Superhydrophobic Surfaces Research Articles

Particle–droplet coalescence and jumping on superhydrophobic surfaces—A direct numerical simulations study

We report here multiphase direct numerical simulations of a recently discovered passive mechanism of self-cleaning on superhydrophobic surfaces. The removal of contaminants is governed by coalescence of a single droplet with a particle of micrometer size, where the droplet initiates spontaneous spreading on the particle and drives particle–droplet jumping. We use an in-house volume of fluid–immersed boundary numerical framework, introduce and thoroughly analyze capillary forces at the particle–droplet contact line, and validate our simulations in relation to previous experimental results. We then perform a comprehensive investigation over a number of different parameters regarding the interaction physics of the droplet with the particle and the substrate. We systematically vary particle, droplet, and surface physical and wetting properties and unveil a range of scenarios related to different energy dissipation mechanisms as a function of the substrate contact angles and contact-angle hysteresis. Detailed parameter studies establish the connection between the droplet, substrate and particle properties, and the outcome and efficiency of the particle-launching process. We particularly highlight the effects of the particle–droplet size ratio and the wettability of the particle. We reveal and discuss the corresponding dissipation mechanisms and quantify the energy efficiencies of the jumping process in the treated parameter space.

One-step fabrication of robust PDMS-MWCNTs composite superhydrophobic coatings with hierarchical micro-nanostructures for anticorrosive applications

Superhydrophobic coatings hold considerable promise for safeguarding metals against corrosion. However, realizing coatings with excellent superhydrophobicity, mechanical strength and corrosion resistance remains a great challenge. In this study, a robust superhydrophobic surface was achieved through the laser processing of multi walled carbon nanotubes (MWCNTs)/polydimethylsiloxane (PDMS) coatings, resulting in a water contact angle (WCA) of approximately 165°. Notably, the coating demonstrated outstanding abrasion resistance in sandpaper abrasion, steel wool abrasion, and falling sand tests. The friction coefficient of the coating is only 0.25. In addition, the robust superhydrophobic coating also has good anti-corrosion. The lowest-frequency impedance value of the composite coatings increases seven orders of magnitude compared with that of carbon steel, and the corrosion inhibition efficiency (η) is 99.83 %. The outstanding corrosion resistance is primarily ascribed to the formation of a dual synergistic barrier by PDMS-MWCNTs, providing effective protection for carbon steel.

Controllable size of graphite nanosheets and functionalized SiO2 microspheres with excellent anti-corrosive/wear function for superhydrophobic coating

The application of superhydrophobic coatings is limited by their complex fabrication process, expensive modification, and lack of durability. This study addresses these issues by proposing a simple and economical method for fabricating strong superhydrophobic coatings by scraping off the fluorinated suspension. This coating comprises epoxy resin (EP), graphene nanosheets, and modified silicon oxide on the Q235 steel substrate. The contact angle and sliding angle of we prepare the coating coating are 152.3° and <10° respectively, exhibiting excellent super-hydrophobicity. After being immersed in 3.5 % sodium chloride solution for 1440 minutes, the coating shows a resistance of 1.14×106 Ω cm2, which is two orders of magnitude higher than that of the coating immersed for 30 minutes (1.08×104 Ω cm2), indicating its excellent corrosion resistance. A 30-day salt spray corrosion test also confirms the good corrosion resistance of the coating. When the coating is polished 100 times under a load pressure of 500 g on 1000-mesh sandpaper, the contact angle is only reduced by 7 times, indicating its good abrasion resistance. In addition, the coating also has good weather resistance and self-repairing ability against ultraviolet radiation. This simple and environmentally friendly manufacturing method has great potential for large-scale practical applications in many fields.

Understanding condensation halo growth on superhydrophobic surfaces: Insights from a vapor diffusive model

Anti-icing technologies are vital across various sectors, from transportation to energy systems. In this study, we investigate the formation of condensation halos during the process of condensation–freezing on superhydrophobic surfaces. Experimental tests were conducted on metallic nanostructured superhydrophobic surfaces, where droplet icing was induced and observed under controlled conditions. The formation of condensation halos, characterized by the sudden appearance and subsequent vanishing of microdroplets around the freezing droplets, was captured and analyzed. A vapor diffusive model coupled with heterogeneous nucleation theory was developed to understand and quantify the growth of condensation halos. The model considers vapor diffusion around the icing droplet and the critical vapor pressure required for nucleation. Experimental observations and theoretical predictions demonstrated a strong dependence of condensation halo size on the icing droplet radius. The study sheds light on the mechanisms underlying condensation halo formation and provides insights into the intricate interplay between droplet size, surface properties, and environmental conditions in condensation–freezing phenomena, offering valuable perspectives for the development of effective anti-icing strategies.

Anticorrosive/antifouling superhydrophobic coatings

Anticorrosive/antifouling superhydrophobic coatings

Publisher's Note: “Droplet impact and rebound dynamics on superhydrophobic surfaces” [Phys. Fluids 36, 072103 (2024)]

Publisher's Note: “Droplet impact and rebound dynamics on superhydrophobic surfaces” [Phys. Fluids <b>36</b>, 072103 (2024)]

Dynamics of bubble formation on superhydrophobic surface under a constant gas flow rate at quasi-static regime

Dynamics of bubble formation on superhydrophobic surface under a constant gas flow rate at quasi-static regime

Wettability step electrode to generate millimeter-scale gas–liquid interface for drag reduction

Superhydrophobic surfaces can seal the gas–liquid interface (GLI) under water to produce the drag reduction effect. Enhancing the stability and slip length of the GLI is an important issue in this context. Herein, we fabricate wettability step electrodes (WSEs) by creating an array of millimeter-scale circular superhydrophobic regions on a hydrophilic graphite plate by using an economical and efficient mask spraying method. When the WSE was electrified as an anode, the oxygen produced by the electrolytic reaction was preferentially precipitated in the superhydrophobic regions and ultimately formed an array of millimeter-scale GLIs. The evolution process of this GLI can be divided into a spreading stage and a growth stage. The results of experiments revealed that the spreading duration of the GLI increased with the diameter of the superhydrophobic regions (D) and decreased with the spacing between adjacent superhydrophobic regions (L). During the growth stage, the height of the GLI decreased with the ratio D/(L + D) and increased over the duration of electrification according to a 1/3 power-law relationship. Finally, we measured the slip characteristic on a single millimeter-scale GLI by particle image velocimetry. The result showed that the effective slip length of the GLI with a streamwise length of 2 mm can exceed 100 μm, thus confirming the potential of the millimeter-scale GLI for drag reduction.

Fabrication of non-wetting surface on zinc substrate with coalescence-induced droplet jumping behavior for atmospheric corrosion protection

In this study, a superhydrophobic surface was constructed on the zinc substrate through a two-step process. The research first examined the correlations between the microstructure and coalescence-induced droplet jumping behavior (CIDJB). Then a droplet jumping phase map was created by considering energy principles. Finally, the performance and mechanism CIDJB in enhancing anticondensation effectiveness and salt spray experiments against atmospheric corrosion were revealed. This study provides insights into designing superhydrophobic surfaces with CIDJB for atmospheric corrosion protection. Further verification of the impact mechanism of environmental factors and stability and durability of CIDJB in practical applications will be investigated.

Synthesis and Characterization of ZnO Nanoparticles and Their Application on Cotton Fabrics to Obtain Superhydrophobic Surfaces

حضرت جسيمات أوكسيد الزنك النانوية ZnO (NPs) بطريقة كيميائية رطبة عند درجتي حرارة مختلفتين (30-90) ̊C باستخدام كلوريد الزنك كبادئ في وسط مائي. تم توصيف جسيمات أوكسيد الزنك النانوي المحضر باستخدام المجهر الإلكتروني الماسح (SEM) ,حيود الأشعة السينية (XRD) ، وطيف الأشعة تحت الحمراء (FTIR) ، أشارت أنماط الأشعة السينية الى بنية سداسية (wurtizte) بمتوسط حجم بلوري (19.5-23nm) لجسيمات أوكسيد الزنك المحضرة عند درجتي الحرارة (30-90 ̊C)على التوالي. أظهرت صور المجهر الالكتروني الماسح أنه مع زيادة درجة الحرارة ، تغير شكل جسيمات أوكسيد الزنك النانوي من الأسلاك النانوية إلى االشكل الكروي . لتصنيع قماش قطني فائق المقاومة للماء. تم غمر القماش في محلول غروواني لأوكسيد الزنك بتركيز (1٪) ,ثم تعديله لاحقًا بحمض الشمع . تم توصيف القماش القطني المعالج باستخدام المجهر الإلكتروني الماسح (SEM) واختبار زاوية التماس (CA) .أظهرت نتائج المجهر الالكتروني الماسح أن جسيمات أوكسيد الزنك النانوي المحضر عند(90 ̊C) خلق خشونة على سطح القطن أفضل من جسيمات أكسيد الزنك النانوي المحضرعند (30 ̊C). أظهرت نتائج اختبار زاوية التماس CA خصائص فائقة الكره للماء لعينات أوكسيد الزنك النانوي المحضر عند(90 ̊C) ، بينما أظهرت عينات أوكسيد الزنك النانوي المحضر عند(30 ̊C) خصائص كارهة للماء فقط بعد التعديل بحمض الشمع. تم استخدام تراكيز مختلفة من حمض الشمع لجعل سطح أوكسيد الزنك كارهاً للماء. أظهرت نتائج زاوية التماس (CA) أن القماش القطني المعالج بأوكسيد الزنك النانوي المحضر عند الدرجة (90 ̊C) المعدل ب 1٪ وزناً من حمض الشمع أظهر سطحاً فائق الكره للماء بزاوية تماس (CA=154̊ ).

Experimental study of two-phase closed thermosyphon with super-hydrophilic and super-hydrophobic surfaces

Experimental study of two-phase closed thermosyphon with super-hydrophilic and super-hydrophobic surfaces

Eco-friendly freestanding superhydrophobic thin films and coatings for corrosion protection

Eco-friendly freestanding superhydrophobic thin films and coatings for corrosion protection

Scaling laws of droplets on vibrating liquid-infused surfaces

Droplets oscillating on vibrating substrates are very interesting scientifically, with applications such as anti-icing, droplet transportation, and measuring dynamic surface tension. Reported here are the dynamics of droplets with different volumes on a vibrating smooth surface infused with liquid of different viscosities. The movement of the three-phase droplet contact line is used to quantify the droplet dynamics, and it is found that this movement is linearly proportional to the amplitude of the substrate and inversely proportional to the viscosity of the liquid infused therein. When the substrate viscosity is relatively low, the droplet volume also affects the contact-line movement. Scaling laws for the contact-line movement are derived involving the Ohnesorge number and the reciprocal of the capillary number. Also elucidated is the relationship between the resonance frequency and the substrate viscosity, and the characteristic droplet morphology under different substrate viscosities is extracted to describe the contact-line movement. Interestingly, the substrate viscosity is controlled in an innovative way to achieve almost the same contact-line movement on the present surface as on superhydrophobic and hydrophilic surfaces.

Aerophilic Surfaces for Sustained Corrosion Protection of Metals Underwater

AbstractCorrosion and biofouling are wetting‐related phenomena that limit the effective use of metals in aqueous media. Nonwettable surfaces can mitigate the adverse effects of wetting by minimizing contact with water. However, current achievements in this field fall short of meeting industrial requirements due to the short lifetime of plastrons. This study proposes a method to measure the protective sustainability of plastron. Superhydrophobic (SHS) and aerophilic (APhS) surfaces are constructed on lightweight aluminum and are initially analyzed by conventional goniometry, which show comparable values. However, the plastron that develops underwater is substantially different. While SHS exhibit unevenly broken plastron, APhS show uniform, continuous plastron. As an example of the sustained protective performance of plastron, the corrosion resistance of SHS and APhS is presented. Potentiodynamic polarization, impedance spectroscopy, and long‐term immersion in seawater show a drastic enhancement in corrosion resistance, exclusively for APhS. In fact, almost no electrochemical signals are measurable, and no pitting corrosion is observed after 415 days of immersion in seawater. Conversely, SHS show no noticeable improvement and corrode faster than bare Al due to plastron loss. Since goniometric measurements do not provide information on plastron, it is essential to analyze the plastron for any non‐wettable surface utilized underwater.

A laser-induced superhydrophobic surface with multiple microstructures for stable drag reduction

To address the issue of water resistance in ship navigation, we developed a drag-reducing surface based on the microstructure of shark skin and a superhydrophobic coating. First, grooves with various microstructures were fabricated on the substrate surface using laser processing. Then, a mixture of octadecyltrichlorosilane, water, and silica solution was uniformly sprayed onto the grooved substrate surface to achieve superhydrophobicity. To enhance and stabilize the drag reduction effect, we optimized the width and spacing of the grooves by adjusting the laser processing parameters, achieving a high drag reduction rate under various flow conditions. In the Reynolds number range of 2 × 105 to 5 × 105, the drag reduction rate of the superhydrophobic coated surface with grooves reached over 66.57 %, which is an improvement of 7 % to 26.51 % compared to the superhydrophobic coated surface without grooves. Additionally, the oxides accumulated on the ribs after laser ablation can slow down the wear of the superhydrophobic coating, significantly enhancing the surface's mechanical durability. This simple and cost-effective surface provides a new approach for laser preparation of drag-reducing surfaces.

Tailored nano-pillar structures on surfaces: facile formation and multifunctional properties

Nano-pillar structured surfaces have prized for exceptional optoelectronic properties and biocompatibility but commonly complex, expensive, and inflexible in production. A systematic investigation of the formation, mechanisms, and properties of nanostructures induced by nano-capillaries remains unexplored. Through this study, the facile formation of nano-pillared surfaces induced by designable nano-capillaries on AAO template demonstrated significant improvements in antibacterial effect, spectral structural color and anti-reflection characteristics. Reverse molding of nano-pillar arrays with aspect ratios of up to 1.2 on polydimethylsiloxane (PDMS) films yielded a super-hydrophobic surface of 150° contact angle (CA). Through the utilization of computational fluid mechanics model, a direct correlation was demonstrated between the aspect ratio of nano-pillars and the viscosity of PDMS. The nano-pillared surfaces had remarkable antibacterial rates of 91% for Escherichia coli and 89% for Staphylococcus aureus. In addition, the structured surfaces exhibited color spanning from light blue to light red by tuning nano-pillars dimensions, incident light angle, and/or adding reflective metallic coating, which offered unique visualization effects for anti-counterfeiting. Moreover, the superior light-trapping and anti-reflection of the nano-pillared surface significantly increased the power conversion efficiency (PCE) 19% from the base performance of the polysilicon solar cells.

Liquid metal droplets impacting superhydrophobic surface in a viscous (non-oxidizing) environment

The hypothesis of the present research is the existence of distinct spatial-temporal characteristics of non-oxidized liquid metal (LM) droplets impacting a solid surface. To provide a quantitative claim to this hypothesis, we created a test matrix based on the well-known impingement regime map bounded by two dimensionless quantities—Weber number (We) and Ohnesorge number (Oh). The range of these quantities is from 10−2 to 102 (We) and 10−3 to 101 (Oh), leading to Reynolds number (Re) (≡We1/2/Oh) to vary from 10−2 to 104. The class of LMs opted for are post-transition metals—eutectic gallium alloys—due to their several desired practical features, such as low melting point, non-toxicity, and low vapor pressure. The research is conducted using numerical experiments performed using C++ OpenFOAM libraries. To ensure the reliability of the code, we tested our work with numerous impingement behaviors of fluids available in the literature. A plethora of droplet behaviors are reported, such as deposition, rebound, bubble entrapment, and splash. Several features of droplet impingement were critically examined, such as temporal spreading factor, maximum spreading factor, and contact time of droplets on the surfaces. Moreover, the conventional scaling laws regarding the impingement behavior of droplets were tested, with new ones proposed where deemed necessary. Furthermore, a distinct route for the entrapment of droplet is observed, caused by the bulging of LM droplet during the recoiling stage. Emphasis is made to form delineations for these impingement characteristics using dimensionless groups (i.e., We, Oh, and Re).

Constructing carbon nanotube (CNTs)/silica superhydrophobic coating with multi-stage rough structure for long-term anti-corrosion and low-temperature anti-icing in the marine environment

The rough superhydrophobic surface is beneficial for capturing more gases underwater, which enhances corrosion and ice resistance for marine equipment. In this work, a PVAc-PVDF-FMCS (PPFMCS) multi-stage rough superhydrophobic coating was manufactured by a cold spraying method, significantly improving the anti-corrosion and anti-icing of aluminium alloy. The point-line structure was designed by cross-linking between multi-walled carbon nanotubes (MWCNTs) and silicon dioxide (SiO2), which facilitated the formation of multi-stage rough surface. The porous skeleton and interfacial adhesion of the PPFMCS coating were attributed to the introduction of polyvinylidenefluoride (PVDF) and polyvinylacetate (PVAc), respectively. The PPFMCS coating had good superhydrophobicity with a water contact angle of 163.5°, while exhibiting the outstanding performance of long-term anti-corrosion and anti-icing. The |Z|0.01 Hz value of the PPFMCS coating was still 9.62 × 109 Ω cm2 in 3.5 wt% NaCl solution for 35 days, only two orders of magnitude lower than that of the original coating, the equivalent circuits were further investigated and the metal protection is evaluated on the shore or in the deep ocean of the South China Sea. The PPFMCS coating was able to obviously delay icing for 5 min at −20 °C, the ice adhesion of its surface was as low as 85.5 kPa. The icing phase transition was analysed by a thermodynamic method. The coating also had good stability and drag reduction performance. This high-performance coating based on point-line structural design is expected to have practical applications in marine transportation, oil exploration and polar exploration.

Droplet Impact on Superhydrophobic Mesh Surfaces.

Reducing the contact time of droplet impacts on surfaces is crucial for various applications including corrosion prevention and anti-icing. This study aims to explore a novel strategy that greatly reduces contact time using a superhydrophobic mesh surface with multiple sets of mutually perpendicular ridges while minimizing the influence of the impacting location. The effects of the impact Weber numbers and ridge spacing on the characteristics of the impact dynamics and contact time are studied experimentally. The experimental results reveal that, for the droplet impact on mesh surfaces, ridges can segment the liquid film into independently multiple-retracting liquid subunits. The retracted subunits provide the upward driving force, which may promote the splashing or pancake bouncing of droplets. At this point, the contact time has a negligible sensitivity for the impacting position and is significantly reduced by up to 68%. Furthermore, the time, dynamic pressure, and energy criteria for triggering splashing and pancake bouncing are proposed theoretically. This work provides an understanding of the mechanism and the design guidelines for effectively reducing the contact time of the impacting droplet on superhydrophobic surfaces.

Lotus leaf and shark skin-inspired micro-, nano-, hierarchical superhydrophobic surfaces for anti-fouling applications

Fouling, the accumulation and adhesion of unwanted contaminants, is a serious issue around the world in many aspects including industrial transport, marine ecological environment, maintenance of infrastructures, etc. which costs hundreds of millions of dollars and lots of manpower and material resources. Multiscale hierarchical surface structures of superhydrophobic surfaces possess various intriguing properties which provides new platforms for fabricating artificial anti-fouling surfaces. In particular, lotus leaf and shark skin with their unique micro-, nano-, hierarchical surfaces structures are highlighted as emerging tools for anti-bacterial, anti-dust, anti-corrosion, anti-icing, drag reduction applications and so on. In this review, we first provide basic information on two famous superhydrophobic states. After that we outlined the biological models and applications of lotus leaf and shark skin respectively. The self-cleaning effect of superhydrophobic lotus leaf due to their multiscale hierarchical surface structures is discussed after which anti-bacterial applications with three kinds of mechanisms and self-cleaning properties are outlined. Biological models of superhydrophobic shark skin are presented followed by their real-life applications in aircrafts and turbine blades. In addition, we discuss the potential drawbacks of recent biomimetic anti-fouling superhydrophobic surfaces like the loss of anti-fouling hydrophobic materials, adaptability to extreme environments, production and use costs and other problems as well as the possibilities of combine superhydrophobic structures and materials with high temperature resistance, oxidation resistance and other characteristics in order to be applied in more industries fields.