A pushing-pulling captive bubble method for precise measurement of dynamic contact angles underwater
Pith reviewed 2026-05-15 01:05 UTC · model grok-4.3
The pith
A pushing-pulling captive bubble method enables precise measurement of dynamic contact angles underwater by vertical surface motion.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The pushing-pulling captive bubble method allows stable and precise measurement of dynamic contact angles underwater without directly changing the bubble volume. A bubble is pushed against and detached from the surface through controlled vertical motion, enabling consistent observation of the contact line while minimizing deformation and lateral shifts. This yields reproducible results across surface types, including cases where prior methods struggled due to trapped air layers.
What carries the argument
pushing-pulling captive bubble method using vertical surface motion to control bubble contact
If this is right
- Dynamic contact angles measured underwater match those in air for smooth and Wenzel surfaces.
- The method achieves reproducibility equal to or exceeding conventional captive bubble techniques.
- Ultrasonic degassing combined with the method permits dynamic angle measurements on microstructured surfaces under fully wetted conditions.
- Different wetting behaviors emerge in water compared to air for microstructured surfaces.
Where Pith is reading between the lines
- The technique may extend to measuring contact angles in other viscous or complex fluids where volume control is challenging.
- It could facilitate studies of how surface microstructures affect underwater wetting hysteresis without air trapping artifacts.
- Applications in marine coatings or biological surfaces might benefit from this stable underwater measurement approach.
Load-bearing premise
Controlled vertical motion of the surface produces contact angles equivalent to volume-change methods without introducing additional motion-induced effects at the contact line.
What would settle it
Observing substantial discrepancies in measured contact angles between this pushing-pulling method and traditional volume-change captive bubble methods on identical surfaces would falsify the claim of equivalence and precision.
read the original abstract
Accurate measurement of dynamic contact angles in aqueous environments is essential for evaluating surface wettability. However, conventional captive bubble methods often suffer from limitations such as bubble instability and interference from needle wetting. In this study, we develop a pushing-pulling captive bubble method that enables stable and precise measurement of dynamic contact angles underwater without directly changing the bubble volume. In this method, a bubble is pushed against and detached from a surface by controlled vertical motion. This procedure allows stable observation of the contact line while suppressing bubble deformation and lateral movement. Dynamic contact angles were measured in both air and water using three types of surfaces: smooth surfaces, sandpaper-polished surfaces prepared to exhibit the Wenzel state in air and the reversed gas-liquid Wenzel state in water, and microstructured surfaces exhibiting hydrophobicity in air. For smooth and Wenzel surfaces, the dynamic contact angles measured in air and water showed similar values. Moreover, the modified captive bubble method exhibited reproducibility comparable to or higher than that of conventional captive bubble methods. For microstructured surfaces, dynamic contact angle measurements in water had previously been difficult because an air layer remained trapped on the surface. In this study, ultrasonic degassing enabled dynamic contact angle measurements under fully wetted conditions, revealing behavior that differed significantly from that observed in air.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces a pushing-pulling captive bubble method for measuring dynamic contact angles underwater. Instead of altering bubble volume, the solid substrate is translated vertically to push the bubble into contact and then detach it, enabling stable contact-line observation while suppressing deformation and lateral motion. The approach is demonstrated on smooth surfaces, sandpaper-polished Wenzel-state surfaces, and microstructured surfaces; dynamic angles in air and water are reported as similar for the first two classes, with reproducibility stated as comparable or superior to conventional captive-bubble protocols. Ultrasonic degassing is used to achieve fully wetted conditions on microstructured surfaces, revealing contact-angle behavior distinct from that observed in air.
Significance. If the central equivalence claim holds, the method offers a practical route to stable underwater dynamic contact-angle measurements on surfaces prone to air trapping, addressing a recognized limitation of needle-based captive-bubble techniques. The reported ability to access fully wetted states on microstructured surfaces and to obtain reproducible data without direct volume control could be useful for studies of underwater wettability and bio-inspired materials.
major comments (2)
- [Abstract / Results] The abstract states that the modified method exhibits reproducibility 'comparable to or higher than' conventional captive-bubble methods, yet no quantitative error bars, standard deviations, number of replicates, or statistical comparisons are supplied for the smooth and Wenzel-surface data. This omission directly undermines the precision and reproducibility claims that are central to the paper's contribution.
- [Methods / Experimental validation] The central methodological claim requires that controlled vertical translation of the substrate produces advancing and receding angles indistinguishable from those obtained by conventional bubble-volume change at the same contact-line speed. No control experiment is described that holds contact-line velocity fixed while switching between the two actuation modes, leaving any systematic offset from surface-induced flow or transient pinning unquantified.
minor comments (1)
- [Abstract] The description of the three surface types in the abstract is concise but would benefit from explicit cross-references to the corresponding preparation protocols and roughness characterization data in the main text.
Simulated Author's Rebuttal
We thank the referee for the constructive feedback on our manuscript. We have addressed the concerns about quantitative support for reproducibility claims and the need for explicit validation of the actuation method. The revisions include added statistical data and expanded methodological discussion to strengthen the central claims without altering the reported results.
read point-by-point responses
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Referee: [Abstract / Results] The abstract states that the modified method exhibits reproducibility 'comparable to or higher than' conventional captive-bubble methods, yet no quantitative error bars, standard deviations, number of replicates, or statistical comparisons are supplied for the smooth and Wenzel-surface data. This omission directly undermines the precision and reproducibility claims that are central to the paper's contribution.
Authors: We agree that the reproducibility claim requires quantitative backing. In the revised manuscript we have added error bars (standard deviation from n=5 independent trials per condition) to all dynamic contact angle data for smooth and Wenzel surfaces, together with a direct numerical comparison of standard deviations between the pushing-pulling protocol and conventional volume-change measurements performed on the same samples. These additions confirm that the modified method yields comparable or lower variability, supporting the original statement with concrete statistics. revision: yes
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Referee: [Methods / Experimental validation] The central methodological claim requires that controlled vertical translation of the substrate produces advancing and receding angles indistinguishable from those obtained by conventional bubble-volume change at the same contact-line speed. No control experiment is described that holds contact-line velocity fixed while switching between the two actuation modes, leaving any systematic offset from surface-induced flow or transient pinning unquantified.
Authors: We acknowledge that an explicit side-by-side control at fixed contact-line velocity would provide the strongest possible validation. Because the present apparatus fixes bubble volume and drives motion solely by substrate translation, a direct switch to volume actuation at identical speed is not straightforward without redesigning the fluid cell. We have therefore added a dedicated paragraph in the Methods section that (i) states the contact-line velocity is set exclusively by the measured substrate speed, (ii) notes that bubble deformation is suppressed by constant volume, and (iii) cites contact-line dynamics literature indicating that surface-induced flow effects are negligible at the low speeds used. We mark this as a partial revision; a full experimental cross-check would require additional hardware and is noted as a possible extension. revision: partial
Circularity Check
No circularity: experimental protocol with direct observations
full rationale
The manuscript describes development and application of a pushing-pulling captive bubble method for dynamic contact angle measurement. No equations, derivations, fitted parameters, or predictions appear in the provided text or abstract. The central claims rest on empirical reproducibility across surface types and environments, with comparisons to conventional methods presented as direct observations rather than internally derived quantities. No self-citation chains, ansatzes, or uniqueness theorems are invoked to justify the method; any external references would be independent support for an experimental technique. The derivation chain is therefore empty, and the result does not reduce to its inputs by construction.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Vertical translation of the substrate produces contact angles free of significant hydrodynamic or inertial artifacts.
- domain assumption Ultrasonic degassing achieves complete removal of trapped gas layers on microstructured surfaces.
discussion (0)
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