{"id":"2ef7f1e0-896d-4f38-94e6-fd57997fb86a","arxiv_id":"2506.03886","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Simulations with limited experimental backing show H2O2 and OH concentrations in plasma-treated water can be steered over orders of magnitude by adjusting power, water vapour, and treatment time.","lead":"This paper uses computer simulations of a helium plasma jet and the water it treats to map how hydrogen peroxide (H2O2) and hydroxyl radicals (OH) build up in the liquid under different power, humidity, and jet-to-water distances. It shows the two species' ratio can be tuned over many orders of magnitude, which matters for plasma-based medicine and enzyme-driven biocatalysis.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The unvalidated linear velocity decay in the unguided effluent sets OH residence times; because OH at the liquid surface is not measured, the claims of OH control and orders-of-magnitude OH/H2O2 ratio control may rest on an ad hoc kinematic profile.","rationale":"The reader's weakest-assumption analysis identified the linear velocity decay in the unguided effluent, and I agree that this is the load-bearing uncertainty. The paper is transparent: it states the assumption, acknowledges the H2O2 overestimate and the distance-trend mismatch, and gives the factor-five caveat. These are reasons to keep the verdict conditional, not to reject. However, the headline claim that OH/H2O2 selectivity can be controlled over orders of magnitude rests primarily on simulated OH, not on the validated H2O2 channel. The linear velocity profile is not a small calibration detail: it sets residence times in the effluent, and Fig. 6 shows OH is actively consumed there. The H2O2 comparisons in Fig. 4 cannot validate the velocity profile because H2O2 is long-lived and its liquid concentration is set by transport, not by effluent residence time. The paper's main qualitative OH results—maximum near 200 ppm water, decrease with effluent length, treatment-time decrease due to H2O2 buildup—could still be correct, but they need either a sensitivity check or an experimental OH measurement. Therefore the appropriate verdict remains conditional: the qualitative framework is plausible and partially validated, but the OH-based selectivity claims need one more test to be secure.","tokens_in":18168,"tokens_out":3557,"duration_ms":35954,"concrete_test":"Rerun the gas-phase plug-flow simulation of Section 2.1 with the linear-decay velocity profile replaced by a constant profile at the capillary-exit velocity, keeping the helium-water chemistry and all other settings fixed. Recompute the OH density at the liquid surface for the water-admixture sweep (100–6400 ppm) and the power sweep of Fig. 5. If the OH trend or the position of the OH maximum changes by more than a factor of about two, or if the claimed orders-of-magnitude ratio control in the abstract depends on the linear profile, then the central OH and ratio claims are not robust to the assumed velocity profile.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is in Section 2.1: the unguided effluent velocity is assumed to decrease linearly from the capillary exit to zero at the liquid surface, with the simulation length adjusted to give an equivalent residence time. This profile is neither measured nor derived, and it directly sets the residence time of every species in the 14–45 mm atmospheric-pressure effluent. Because OH is consumed throughout the effluent (Fig. 6) while H2O2 is stable, the unvalidated velocity profile has a first-order effect on the simulated OH density at the liquid surface—the principal input to the liquid OH concentration. The paper validates H2O2 against experiment, but H2O2 is insensitive to this assumption, so that validation provides no check on the OH channel. The abstract's claim that the OH/H2O2 ratio can be controlled over orders of magnitude depends specifically on the OH trends in Fig. 5, which have no experimental counterpart. A different but equally plausible velocity profile (e.g., constant jet velocity until spreading, or a laminar deceleration) could change both the magnitude and the location of the OH maximum in water-admixture and power space, and thus could alter the qualitative control map, not just the absolute calibration.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript presents a combined gas-phase (0-D plug-flow with GlobalKin) and liquid-phase (1-D reaction-diffusion) model to predict H2O2 and OH concentration profiles in water treated by a radio-frequency-driven helium plasma jet with a glass capillary. The model is used to explore how water admixture, deposited power, and jet-to-liquid distance affect H2O2 and OH in the liquid. H2O2 predictions are compared with colorimetric measurements after 900 s treatment, showing qualitative agreement for power and water-admixture trends but a factor-of-five overestimate in absolute concentration and a failure to reproduce the experimental decrease of H2O2 with jet-to-liquid distance. OH predictions are not experimentally validated; the model predicts OH is confined to nanometre depths and is consumed by H2O2, leading to the claim that the OH/H2O2 ratio can be controlled over orders of magnitude by the operating parameters. The paper emphasizes effluent chemistry as the key control point for OH delivery.","tokens_in":18458,"tokens_out":6480,"duration_ms":54414,"significance":"If valid, the framework is attractive because it couples a low-cost 0-D gas model to a 1-D liquid model and makes specific, falsifiable predictions about OH/H2O2 selectivity that are relevant to plasma-driven biocatalysis. The paper's strengths are its transparency about assumptions, its use of an experimentally calibrated effective diffusion coefficient for H2O2, and its direct comparison with measured H2O2 concentrations. However, the predictive value is currently limited by the unvalidated velocity profile in the unguided effluent, the hand-chosen lateral FWHM used in the volume averaging, and the absence of any experimental check of the OH predictions. These limitations affect the central quantitative claims (absolute concentrations and the 'orders of magnitude' ratio control) and therefore the paper currently reads as a useful hypothesis-generating study rather than a validated predictive model.","major_comments":[{"comment":"Section 2.1 states that the gas flow velocity in the unguided effluent is assumed to decrease linearly from the capillary exit to zero at the liquid surface, with the simulation length adjusted for an equivalent residence time. This kinematic profile is neither measured nor derived from fluid dynamics, yet it sets the residence time of OH in the effluent, where OH is consumed (Fig. 6). The H2O2 validation in Fig. 4 cannot constrain this assumption because H2O2 is nearly stable in the effluent. Since the OH flux to the liquid, and hence the simulated liquid OH concentration and penetration depth (Fig. 5), are first-order sensitive to this residence time, the paper's central claim that the OH/H2O2 ratio can be controlled over orders of magnitude relies on an untested assumption. Please provide a sensitivity analysis over plausible alternative velocity profiles (e.g., constant jet velocity with lateral spreading, or parabolic decay) and/or measurements of the gas velocity or OH density in the effluent to bound the uncertainty.","section":"Section 2.1, Figs. 5-9"},{"comment":"The comparison between simulated and experimental H2O2 concentrations requires an assumed Gaussian lateral profile with FWHM = 3 mm, described in Section 3.1 as a coarse estimate. The integrated concentration is strongly dependent on this FWHM, and the paper acknowledges that the value is uncertain. The reported factor-of-five overestimate is therefore not a well-constrained quantitative discrepancy; a different FWHM would change the simulation curve. Please quantify the sensitivity of the integrated H2O2 concentration to the FWHM over a reasonable range (e.g., 1-10 mm) and report the resulting uncertainty band. Without this, the claim of 'good qualitative agreement' cannot be distinguished from calibration.","section":"Section 3.1, Fig. 4"},{"comment":"The OH concentration and penetration depth predictions (Fig. 5) have no experimental validation, yet they underpin the abstract's statement that the OH/H2O2 ratio can be controlled over orders of magnitude. The two-reaction liquid scheme (Reactions 1-2) omits other OH scavengers (e.g., dissolved oxygen, buffer species, or products of the plasma-liquid interaction), and the gas-phase velocity assumption discussed above directly controls OH delivery. The authors should either (i) provide an experimental OH measurement (e.g., using a fluorescent probe or a scavenger assay) for at least one operating condition, or (ii) perform and report a sensitivity analysis showing how the qualitative trends in Fig. 5 survive variations in the velocity profile and the liquid reaction set. As it stands, the 'orders of magnitude' control claim is a model extrapolation.","section":"Section 3.2"},{"comment":"The model predicts H2O2 concentration in the liquid to be independent of jet-to-liquid distance, whereas the experiment shows a clear decrease with distance (Fig. 4c). The authors attribute this to unmodeled lateral spreading of the gas jet; this is reasonable, but it also implies that the OH-distance predictions in Fig. 5c are subject to the same unmodeled spreading, which would dilute the OH flux and change its residence time. Because the distance variation is one of the three control parameters claimed in the abstract, the failure to reproduce this trend for H2O2 should be treated as a major limitation for the OH-distance predictions as well. Please either include a simple model of jet spreading (e.g., an effective dilution factor as a function of distance) or restrict the control-parameter claims to power and water admixture.","section":"Section 3.3.3, Fig. 9"}],"minor_comments":[{"comment":"'an UV cuvette' should be 'a UV cuvette'.","section":"Section 2.2"},{"comment":"'ammonium metavandate' is a typo for 'ammonium metavanadate'; it appears twice in the description of the spectrophotometric measurement.","section":"Section 2.2"},{"comment":"The sentence 'The H2O2 remains unchanged...' should read 'The H2O2 concentration remains unchanged...'.","section":"Section 3.1"},{"comment":"Many axis labels and legend entries are difficult to read in the submitted resolution; please ensure all panels are legible.","section":"Figures 2, 3, 5, 7-9"},{"comment":"The statement that OH is 'eight to thirteen orders of magnitude' lower than H2O2 in Sec. 3.3.1 is inconsistent with 'around fourteen orders of magnitude' in Sec. 3.3.2; please reconcile the numbers for comparable conditions.","section":"Sections 3.3.1 and 3.3.2"},{"comment":"The definition of the averaged OH concentration ('averaged over the distance between the liquid surface and the penetration depth') would benefit from being written as an explicit equation, since this quantity is central to Fig. 5.","section":"Section 3.2"}],"recommendation":"major_revision","confidential_remarks":"The paper is honest about its limitations and the modeling framework is potentially useful, but the abstract oversells the predictive power given the unvalidated velocity profile and the missing OH experiment. I recommend major revision with the sensitivity analyses described in my comments; if the authors provide those, the paper could become acceptable. There is no concern about novelty overlap or citation ethics."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a solid, workmanlike extension of the group's earlier model [37], giving a parametric map of H2O2 and OH in plasma-treated water. The H2O2 part has experimental support; the OH part is an unvalidated prediction that rests on an ad hoc velocity profile. So the \"orders of magnitude ratio control\" claim is interesting but not yet demonstrated.\n\nWhat's new: systematic sweeps of water admixture, power, and jet-to-liquid distance; new H2O2 measurements for the capillary jet; a reaction-pathway analysis of the effluent showing OH is consumed en route, so the OH flux to the liquid is not a simple function of plasma-region OH. The liquid model is transparent: two reactions, an effective diffusion coefficient fitted to prior experiments, and Henry's law flux boundary conditions. The paper is honest about its limitations, including the factor-five H2O2 overestimate, the unreproduced distance trend, and the hand-picked 3 mm FWHM for lateral averaging. That transparency is real and should count in its favor.\n\nSoft spots: the load-bearing one is the linear velocity decay assumption in the unguided effluent (Section 2.1). It sets the residence time of every species in the 14-45 mm effluent, and OH is consumed throughout that region while H2O2 is not. Since OH is not measured at the liquid surface or in the liquid, the H2O2 validation doesn't exercise the OH channel. A different velocity profile (constant jet until spreading, or laminar deceleration) would change both the magnitude and possibly the argmax of the simulated OH in water-admixture and power space, which is exactly where the ratio-control claim lives. The absence of code and data also makes it hard to test the sensitivity directly. The factor-five H2O2 overestimate and the distance mismatch are secondary but signal that the quantitative calibration is incomplete.\n\nNone of this sinks the paper. The qualitative physics—H2O2 transport-dominated, OH scavenged by H2O2—is plausible and consistent with the broader literature. The authors flag the key caveats themselves. But the headline claims about OH/H2O2 ratio control should be presented as model predictions awaiting OH measurements.\n\nWho it's for: plasma-liquid experimentalists and modelers, especially people working on plasma biocatalysis or medicine who need tuning knobs for H2O2 vs OH. A serious referee should engage; the velocity assumption needs either experimental backing or a sensitivity study, and OH measurements should be the top priority.\n\nRecommendation: send to peer review. The paper is honest, useful, and the central H2O2 story is partially validated. Ask the authors to address the velocity profile and provide code and data.","headline":"A useful parametric map for H2O2/OH in plasma-treated water, but the OH side rests on an unvalidated velocity assumption and needs experimental confirmation before the order-of-magnitude control claim can be trusted.","tokens_in":18975,"tokens_out":2755,"would_cite":true,"duration_ms":25714,"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":"Simulation shows that plasma-treated water's OH/H2O2 balance is tunable over orders of magnitude by power, humidity, jet distance, and treatment time.","keywords":["plasma-liquid interaction","reactive oxygen species","hydrogen peroxide","hydroxyl radical","plasma jet","reaction-diffusion model","plasma effluent chemistry","plasma-driven biocatalysis"],"falsifier":"Measure the axial velocity profile of the effluent between the capillary exit and the liquid surface, for example with particle image velocimetry, and compare the OH flux computed from the real profile with the flux from the assumed linear fall-off; a mismatch would change predicted liquid OH concentrations and penetration depths directly.","tokens_in":18000,"feed_emoji":"🧪","tokens_out":7566,"duration_ms":62869,"temperature":0.7,"pith_summary":"Plasma jets that treat water produce two key reactive species, hydrogen peroxide (H2O2) and the hydroxyl radical (OH), and applications such as plasma-driven biocatalysis need one without too much of the other. This paper uses a 0-D plug-flow gas-phase kinetics simulation coupled to a 1-D reaction-diffusion liquid model to show that the depth profiles of both species are set by a simple division of labour: H2O2 is long-lived and its penetration into water is governed by transport, while OH is short-lived and is consumed mainly by reacting with H2O2. As a result, every operating knob—plasma power, water-vapour admixture, jet-to-liquid distance, and treatment time—moves both the absolute concentrations and the penetration depths in opposite directions for the two species, and the OH/H2O2 ratio in the liquid spans orders of magnitude. The simulations reproduce measured H2O2 trends as power and water admixture are varied, and they identify why OH density at the end of the plasma region is a poor predictor of OH delivered to the liquid: effluent chemistry consumes OH before it reaches the surface. If the picture is right, the same plasma device can be tuned to favour H2O2 (high humidity, higher power, long treatment) or OH (low humidity, low power, short treatment) without changing the reactor.","feed_headline":"OH-to-H2O2 ratio in plasma-treated water spans orders of magnitude","feed_subtitle":"Long treatment, high humidity and higher power favor H2O2; short treatment and low humidity favor OH.","key_machinery":"The argument is carried by a two-stage simulation chain. Gas phase: a 0-D plasma-chemistry plug-flow model (GlobalKin) with a validated He/H2O reaction scheme marches species densities from the powered electrode region through the capillary and unguided effluent to the liquid surface, assuming the unguided flow velocity falls linearly to zero at the surface. The densities just above the interface are converted into liquid fluxes using Henry's-law-based boundary layer expressions from Semenov et al. Liquid phase: a 1-D reaction-diffusion equation solved with MATLAB's pdepe, using only two liquid reactions—OH + OH → H2O2 and OH + H2O2 → HO2 + H2O—and an experimentally derived effective diffusion coefficient for H2O2 that accounts for gas-flow-driven convection, while OH uses its molecular diffusion coefficient. The mechanism that does the explanatory work is the second reaction: as H2O2 accumulates in the liquid, it scavenges OH and shrinks OH's penetration depth, coupling treatment time to selectivity.","core_discovery":"The paper's central claim is that the concentration profiles of H2O2 and OH in plasma-treated water are not set by plasma chemistry alone but by a gas-to-liquid relay in which the two species separate: H2O2 survives the effluent, crosses the interface, and spreads through the liquid like a diffusing plume, whereas OH is largely consumed twice—once in the effluent by reactions with O, H2, HO2, and H2O2, and again in the liquid by H2O2 itself. The liquid simulations therefore show H2O2 building up with treatment time and penetrating to millimetre depths, while OH stays within tens to hundreds of nanometres of the surface and its concentration falls as H2O2 accumulates. Trends in H2O2 in the liquid mirror trends in the gas-phase H2O2 density above the liquid for water-admixture and power variations, and the absolute simulations lie within a factor of about five of measured H2O2 concentrations; the experimental distance dependence is not captured by the model. The most striking consequence is that the OH/H2O2 ratio is a tunable property: low water admixture, low power, and short treatment times maximise selectivity for OH, while high water admixture, high power, and long treatment times maximise selectivity for H2O2.","pith_inferences":["A direct test of the assumed linear velocity profile in the unguided effluent would be the quickest way to harden or correct the absolute OH fluxes, since the residence time of OH in the effluent is set by that profile.","The factor-five overestimate of absolute H2O2 and the missed distance trend suggest that the missing physics is jet spreading and lateral transport, not the liquid reaction scheme; a 2-D or 3-D gas-flow/interface model should close most of the gap without changing the two-reaction liquid chemistry.","The two-species liquid model omits other plasma-produced species such as nitrogen oxides, ozone, and superoxide; in applications with ambient air entrainment, those could compete for OH and shift the reported selectivity map.","Because treatment time alone flips the OH/H2O2 balance, real-time monitoring of H2O2 during treatment could enable feedback control that holds a target ratio as the liquid composition evolves."],"forward_implications":["For a fixed jet setting, prolonging treatment increases the liquid H2O2 concentration and its penetration depth while decreasing the OH concentration and its penetration depth, because accumulated H2O2 consumes OH.","The H2O2 trend in the liquid can be read off the H2O2 density in the gas phase above the liquid for changes in power and water admixture, making gas-phase measurement a practical proxy for liquid outcome.","Operating maps with low water admixture, low deposited power, and short treatment times give the highest OH selectivity; high water admixture, higher power, and long treatment times give the highest H2O2 selectivity.","OH densities at the end of the plasma region cannot be used to predict OH delivery into the liquid; the effluent region's consumption chemistry must be included.","For plasma-driven biocatalysis, where H2O2 is wanted and OH is damaging, long treatment times, lower powers, higher water admixtures, and larger jet-to-liquid distances are the favourable settings."],"supporting_citations":[{"why":"Supplies the gas-to-liquid simulation method, the experimentally derived effective diffusion coefficient for H2O2, and the molecular diffusion coefficient for OH used in the liquid model.","marker":"[37]"},{"why":"Provides the flux expressions for gas-to-liquid transfer of species at the plasma-liquid interface.","marker":"[19]"},{"why":"Supplies the helium-water vapour gas-phase reaction scheme used in the plasma and effluent simulations.","marker":"[43]"},{"why":"Earlier study of the same capillary jet's H2O2 production and transport that the base case builds on.","marker":"[29]"},{"why":"Describes the GlobalKin code used for the 0-D plasma kinetics and plug-flow gas-phase simulation.","marker":"[35]"},{"why":"Describes the ammonium metavanadate spectrophotometric H2O2 measurement used to validate the simulations.","marker":"[47]"},{"why":"Measured H2O2 spatial distribution widths that motivate the 3 mm FWHM Gaussian profile assumed when comparing simulation with stirred-cuvette experiments.","marker":"[26]"}],"fun_headline_variants":["H2O2 diffuses deep, OH clings to surface in plasma water","Tuning OH vs H2O2 ratio in plasma-treated water","Plasma water: control OH and H2O2 with power, humidity, time","OH depth limited by H2O2 sink in plasma-treated water","H2O2 plume penetrates water; OH stays near the surface"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The simulated OH delivery to the water depends on an unmeasured assumption that the unguided gas flow slows linearly from the capillary exit to zero at the liquid surface.","fun_headline_variants_meta":{"raw":{"variants":["H2O2 diffuses deep, OH clings to surface in plasma water","Tuning OH vs H2O2 ratio in plasma-treated water","Plasma water: control OH and H2O2 with power, humidity, time","OH depth limited by H2O2 sink in plasma-treated water","H2O2 plume penetrates water; OH stays near the surface"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000684,"raw_usage":{"total_tokens":3239,"prompt_tokens":1213,"completion_tokens":2026,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":829,"completion_tokens_details":{"reasoning_tokens":1927}},"tokens_in":829,"tokens_out":2026,"duration_ms":13934,"temperature":1.0,"reasoning_tokens":1927,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T10:53:46.720561+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the axial velocity profile of the effluent between the capillary exit and the liquid surface, for example with particle image velocimetry, and compare the OH flux computed from the real profile with the flux from the assumed linear fall-off; a mismatch would change predicted liquid OH concentrations and penetration depths directly.","supporting_citations":[{"cited_title":"Poggemann, S","cited_arxiv_id":null,"evidence_quote":"Supplies the gas-to-liquid simulation method, the experimentally derived effective diffusion coefficient for H2O2, and the molecular diffusion coefficient for OH used in the liquid model."},{"cited_title":"Semenov, K.-D","cited_arxiv_id":null,"evidence_quote":"Provides the flux expressions for gas-to-liquid transfer of species at the plasma-liquid interface."},{"cited_title":"Brisset, A.R","cited_arxiv_id":null,"evidence_quote":"Supplies the helium-water vapour gas-phase reaction scheme used in the plasma and effluent simulations."},{"cited_title":"Sch¨ uttler, A.L","cited_arxiv_id":null,"evidence_quote":"Earlier study of the same capillary jet's H2O2 production and transport that the base case builds on."},{"cited_title":"Lietz and M.J","cited_arxiv_id":null,"evidence_quote":"Describes the GlobalKin code used for the 0-D plasma kinetics and plug-flow gas-phase simulation."},{"cited_title":"Sch¨ uttler, L","cited_arxiv_id":null,"evidence_quote":"Describes the ammonium metavanadate spectrophotometric H2O2 measurement used to validate the simulations."},{"cited_title":"Harris, L","cited_arxiv_id":null,"evidence_quote":"Measured H2O2 spatial distribution widths that motivate the 3 mm FWHM Gaussian profile assumed when comparing simulation with stirred-cuvette experiments."}],"review_version":1}