimming.","tokens_in":26821,"feed_emoji":"🦠","tokens_out":14397,"duration_ms":123208,"temperature":0.7,"pith_summary":"This paper tries to establish that the swimming of bacteria inside a droplet, not just their presence or surface chemistry, controls how strongly the droplet adheres to a solid surface. Comparing droplets loaded with live and dead E. coli, the authors find that live droplets adhere less to a superhydrophobic surface—at the highest concentration the adhesion was roughly 9 µN for live cells versus 39 µN for dead cells—and that adhesion keeps falling as the live-cell concentration rises, even though live droplets have lower surface tension. Internal flow imaging shows the live bacteria swim against the evaporation-driven current that would otherwise carry them to the contact line, and at higher concentrations their motion becomes collectively organized and spatially coherent. Sliding experiments on a moderately wetting surface show live droplets depin and slide at lower tilt angles, with frequent stick-slip motion, while proxy droplets matched in surface tension or loaded with inert microparticles do not reproduce this behavior. The paper concludes that bacterial motility alone can reorganize internal flow and weaken contact-line pinning.","feed_headline":"Live E. coli droplets stick far less than dead ones","feed_subtitle":"Swimming opposes capillary flow and frees the contact line, so motile contaminants may be easier to remove.","key_machinery":"The load-bearing mechanism is flagellar run-and-tumble motility, the alternating straight swimming and reorientation that lets E. coli exert its own forces on the surrounding fluid and swim upstream (positive rheotaxis). The paper tracks the competition between this active swimming and the evaporation-driven capillary flow with micro-particle image velocimetry, and quantifies the resulting organization with a temporally resolved two-point Pearson correlation of velocity components: live suspensions keep peak correlations near 0.96 that decay slowly across the roughly 200 µm observation window, while dead suspensions decorrelate within about 40 µm. The proposed link to adhesion is that these coordinated, intermittently reversing flows agitate and repeatedly depin the triple contact line, an effect quantified on the force side by the cantilever relation $F_{\\mathrm{adh}} = k\\Delta X$, where $k$ is the cantilever spring constant and $\\Delta X$ the maximum deflection at detachment.","core_discovery":"The central claim is that motile E. coli inside a sessile droplet actively oppose the evaporation-driven capillary flow that normally carries particles toward the triple contact line, and that this opposition—amplified into coordinated, direction-reversing collective motion at higher concentrations—mechanically disturbs the contact line, promotes depinning, and therefore lowers the force needed to detach the whole droplet. Direct force measurements support the claim: at about $10^7$ CFU/mL, live droplets showed an adhesion near 9 µN against roughly 39 µN for dead droplets, with live-droplet adhesion decreasing statistically significantly as concentration increased and dead-droplet adhesion increasing slightly. Micro-PIV and particle tracking show that dead bacteria passively ride the capillary flow toward the contact line, whereas live bacteria exhibit around-zero mean net velocity, frequent directional reversals, and long-range spatial correlations. The paper argues this is positive rheotaxis: flagellated cells swim upstream against the shear near the wall, and at high density their coordinated swimming promotes frequent contact-line depinning events that reduce adhesion.","pith_inferences":["The paper does not test whether the effect extends to other motile species, but if it does, the viability of airborne pathogens changes their physical persistence on surfaces: a droplet of living, swimming bacteria should roll off a repellent surface more readily than one containing inactivated cells, which would alter fomite-contamination patterns.","A cleaner control than alcohol-killed cells would be viable but non-motile mutants (for example, flagellar-knockout strains) or dead cells prepared without lysis; the paper's own limitation note acknowledges that aggregates in the dead sample could contribute to pinning, so the true size of the motility effect is not yet isolated.","The correlation data imply a quantitative prediction the authors did not make: the adhesion reduction should scale with an activity ratio of swimming strength to capillary flow strength, so that on surfaces or under humidity conditions with weaker evaporation-driven flow, the same bacterial concentration should produce even larger relative adhesion reductions.","If the depinning mechanism is robust, it could be exploited deliberately—for example, by maintaining conditions that sustain motility in deposited droplets—as a passive cleaning strategy, rather than designing surfaces only for low surface energy."],"forward_implications":["On superhydrophobic surfaces, conventional contact angle measurements cannot predict how strongly a biologically active droplet sticks, because a one-pixel baseline error can change the inferred work of adhesion by roughly 35 percent or more.","Within the first hundred seconds after deposition, droplets carrying live motile bacteria are easier to remove than droplets carrying inert particles of the same size, and this removal advantage grows with bacterial concentration.","Because increasing the concentration of live bacteria lowers adhesion while the opposite holds for dead cells, contamination load does not simply make a droplet harder to clean at early timescales.","Sliding behavior of live droplets is governed by frequent depinning events rather than by macroscopic receding angle or hysteresis, so droplet roll-off models based only on these angles will misclassify active biological droplets.","The cantilever-deflection method, rather than goniometry, is needed to resolve subtle adhesion differences between live and dead bacterial suspensions."],"supporting_citations":[{"why":"Shows that biosurfactant-producing bacteria can alter the capillary flow in an evaporating droplet and reverse the coffee-ring effect, providing the starting point the paper extends to droplet adhesion.","marker":"[24]"},{"why":"Identifies evaporation-driven capillary flow as the transport that carries suspended particles to the contact line, the passive baseline against which live bacterial motion is compared.","marker":"[27]"},{"why":"Provides the prior demonstration of active depinning of bacterial droplets, the phenomenon the paper connects to early-stage adhesion.","marker":"[26]"},{"why":"Supplies the rheotaxis mechanism by which flagellated bacteria swim upstream against shear flow, the explanation for live cells resisting capillary transport.","marker":"[37]"},{"why":"Establishes the wetting signature of microparticle and bacteria-laden droplets, the inert-particle baseline used to interpret dead-cell pinning.","marker":"[46]"},{"why":"Introduces the cantilever-deflection adhesion measurement technique whose force data carry the paper's central result.","marker":"[47]"},{"why":"Documents the systematic errors of contact angle goniometry on superhydrophobic surfaces, justifying the switch to direct force measurement.","marker":"[49]"},{"why":"Shows that inert microbeads raise contact angle hysteresis through pinning, the mechanism the paper invokes for dead-cell adhesion increasing with concentration.","marker":"[50]"}],"fun_headline_variants":["Swimming E. coli cut droplet adhesion by 4x","Motile E. coli droplets: less stick, more slip","Bacterial swimming disrupts capillary flow, reduces adhesion","Motile bacteria make droplets less sticky","Self-cleaning insight: live E. coli droplets adhere less"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The live-versus-dead comparison assumes that dead bacteria are an inert stand-in for non-motile cells, but the alcohol-killing step leaves clumps of cell debris that could themselves pin the contact line and raise adhesion, so part of the measured difference may come from clumping rather than from the absence of swimming.","fun_headline_variants_meta":{"raw":{"variants":["Swimming E. coli cut droplet adhesion by 4x","Motile E. coli droplets: less stick, more slip","Bacterial swimming disrupts capillary flow, reduces adhesion","Motile bacteria make droplets less sticky","Self-cleaning insight: live E. coli droplets adhere less"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00062,"raw_usage":{"total_tokens":2927,"prompt_tokens":1047,"completion_tokens":1880,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":663,"completion_tokens_details":{"reasoning_tokens":1801}},"tokens_in":663,"tokens_out":1880,"duration_ms":13216,"temperature":1.0,"reasoning_tokens":1801,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T15:40:37.516904+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A decisive test would measure cantilever adhesion for viable non-motile mutants at the same concentration and on the same superhydrophobic surface: the paper's claim predicts that droplets with mutant cells stick near the dead-cell value (about 39 µN at $10^7$ CFU/mL) rather than the live-cell value (about 9 µN). If the mutant droplets also show low adhesion, the reduction is not caused by swimming.","supporting_citations":[],"review_version":2}