Immobilization of water drops on hydrophobic surfaces by contact line pinning at non-lithographically generated polymer microfiber rings

Martin Steinhart; Peilong Hou

REVIEW 2 major objections 3 minor 26 references

Immobilization of water drops on hydrophobic surfaces by contact line pinning at non-lithographically generated polymer microfiber rings

T0 review · 2 major / 3 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Rings of ruptured block-copolymer fibers protruding from perfluorinated macroporous silicon pin water drops in place, so the drops stay put even when the surface is tilted vertical.

desk verdict A nonlithographic polymer-ring trick that probably works as advertised, but the pinning mechanism is asserted, not shown — worth refereeing, not desk-rejecting. read the letter →

arxiv 1908.05795 v1 pith:TVE53SDW submitted 2019-08-15 physics.app-ph cond-mat.mtrl-sci

Peilong Hou , Martin Steinhart This is my paper · ORCID

classification physics.app-phcond-mat.mtrl-sci

keywords Wettingblockcopolymersswellingporousmaterialscontactlinepinningmacroporoussiliconwaterdropshydrophobicsurfaces

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

Water drops usually roll off hydrophobic surfaces, which limits their use as small reaction vessels even though such surfaces are chemically inert and resist fouling. This paper reports a nonlithographic way to stop the rolling: a narrow ring of polymer fibers left behind after a swollen block-copolymer film is peeled from macroporous silicon pins the line where water, air, and surface meet. With rings of this kind, drops of 20 and 50 microliters stay in place while the substrate is tilted to vertical, whereas on the same surface without a ring they roll off immediately. The practical value is a simple route to drop-based lab-on-chip devices on inert, anti-fouling hydrophobic surfaces without photolithography.

What carries the argument

The key object is the fiber ring itself: an annulus roughly 0.2 mm wide made of ruptured PS-b-P2VP fibers standing in the macropores of silicon whose surface is covered by perfluorinated chains. The ring forms only where selective swelling has expanded the polymer enough to wedge the fibers in the pore necks but not enough to push them out, so peeling the film breaks them there. The protruding fibers, with a P2VP-rich outer surface, present both a chemical contrast (P2VP is slightly hydrophilic on a hydrophobic background) and a topographic obstacle; together they pin the three-phase contact line, the line where water, air, and substrate meet, so the drop cannot move or dewet the area inside the ring.

What would settle it

Cool a pinned drop on a vertically held sample until it freezes, then cut through it and image the contact region by cryo-SEM. If the fiber stubs are flattened, pulled out, or not touching the drop while the drop is still held, the pinning mechanism is not the protruding fibers. Alternatively, measure the tilt angle at which progressively larger drops detach: if drops detach at the same angle with and without fiber rings, the rings are not doing the holding.

Watch

Extended reading notes

Core claim

The paper's central claim is that contact line pinning at rings of ruptured PS-b-P2VP fibers protruding from perfluorinated macroporous silicon immobilizes water drops. The rings are made by dropping a solution of polystyrene-block-poly(2-vinylpyridine) onto hydrophobically modified macroporous silicon, letting the solvent evaporate so the pores fill with polymer rods, swelling the P2VP domains with hot ethanol, and then peeling off the film. In an annular zone between the outer rim and the center, the swollen fibers are wedged in the pore necks and break, leaving a ring of protruding fiber stubs. These stubs act as both chemical and topographic barriers: they are partly P2VP, which is slightly hydrophilic, and they likely pierce the drop. The paper shows that drops of 20 microliters are held by rings of about 4.1 mm diameter, and 50 microliter drops by rings of about 6.5 mm diameter, with the contact line coinciding with the ring; without rings the same drops roll off when the surface is tilted.

Load-bearing premise

The claim rests on the ruptured polymer fibers staying anchored in the pores and protruding far enough to touch and pin the water's edge; the paper infers this from electron microscopy and drop behavior but does not directly image the fiber–drop contact or test the fibers through repeated wetting cycles.

Editorial extensions

If this is right

On hydrophobized macroporous silicon with a fiber ring, water drops of 20 microliters (4.1 mm rings) and 50 microliters (6.5 mm rings) remain immobilized when the substrate is turned vertical, despite gravity and manual shaking.
Drops must be large enough to reach the ring: 5 microliter drops inside a 4.1 mm ring do not touch it, and only drops whose contact line coincides with the ring are held; above a certain volume, such as 40 microliters for 4.1 mm rings, the drop is no longer confined.
The ring diameter and width are set by the amount of polymer solution deposited and by the swelling step, so no lithography is needed; placement of drops into rings and ring fabrication can be automated.
Because the ring is both chemical and topographic, the method extends drop immobilization to inert, perfluorinated surfaces that would otherwise shed water drops, opening the way to drop-based reaction compartments that repel adsorbates.

Reading between the lines

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

The same swelling-and-peeling route should transfer to other porous substrates whose pore necks are narrower than the fiber cross-section, so the essential geometric requirement may be a pore shape that grips the fibers rather than silicon itself.
If the topographic barrier dominates, rings made from a purely hydrophobic polymer might pin drops as well; comparing P2VP rings with such rings would separate chemical from geometric contributions.
Repeated wetting-drying or tilting cycles would test whether the stubs stay anchored and protruding; the paper demonstrates holding in single-use trials without reporting cycling endurance.
Since ring formation reflects a competition between swelling-driven ejection and wedging, tuning swelling time, temperature, or film thickness should control ring width and the maximum drop volume that can be held.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Bottom line: the fabrication route is the real contribution. Drying a PS-b-P2VP drop on perfluorinated macroporous silicon, swelling in hot ethanol, and peeling off the film leaves a ~0.2 mm wide annulus of ruptured fibers, and those rings hold 20–50 µL water drops on a surface that otherwise lets them roll off. That is a simple, mask-free way to get drop confinement on a hydrophobic, anti-fouling substrate, and the paper shows it with matched controls, six repeats per data point, and SEM evidence for the ring structure. The central phenomenon is plausible.

The soft spot is the mechanism. The abstract and conclusion say the protruding fibers 'likely pierce into the water drops' and pin the contact line that way. There is no image, no force measurement, and no cycling test showing fibers actually contact water or stay anchored. P2VP is slightly hydrophilic (66°), so the ring is also a chemical heterogeneity on a 127° perfluorinated surface. A flattened or residual polymer annulus could pin the contact line without any protruding-fiber effect. More troubling, a 5 µL drop inside the 4.1 mm ring, which should not touch the ring, gives 108° whereas bare PFDTS-mSi gives 127°. That suggests the fabrication process alters the surface inside the ring, so the 'without ring' control is not a clean counterfactual for the ring interior. The stress-test note is right to flag this.

Other weaknesses are minor but worth listing: no roll-off angle or pinning force is reported; only two ring sizes are demonstrated; and one standard deviation is rounded to zero, which is at best an odd convention. Self-citations are fine here — the swelling and wetting background comes from the group's own prior work, but the annular-ring result and the immobilization demonstration are new.

Who this is for: people doing open microfluidics, drop reactors, or cell trapping on hydrophobic surfaces. They will find the fabrication simple and the demonstration useful. It is not a field-changing paper, and the mechanism needs more work, but the phenomenon deserves a proper referee. I would send it out and ask for direct evidence of fiber-drop contact or, failing that, a revised mechanism claim that does not lean on unverified piercing.

Referee Report

2 major / 3 minor

Summary. The paper reports a lithography-free route to immobilize water drops on perfluorinated macroporous silicon by pinning the contact line at rings of ruptured PS-b-P2VP fibers. The rings are formed by dropping a PS-b-P2VP/THF solution onto PFDTS-modified mSi, swelling the P2VP domains in hot ethanol, and detaching the circular film with tweezers, leaving protruding fiber fragments in an annular region. The authors show by SEM that fibers protrude from the macropores, and demonstrate with photographs and movies that drops of 20 µL (4.1 mm rings) and 50 µL (6.5 mm rings) remain attached when the substrate is tilted to vertical, while drops on unmodified mSi roll off. Apparent contact angles are reported as a function of drop volume for two ring sizes, with n=6 measurements per data point.

Significance. The fabrication method is simple and nonlithographic, and the matched control (with versus without rings) is a clear strength. If the mechanism is supported, the work offers a practical way to exploit hydrophobic, anti-fouling surfaces in droplet-based lab-on-chip devices. The drop-immobilization phenomenon itself is credible from the photographs and videos. However, the paper's central mechanism—that protruding fibers pierce the drop and pin the contact line—is not directly evidenced, and the contact-angle data suggest the ring-fabrication process alters the surface inside the rings, so the control is not perfectly clean. These gaps are fixable with additional characterization or careful rewording.

major comments (2)

[§2.3, last paragraph] The sentence 'the PS-b-P2VP fibers protruding from the mSi macropores likely pierce into the water drops' is the only support for the specific protrusion/piercing mechanism, and it is explicitly speculative. No image, force measurement, or repeated-wetting test shows that protruding fibers contact the drop or remain anchored after water contact. Because the ring is also a chemical heterogeneity (P2VP is hydrophilic, 66°), the observed immobilization could be caused solely by a residual polymer boundary or by flattened fibers. The central claim in the abstract that contact line pinning occurs 'at rings of ... fibers protruding from mSi macropores' is therefore stronger than the evidence. Please provide direct evidence (e.g., cryo-SEM or confocal imaging of the contact line, or pinning-force/hysteresis measurements) or revise the mechanism statement to describe pinning at the polymer ring without asserting protrusion/piercing.
[§2.3, Figure 7 and §2.1] The paper reports 108°±1° for 5 µL drops inside 4.1 mm rings and 123°±1° for 20 µL drops inside 6.5 mm rings that do not contact the ring, while bare PFDTS-mSi has a contact angle of 127°±4°. The 5 µL value in particular indicates that the area inside the rings is less hydrophobic than the bare control, implying that the ring-fabrication process leaves residual polymer or otherwise alters the surface inside the ring. Consequently, the 'without ring' roll-off control is not a clean counterfactual for the ring interior. Please characterize the interior surface (e.g., XPS or ToF-SIMS, or contact-angle measurements after removal of the polymer) and clarify whether the pinning requires protruding fibers or only a chemical pattern.

minor comments (3)

[§2.1] The report of a standard deviation 'rounded to 0 µm' is not informative; please report the measurement resolution or the unrounded value.
[Figure 6 caption] The word 'sustrate' should be 'substrate'.
[Figure 7 caption] The phrase 'guides to the eyes' should be 'guides to the eye'.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the immobilization claim is an empirical, control-based demonstration, and the self-cited references supply background rather than the conclusion.

full rationale

This paper contains no analytic derivation, fitted parameter, or quantitative model whose output is fed back into its own inputs. The central claim that rings of ruptured PS-b-P2VP fibers pin the contact line and immobilize water drops is supported by direct wetting experiments: drops with and without rings are compared on identical hydrophobically modified macroporous silicon, and the without-ring control rolls off while the with-ring drop remains arrested upon tilting to vertical (Figures 5d-g and 6d-g, Supporting Movies 1 and 2). The paper's own caution that the fibers 'likely pierce into the water drops' (Section 2.3) is a mechanistic inference rather than a derived prediction, and its unverified status is a correctness or evidence concern, not circularity. Self-citations appear for established background steps: PFDTS grafting procedures (refs. 20, 21), selective-swelling-induced pore formation (refs. 18, 19), and the hydrophilicity of P2VP (refs. 25, 26). These are used to describe fabrication and materials properties, not to justify the drop-immobilization result itself. No equation or defined quantity reduces to another definition, and no fitted value is renamed as a prediction. Therefore the paper is self-contained against external benchmarks for its main empirical conclusion, and the appropriate circularity score is 0.

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

The central claim rests on a specific commercial substrate, a solvent-swelling mechanism taken from prior work, a cited wetting property of P2VP, and an untested assumption that the fibers contact the drop. These are inputs from the literature or from the experimental setup rather than derived results.

free parameters (4)

PS-b-P2VP solution concentration = 0.1 g/mL in THF
A hand-selected process condition; no concentration range is tested, and ring formation depends on it.
Deposited solution volume = 20 µL and 60 µL; 5 and 10 µL fail
Controls the ring outer diameter (4.1 mm and 6.5 mm); the paper states size control is limited and smaller volumes do not yield rings.
Selective swelling conditions = Ethanol at 60 °C for 1 h
Chosen protocol for swelling-induced pore formation; central to ring formation; no time or temperature variation is reported.
PFDTS grafting conditions = 5 h at 100 °C with 0.2 mL PFDTS
Protocol from prior literature; used to render the mSi hydrophobic; the exact values are assumed inputs.

assumptions (4)

domain assumption Macroporous silicon with the specified geometry (pore depth 1.8 µm, neck 530 nm, widening to 710 nm, lattice constant 1.5 µm) can be hydrophobized by PFDTS grafting and is a representative hydrophobic substrate.
The substrate properties are taken from a commercial supplier and prior protocols; the claimed behavior is demonstrated only on this substrate.
domain assumption Ethanol selectively swells P2VP minority domains in PS-b-P2VP, generating nanopores and volume expansion at 60 °C.
The mechanism of selective-swelling-induced pore formation is adopted from refs [18,19], not re-established in this paper.
domain assumption After swelling-induced pore formation, the protruding PS-b-P2VP fibers have a P2VP outer surface with a water contact angle of 66±1°, making them slightly hydrophilic.
Taken from refs [25,26]; this is part of the proposed chemical barrier and pinning explanation.
ad hoc to paper The protruding PS-b-P2VP fibers pierce into or contact the water drop surface at the contact line.
Stated only as likely in Section 2.3; no direct observation is provided, and the proposed pinning mechanism depends on fiber-drop contact.

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Cite this review

Pith. "Pith review of Immobilization of water drops on hydrophobic surfaces by contact line pinning at non-lithographically generated polymer microfiber rings." pith.science (2026). https://pith.science/paper/TVE53SDW

@misc{pith2026190805795,
  author       = {Pith},
  title        = {Pith review of: Immobilization of water drops on hydrophobic surfaces by contact line pinning at non-lithographically generated polymer microfiber rings},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TVE53SDW}},
  note         = {Machine review of arXiv:1908.05795}
}

read the original abstract

Water drops used as reaction compartments are commonly immobilized on hydrophilic areas bordered by hydrophobic areas. For many applications, such as the trapping of non-adherent cells, it is desirable to exploit the inertness and the anti-fouling behavior of hydrophobic surfaces as well as their repulsive behavior towards adsorbates in lab-on-chip configurations. However, the immobilization of water drops on hydrophobic surfaces has remained challenging. We report a nonlithographic approach to arrest water drops on hydrophobically modified macroporous silicon (mSi) with per uorinated surface. Contact line pinning at rings of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) fibers protruding from the mSi macropores immobilizes water drops when the hydrophobically modified mSi is moved or tilted and prevents dewetting within the PS-b-P2VP fiber rings. Without PS-b-P2VP fiber rings, water drops readily roll off. The PS-b-P2VP fiber rings were prepared by dropping PS-b-P2VP solution onto hydrophobically modified mSi. Selective swelling of the P2VP in the thus-formed circular PS-b-P2VP films with hot ethanol followed by detachment of the latter yielded hydrophobically modified mSi exhibiting annular areas, in which ruptured PS-b-P2VP fibers protruded from the mSi macropores. For example, PS-b-P2VP fiber rings with diameters of 6.5 mm and widths of about 0.2 mm immobilize water drops with a volume of 50 microliters.

Figures

Figures reproduced from arXiv: 1908.05795 by the authors.

**Figure 2.** Nonlithographic generation of PS-b-P2VP fiber rings on hydrophobically modified mSi (grey). a) PS-b-P2VP/THF solution (ocher) is dropped onto hydrophobically modified mSi so that b), c) a transparent circular film of solid PS-b-P2VP (blue) connected to PS-b-P2VP fibers located in the mSi macropores forms. d), e) The circular PS-b-P2VP film subjected to selective-swelling induced pore formation gets opaque and mesop… view at source ↗

**Figure 3.** Idealized reaction scheme of the grafting of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTS) [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: Scanning electron microscopy images of a PS-b-P2VP fiber ring obtained by dropping 20 µL PS-b-P2VP solution onto hydrophobically modified mSi. a) Large-field view; the approximate positions at which panels b)-e) were taken are indicated. b), c) Outermost rim at the to…

**Figure 5.** Figure 5: Immobilization of water drops on hydrophobically modified mSi by PS- [PITH_FULL_IMAGE:figures/full_fig_p010_5.png]

**Figure 6.** Figure 6: Immobilization of water drops on hydrophobically modified mSi by PS- [PITH_FULL_IMAGE:figures/full_fig_p011_6.png]

**Figure 7.** Figure 7: Apparent contact angles of water drops deposited into PS- [PITH_FULL_IMAGE:figures/full_fig_p012_7.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

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