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REVIEW 4 major objections 6 minor 2 references

Wetting behavior of supercooled water droplets impinging on nanostructured graphite surface: A molecular dynamics study

T0 review · 4 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read On a graphite surface cut with nanoscale grooves, water droplets that hit slowly enough never enter the grooves, and once they do, temperature controls how quickly they pull back.

desk verdict A plausible MD parameter study whose headline threshold claim is contradicted by its own static reference and whose impinging-velocity protocol is underspecified. read the letter →

arxiv 1908.00560 v1 pith:A6BCN4UC submitted 2019-08-01 physics.chem-ph cond-mat.softphysics.comp-ph

Amir Afshar , Dong Meng This is my paper · ORCID
classification physics.chem-phcond-mat.softphysics.comp-ph
keywords moleculardynamicssupercooledwaternanodropletimpactwettingnanostructuredgraphitepenetrationthresholdice-phobicsurfacesTIP4P/Ice
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using molecular dynamics, this paper asks whether supercooled water nanodroplets can be kept out of nanoscale grooves carved into graphite. It reports a critical impinging velocity, identified in its simulations as about 1.5 Mach, below which no water penetrates the groove and above which penetration grows with impact speed and groove width. The droplets that do enter eventually retract, and the time they need to pull back is governed mainly by droplet temperature rather than by impact velocity or groove size. If these findings hold, groove geometry could be engineered so that water stays on the surface at practical impact speeds, a design consideration for ice-phobic nanostructured substrates.

What carries the argument

The argument is carried by the penetration depth H, a density-weighted measure of how far water oxygen atoms have entered the groove (Eq. 4), tracked over 10 ns simulations of a supercooled TIP4P/Ice water droplet striking fixed zig-zag graphene pillars with groove widths d = 2.0 nm and 4.7 nm. A collective impinging velocity expressed in Mach numbers (M, multiples of the speed of sound, though the paper does not state which sound speed is used) is imposed on the thermal velocities of the equilibrated droplet; the threshold where wetting behavior fundamentally changes is then identified as 1.5M, below which penetration is not observed. Retraction is quantified by comparing dynamic and static wetting profiles to define a retracting time. The force-field calibration, especially the water-carbon Lennard-Jones parameters drawn from Werder et al., sets the hydrophobicity that drives retraction.

What would settle it

Repeat the simulations with the droplet given a physical centre-of-mass velocity, computed from the speed of sound in supercooled water, and with a thermostat that removes the added kinetic energy; if penetration appears below 1.5M or the threshold shifts with the initialization scheme, the claimed v* is an artifact of the Mach-number boost.

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Extended reading notes

Core claim

The central claim is that dynamic wetting of a nanostructured graphite surface has a velocity threshold. In the simulations, impinging speeds of 0.5M and 1.0M produce wetting profiles indistinguishable from each other and from the static case, while 1.5M marks a fundamental change: above it, maximum penetration depth grows with both impinging velocity and groove width (2.0 nm versus 4.7 nm). Below the threshold, grooves are not penetrated, and the authors infer that grooves smaller than a size-related cutoff can stop water ingress even at speeds around 0.5M, attributing the resistance to interfacial tension. The time required for the droplet to retract from the groove is then shown to depend most strongly on temperature, with 250 K droplets retracting markedly more slowly than 265 K droplets, an effect the authors trace to the TIP4P/Ice model's viscosity behavior.

Load-bearing premise

The load-bearing assumption is that adding a single speed expressed as a Mach number to every water molecule's thermal motion is a well-defined model of droplet impact, since the paper gives no sound speed, conversion formula, or control of the added kinetic energy.

Editorial extensions

If this is right

  • Below the critical impinging velocity, water does not enter surface grooves, so nanostructured groove geometry can act as a barrier to wetting at impact speeds near 0.5M.
  • Above the threshold, maximum penetration depth increases with both impinging velocity and groove width, so larger textures are more easily wetted under fast impacts.
  • Retraction time is controlled mainly by droplet temperature, meaning colder supercooled droplets linger longer inside grooves regardless of impact speed.
  • After a sufficient retracting time, the dynamic wetting profile approaches the static wetting profile, so the impact outcome eventually relaxes to equilibrium wetting.
  • The calibrated hydrophobic graphite surface reproduces experimentally reported contact angles of 90-95 degrees, supporting the use of this model for ice-phobic surface design.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A testable extension is that the critical velocity should scale with interfacial tension divided by groove width, reflecting a race between impact inertia and capillary resistance; the paper does not derive this scaling.
  • The Mach-number boosts used here correspond to speeds of hundreds of metres per second, far above typical practical droplet impacts, so the design rule that narrower grooves block water may hold for much wider grooves at real-world speeds.
  • The strong temperature control of retraction time points to shear viscosity of supercooled water as the underlying transport property; one could test this by varying viscosity independently of temperature in the water model.
  • If the threshold is real, groove width could be chosen as a design parameter for anti-icing surfaces, but this would need confirmation at larger scales and with other water models before becoming an engineering rule.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. This manuscript reports molecular dynamics simulations of supercooled water nanodroplets impinging on a grooved graphite substrate, using the TIP4P/Ice water model and OPLS carbon parameters. The authors vary the groove spacing (d = 2.0 nm and 4.7 nm), the droplet temperature (250 K and 265 K), and an impact velocity expressed as Mach numbers (0.5M–2.5M). They compute a density-weighted penetration depth H, obtain wetting profiles over time, and define a critical impact velocity v* = 1.5M where wetting behavior changes significantly. They report that the droplet retracts from the grooves after maximum penetration, that retraction time depends mainly on temperature, and that grooves smaller than a certain size can stop water penetration at low impact velocities.

Significance. If the threshold claim were fully supported, the work would provide a useful atomistic-scale design rule for ice-phobic nanostructured surfaces. The study has several strengths: it uses a water model appropriate for supercooled conditions, calibrates the water–carbon interaction to reproduce the experimental contact angle, and systematically compares static and impinged droplets. The observation of retraction from grooves and the temperature dependence of retraction time are plausible and potentially interesting. However, the headline conclusion about a penetration threshold is over-stated relative to the evidence presented, and the impact velocity initialization is not defined well enough to ensure reproducibility. These issues can likely be fixed by rewriting the claims and adding technical details.

major comments (4)
  1. [Abstract and Section 3.1] The abstract's claim that below v* 'penetration is not observed' is inconsistent with the manuscript's own data and analysis. Section 3.2 and Figure 8 explicitly subtract the maximum penetration depth of the non-impinged droplet from the impinged values, and Section 3.1 states that wetting profiles for velocities equal to or less than 1.0M are similar to the static wetting profile. Since the static droplet has a nonzero H, low-velocity droplets do penetrate the grooves; v* is at best a threshold for enhanced penetration, not for the onset of penetration. This misstatement affects the abstract, the design recommendation, and the conclusion.
  2. [Section 2] The impinging velocity is defined only as 'Much numbers (M)' added to thermal velocities, with no speed of sound, no conversion formula, no specification of the direction of the collective velocity, and no information on how the NVT thermostat interacts with this imposed velocity. Consequently, the magnitude of the impact velocities is not defined and the simulations are not reproducible. The authors should provide actual velocities in m/s for each M value and a detailed initialization protocol.
  3. [Equation (3) and Table 3] Equation (3) as printed reads ε_c−o = sqrt(ε_c−c + ε_o−o), but the values in Table 3 (0.50840 kJ/mol for the mixing rule) correspond to the geometric mean sqrt(ε_c−c · ε_o−o). A reader implementing the printed formula would obtain an interaction strength roughly a factor of two too large, which would change the predicted contact angle. The equation should be corrected to use the product of the well depths.
  4. [Section 3.1] The threshold v* = 1.5M is introduced as a 'fundamental change' without a quantitative criterion. To support a threshold claim, the authors should define an operational measure (e.g., a minimum increase in MDP over the static value) and apply it consistently to both groove sizes and temperatures.
minor comments (6)
  1. [Section 2] The term 'Much numbers' should be 'Mach numbers' throughout the manuscript.
  2. [Section 1 (Introduction)] The statement that 'there have so far been no MD simulation incorporated to investigate the dynamic wetting of impinged water droplet on solid surfaces' is incorrect; many prior MD studies of droplet impact on solid surfaces exist. This sentence should be revised.
  3. [References] Reference 12 (Park et al., J. Hydrol. 2011) is unrelated to wetting and should be replaced with a proper citation for the effect of interfacial energy on wettability.
  4. [Throughout] There are several typographical errors, including 'imping velocity' instead of 'impinging velocity' and 'no significant different' instead of 'no significant difference.'
  5. [Section 3.2] The phrase 'retracting as a function of "pure" depth of penetration' is unclear; please define 'pure' as the relative MDP used in Figure 8.
  6. [Equation (5)] In Equation (5), please clarify that L_y and L_z are the groove dimensions, not the simulation box dimensions.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: MD wetting predictions are simulation outputs calibrated only against an external contact-angle benchmark.

full rationale

The paper reports molecular-dynamics observations of supercooled water droplet penetration into nanotextured graphite grooves and does not derive its central wetting claims from the same quantities being predicted. The only calibrated parameter is the water-carbon Lennard-Jones well depth, which was tuned so that the simulated static contact angle matches the experimental 90-95 degree value on graphite (Section 2, Table 3, Figure 2, citing Wang et al. and Shin et al.). That calibration is an external benchmark, not the target result: penetration depth, retraction time, and the 1.5M threshold trend are independent simulation outputs, so the wetting predictions are not forced by construction. The self-citations [22], [27], and [32] are methodological references (combining-rule usage, ice fracture setup, and prior impinging-velocity setup) and are not used as load-bearing justifications for the central claim. The paper's definition of 'critical impinging velocity' as 1.5M (Section 3.1) is an empirical labeling of the simulation profiles, not a fitted parameter renamed as a prediction. The abstract's phrase 'below which penetration is not observed' is internally inconsistent with the body's own baseline subtraction (Section 3.2, Figure 8 subtracts the non-impinged MDP, implying the static droplet itself penetrates), but that is an overstatement/correctness issue, not a circular derivation. No equation is shown to be equivalent to its own input, and no prediction reduces to a fitted value. Hence no significant circularity; score 0.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claims rest on a calibrated force field, a standard water model, a fixed-substrate assumption, an underspecified Mach-velocity initialization, and a paper-specific penetration definition. No new physical entities are introduced.

free parameters (1)
  • Water-carbon Lennard-Jones well depth (epsilon_C-O) = 0.392 kJ/mol (Werder et al. value; mixing-rule value 0.5084 kJ/mol rejected)
    Section 2 and Figure 2: chosen to reproduce the experimental contact angle of 90 to 95 degrees on graphite. All wetting results are generated with this calibrated interaction, so the claims are conditional on this input.
assumptions (4)
  • domain assumption TIP4P/Ice force field reproduces supercooled water and ice properties relevant to this study (melting point 269.8 K, density 0.906 g/cm3).
    Section 2: accuracy of the water model is imported from prior literature [13,18], not verified for the impact conditions used.
  • domain assumption Graphene sheets can be kept fixed without affecting wetting behavior of supercooled water on graphite.
    Section 2: the authors rely on references [6,26] for this assumption rather than testing it in their own grooved geometry.
  • ad hoc to paper Imposing a collective velocity corresponding to Mach numbers between 0.5M and 2.5M onto thermal velocities is a well-defined model of droplet impact.
    Section 2: no sound speed, conversion formula, or thermostat handling is given; this is a load-bearing methodological premise for all velocity-dependent claims.
  • ad hoc to paper The density-weighted penetration depth H defined by Equation (4) captures the physically meaningful groove penetration of the droplet.
    Section 3.1: the definition is introduced for this paper without benchmarking against alternative penetration measures or validating the factor of 2.

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Cite this review

Pith. "Pith review of Wetting behavior of supercooled water droplets impinging on nanostructured graphite surface: A molecular dynamics study." pith.science (2026). https://pith.science/paper/A6BCN4UC

@misc{pith2026190800560,
  author       = {Pith},
  title        = {Pith review of: Wetting behavior of supercooled water droplets impinging on nanostructured graphite surface: A molecular dynamics study},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/A6BCN4UC}},
  note         = {Machine review of arXiv:1908.00560}
}
read the original abstract

In this work, we studied the wetting behavior of impinged and stagnant supercooled water nanodroplet at the atomistic scale using molecular dynamics simulations. We show that water droplet represents a retraction behavior from surface groove nanostructure after indicating maximum penetration. There is a threshold for the value of impinging velocity, v*, below which penetration is not observed which depends on size of the surface roughness. This indicates grooves smaller than a certain size can stop water from penetration, considering v~0.5M in practical situations, which is such a useful information for design of nanostructured ice-phobic substrates. The resistance presumably comes from interfacial tension between solid substrate and liquid water droplet. The extent of water penetration significantly depends on droplet impinging velocity and the size of surface roughness, for impinging velocities higher than V*. Our result indicates the dominant role of droplet temperature on time required droplet recede from surface groove structure rather than droplet impinging velocity and the surface groove nanostructure.

Figures

Figures reproduced from arXiv: 1908.00560 by the authors.

Figure 1
Figure 1. Fig.1 [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

2 extracted references · 1 canonical work pages

  1. [1]

    Wetting and Spreading

    (1) Bonn, D.; Eggers, J.; Indekeu, J.; Meunier, J.; Rolley, E. Wetting and Spreading. Rev. Mod. Phys. 2009, 81 (2), 739–805. https://doi.org/10.1103/RevModPhys.81.739. (2) Sui, Y.; Ding, H.; Spelt, P. D. M. Numerical Simulations of Flows with Moving Contact Lines. Annu. Rev. Fluid Mech. 2014, 46 (1), 97–119. https://doi.org/10.1146/annurev-fluid-010313- 1...

    arXiv 2009

  2. [2019]

    Wettability and Surface Free Energy of Graphene Films

    (28) Wang, S.; Zhang, Y.; Abidi, N.; Cabrales, L. Wettability and Surface Free Energy of Graphene Films. Langmuir 2009, 25 (18), 11078–11081. https://doi.org/10.1021/la901402f. (29) Shin, Y. J.; Wang, Y.; Huang, H.; Kalon, G.; Wee, A. T. S.; Shen, Z.; Bhatia, C. S.; Yang, H. Surface-Energy Engineering of Graphene. Langmuir 2010, 26 (6), 3798–3802. https:/...

    work page doi:10.1021/la901402f 2009

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