Stability of textured water-repellent surfaces
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Дата
2026
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Науковий керівник
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Видавець
Igor Sikorsky Kyiv Polytechnic Institute
Анотація
Zuo Yong. Stability of textured water-repellent surfaces. - Qualified scientific work on the rights of the manuscript. Dissertation for the degree of Doctor of Philosophy in the specialty 161 - Chemical Technologies and Engineering and Knowledge branch 16 – Chemical Engineering and bioengineering - National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", Kyiv, 2026. The environmental durability of superhydrophobic surfaces remains a critical barrier to their practical application, particularly for the corrosion protection of aluminum alloys in demanding marine, aerospace, and industrial environments. Despite decades of research inspired by natural superhydrophobic systems such as the lotus leaf, the translation of laboratory-scale superhydrophobic coatings into real-world applications has been severely limited by rapid performance degradation under environmental stress. This dissertation confronts this challenge not through empirical trial-and-error, but through a mechanism-driven approach that integrates multi-scale surface characterization, accelerated degradation testing, rational composite design, and computational electrochemistry to establish a predictive engineering framework for durable superhydrophobic coatings. The research began by moving beyond the limitations of static contact angle measurements, which provide only superficial insight into wetting behavior. A comprehensive characterization methodology was established that combines OwensWendt surface energy decomposition with dynamic contact angle analysis. This approach enabled quantitative assessment of both the dispersive and polar components of surface energy, revealing that stabilizing the Cassie-Baxter wetting state - where air pockets are trapped beneath water droplets - hinges on reducing the polar surface energy component to below 1 mJ/m2 through silane functionalization. This quantitative threshold, inaccessible through static contact angle measurements alone, provides a clear design target for surface chemical modification. The multi-scale investigation spanned textures from 100 nm laser-induced periodic surface structures to 750 μm milled grooves, systematically delineating the operational limits of classical wetting models. Notably, the Wenzel model, which assumes complete liquid penetration into surface roughness, was found to fail for large-scale microstructures due to contact line pinning at texture edges, a phenomenon that classical theory does not account for. Having established a robust characterization framework, the research turned to identifying the primary failure pathway under environmental stress. Accelerated weathering protocols combining UV irradiation (340 nm, 0.89 W/m²) with elevated temperature (60°C) and humidity (50% RH) were employed to simulate months of outdoor exposure in compressed timescales. Through in-situ chemical analysis using Fourier-transform infrared spectroscopy, polymer photo-oxidation was unambiguously identified as the dominant degradation mechanism, evidenced by a 60-80% decrease in aliphatic C-H stretching vibrations (2850-2950 cm-1 ) and a fivefold increase in carbonyl C=O absorption (1731 cm-1 ). This mechanistic insight was critical: it demonstrated that particle erosion and mechanical delamination, often assumed to be primary failure modes, were in fact secondary to chemical degradation of the polymer binder. The implication was clear: durability could be enhanced not by strengthening adhesion or increasing coating thickness, but by minimizing the polymer binder content while maximizing the density of a hierarchical inorganic particle network that is inherently resistant to photo-oxidation. This mechanism-driven approach culminated in an optimized composite coating formulation comprising 5 wt.% acrylic copolymer binder, 90 wt.% silanefunctionalized calcium carbonate microparticles (d₅₀ ≈ 5 μm), and 5 wt.% fumed silica nanoparticles (d₅₀ ≈ 12 nm). This hierarchical architecture combines the mechanical integrity provided by the polymer with the chemical stability of the inorganic particles, while the bifractional particle size distribution creates a dense, tortuous network that impedes electrolyte penetration. The coating demonstrated exceptional performance in 3.0 wt.% NaCl electrolyte under continuous immersion. Potentiodynamic polarization measurements revealed a +797 mV noble shift in corrosion potential, from -660 mV for bare AA6061 aluminum to +137 mV vs. saturated calomel electrode (SCE) for the coated substrate. Electrochemical impedance spectroscopy at 0.01 Hz showed impedance magnitudes exceeding 688 Ω·cm², representing a 2.3-fold improvement over unoptimized formulations. Critically, superhydrophobicity, with apparent contact angles exceeding 130°, was maintained after 72 hours of continuous immersion, a performance benchmark rarely reported in the literature for polymer-based coatings. To move beyond purely empirical optimization, a multi-physics computational model was developed and experimentally validated. The model coupled Butler-Volmer electrode kinetics with porous media transport, accounting for both the tortuous diffusion path through the particle network and the electrochemical reactions at the aluminum-electrolyte interface. The model reproduced experimental polarization curves within 10% accuracy across the entire potential range, providing confidence in its predictive capability. More importantly, the model enabled mechanistic decomposition of the protection mechanisms, revealing a synergistic effect: a 3.2-fold increase in solution resistance from the tortuous particle network combined with a 5.8- fold increase in charge transfer resistance from surface passivation by the hydrophobic silane layer. This quantitative insight, inaccessible through experiment alone, demonstrates that durability arises not from a single dominant mechanism but from the synergistic interaction of multiple protective barriers. The scientific novelty of this dissertation lies in the development of a comprehensive framework for understanding the formation, stability, and protective performance of superhydrophobic composite coatings through combined experimental and computational approaches. For the first time, it has been demonstrated that the Owens–Wendt surface energy decomposition method, when coupled with dynamic contact angle analysis, provides substantially greater predictive capability than conventional static measurements, revealing that stabilization of the Cassie–Baxter wetting regime is primarily governed by a drastic reduction in the polar component of surface energy from approximately 20 mJ/m² to below 1 mJ/m² after silane functionalization, which enables apparent contact angles exceeding 150°. The work also advances the understanding of the applicability limits of classical wetting models through a systematic investigation of textured surfaces spanning nearly three orders of magnitude in characteristic size, from laser-induced periodic surface structures of about 100 nm to micromilled features approaching 750 μm, demonstrating that the Wenzel model becomes invalid for textures with periodicities above roughly 200 μm due to contact line pinning at structural edges, a mechanism not captured by equilibrium thermodynamic descriptions of wetting. Furthermore, the dominant degradation mechanism of superhydrophobic composite coatings under environmental exposure has been identified for the first time by combining ultraviolet aging with chemical analysis, showing that coating failure is primarily driven by photo-oxidative transformation of the polymer matrix rather than by particle erosion or delamination. Infrared spectroscopy revealed a pronounced decrease in aliphatic C–H stretching vibrations in the range of 2850–2950 cm⁻¹ accompanied by a significant increase in carbonyl absorption near 1731 cm⁻¹, confirming the conversion of hydrophobic groups into oxygen-containing polar functionalities. Based on these findings, a mechanistic design principle for durable superhydrophobic coatings has been established, demonstrating that optimal performance is achieved by minimizing the polymer fraction while maintaining a dense inorganic particle network that defines the hierarchical surface morphology. An optimized composite formulation containing 5 wt.% acrylic copolymer, 90 wt.% silane-functionalized calcium carbonate microparticles, and 5 wt.% fumed silica nanoparticles was developed and experimentally validated, providing a substantial positive shift of corrosion potential, increased impedance magnitude at low frequencies, and preservation of high hydrophobicity after prolonged immersion in a sodium chloride electrolyte. In addition, a multi-physics computational model has been developed and validated that quantitatively reproduces electrochemical polarization behavior and allows mechanistic separation of the protective contributions of the coating structure. The modeling results demonstrate that the enhanced corrosion resistance arises from synergistic effects including increased solution resistance associated with tortuous ionic diffusion pathways through the particle network and elevated charge transfer resistance resulting from surface passivation induced by silane treatment. Overall, the obtained results reveal that a dense inorganic particle framework simultaneously generates the hierarchical roughness required for superhydrophobic wetting and acts as a diffusion barrier that suppresses ionic transport, while the reduced polymer content limits water uptake and hydrolytic swelling. This integrated understanding establishes a mechanism-oriented approach to the design of durable superhydrophobic protective coatings. The practical significance of the obtained results is substantial and multifaceted. First, this work delivers a viable, environmentally benign alternative to hexavalent chromium-based conversion coatings, which remain the industry standard for aluminum corrosion protection despite their well-documented carcinogenicity and environmental toxicity. The optimized superhydrophobic coating matches or exceeds the corrosion protection performance of chromate coatings while eliminating hazardous waste generation and worker exposure risks. Second, the mechanism-driven design methodology developed here is transferable to other composite material systems, providing a template for rational materials engineering in applications ranging from self-cleaning surfaces to anti-icing and anti-fouling coatings. Third, the validated computational model enables predictive design and optimization without exhaustive experimental testing, significantly reducing development time and cost. Fourth, the quantitative surface energy thresholds and formulation guidelines established in this work provide actionable design rules for industrial coating formulation, accelerating the translation of superhydrophobic technology from laboratory curiosities to commercial products. In summary, this dissertation represents a critical step in advancing materials science from empirical discovery to predictive engineering. By integrating surface science, polymer chemistry, electrochemistry, and computational modeling, it establishes a comprehensive framework for designing durable superhydrophobic coatings that perform reliably in complex operational environments. The work demonstrates that durability is not an intrinsic material property to be discovered, but an engineered system-level behavior that emerges from the rational integration of hierarchical structure, interfacial chemistry, and protective mechanisms.
Опис
Ключові слова
superhydrophobic, polymer, composite, surface, modification, nanoparticles, silica, morphology, corrosion, contact angle, UV resistance, degradation, wetting, супергідрофобний, полімер, композит, поверхня, модифікація, наночастинки, кремнезем, морфологія, корозія, кут змочування, УФ-стійкість, деструкція, змочування
Бібліографічний опис
Zuo Yong. Stability of textured water-repellent surfaces : dissertation submitted for the Doctor of Philosophy degree : 161 Chemical Technologies and Engineering / Zuo Yong. – Kyiv, 2026. – 154 p.