Large Contact Angle Research Articles

Hierarchical Micro/Nanostructures with Anti-Reflection and Superhydrophobicity on the Silicon Surface Fabricated by Femtosecond Laser

In this paper, hierarchical micro/nano structures composed of periodic microstructures, laser-induced periodic surface structures (LIPSS), and nanoparticles were fabricated by femtosecond laser processing (LP). A layer of hydrophobic species was formed on the micro/nano structures through perfluorosilane modification (PM). The reflectivity and hydrophobicity’s influence mechanisms of structural height, duty cycle, and size are experimentally elucidated. The average reflectivity of the silicon surface in the visible light band is reduced to 3.0% under the optimal parameters, and the surface exhibits a large contact angle of 172.3 ± 0.8° and a low sliding angle of 4.2 ± 1.4°. Finally, the durability of the anti-reflection and superhydrophobicity is also confirmed. This study deepens our understanding of the principles of anti-reflection and superhydrophobicity and expands the design and preparation methods for self-cleaning and anti-reflective surfaces.

Universal droplet propulsion by dynamic surface-charge wetting

Controllable droplet propulsion on solid surfaces plays a crucial role in various technologies. Many actuating methods have been developed; however, there are still some limitations in terms of the introduction of additives, the versatilities of solid surfaces, and the speed of transportation. Herein, we have demonstrated a universal droplet propulsion method based on dynamic surface-charge wetting by depositing oscillating and opposite surface charges on dielectric films with unmodified surfaces. Dynamic surface-charge wetting propels droplets by continuously inducing smaller front contact angles than rear contact angles. This innovative imbalance is built by alternately storing and spreading opposite charges on dielectric films, which results in remarkable electrostatic forces under large gradients and electric fields. The method exhibits excellent droplet manipulation performance characteristics, including high speed (~130 mm/s), high adaptability of droplet volume (1 μL–1 mL), strong handling ability on non-slippery surfaces with large contact angle hysteresis (CAH) (maximum angle of 35°), significant programmability and reconfigurability, and low mass loss. The great application potential of this method has been effectively demonstrated in programmable microreactions, defogging without gravity assistance, and surface cleaning of photovoltaic panels using condensed droplets.

Gravitational effects on optical lens fabrication: First insides on Earth- and microgravity experiments

Abstract Understanding the effects of gravity on manufacturing processes is a pioneering extension of the process parameter space used to date. Until now, the improvement of manufacturing technologies has mainly focused on process parameters such as temperature, pressure, and material composition, as access to variable gravity environments is limited. The Einstein-Elevator opens up new possibilities for the variation of these process parameters and the development of in-space manufacturing technologies. Together with the research of innovative production processes for optical components within the PhoenixD Cluster of Excellence, this creates an entirely new field of research. The research presented here focuses on investigating gravity's effects on dispensed optical lens production. Using a jet dispenser, sessile droplets are produced during a flight phase in the Einstein-Elevator and cured directly by UV polymerization. As part of this study, optical lenses were produced and compared under microgravity and Earth's gravitational conditions. Geometric properties such as height and contact angle of the lenses produced were analyzed. It was found that lenses fabricated under microgravity have a larger contact angle than those fabricated under Earth gravity. Similarly, the height increases with decreasing gravity. These results are consistent with the theoretical assumptions described, although generalized theories to describe the morphology of a sessile droplet are not yet available. The case study findings on the influence of gravity as a process parameter on drop morphology represent a fundamental improvement for additive manufacturing technologies, especially for in-space manufacturing.

Interfacial and foaming properties of plant and microbial proteins: Comparison of structure-function behavior of different proteins

Many plant proteins are amphiphilic molecules that can adsorb to air-water interfaces and form protective coatings around gas bubbles. In this study, the composition, structure, physicochemical properties, air-water interfacial properties, and foaming properties of 16 plant and microbial proteins were characterized. We found a correlation between the composition, structure, physicochemical properties, and foaming properties of the proteins. The foaming capacity of them showed a highly significant positive correlation (p ≤ 0.01) with their foaming stability, α-helix content, surface hydrophobicity, and free sulfhydryl content. The foaming capacity and foaming stability showed highly significant negative correlations with disulfide bond content (p ≤ 0.01). We found wheat gluten protein (WGP) and mung bean protein (MBP) had higher foaming capacity (102.67 ± 8.08 % and 89.33 ± 4.72 %), which could be attributed to higher surface hydrophobicity (179.68 ± 1.40 and 130.28 ± 1.41) and larger contact angle (82.369 ± 0.016° and 82.949 ± 0.228°).

Superhydrophobicity, Photocatalytic Self-Cleaning and Biocidal Activity Combined in a Siloxane-ZnO Composite for the Protection of Limestone.

The erosion phenomena of the natural stone in cultural heritage are induced by various sources. Consequently, the development of multifunctional protective materials that combine two or more useful properties is an effective strategy in addressing the synergistic effects of various erosion mechanisms. A multifunctional coating, consisting of a silane-based precursor and zinc oxide (ZnO) nanoparticles (NPs), is produced and tested for the protection of limestone. The hybrid coating combines the following three properties: superhydrophobicity, including water-repellency, photocatalytic self-cleaning and biocidal activity. The relative concentration of the NPs (0.8% w/w), used for the suggested composite coating, is carefully selected according to wetting studies, colourimetric measurements and durability (tape peeling) tests. The non-wetting state is evidenced on the surface of the composite coating by the large contact angle of water drops (≈153°) and the small contact angle hysteresis (≈5°), which gives rise to a physical self-cleaning scenario (lotus effect). The photocatalytic chemical self-cleaning is shown with the removal of methylene blue, induced by UV-A radiation. Moreover, it is shown that the suggested coating hinders the incubation of E. coli and S. aureus, as the inhibitions are 94.8 and 99.9%, respectively. Finally, preliminary studies reveal the chemical stability of the suggested coating.

Facile Synthesis of a Layered Bismuth-Based MOF With Superhydrophobic-Oleophilic Property and High Oil-Water Separation Performance.

Metal organic frameworks (MOFs) are a class of potential superhydrophobic-oleophilic materials. The organic ligands in superhydrophobic MOFs usually contain long alkyl chains, fluorine groups or aromatic rings with large π conjugation, the preparation of which suffers from high cost, complex operation and so on. In addition, the topological structure of MOFs plays an important role in the hydrophobicity, which may be ignored previously. Here we report a superhydrophobic-oleophilic MOF (BiPPA2) obtained by a facile and fast method, which not only displays a large water contact angle of up to 163° and a small sliding angle of nearly equal to 0°, but also exhibits high sorption capacity for multiple oils and organic solvents, well reusability and high oil retention. In addition, BiPPA2 is stable in a wide pH range (0.5-11.0). Finally, the single crystal structure of BiPPA2 is resolved to reveal the intrinsic reason for the super-hydrophobicity. This work may inspire the further design of pristine superhydrophobic MOFs based on a simple method, which enriches the family of superhydrophobic MOFs and has great significance for practical applications.

Engineering of Hierarchical Porous Composite Film (ZIF-8@PVDF/PMMA) with Superhydrophobicity for Highly Efficient Water Harvesting.

Atmospheric water harvesting has attracted much attention because of its potential to escalate the global freshwater shortage. However, the water collection efficiency is hindered by the trade-off between fast droplet nucleating and rapid droplet dripping due to the opposite requirements in the chemistry and the morphology of surfaces. Herein, the hierarchical porous composite film (ZIF-8@PVDF/PMMA, HPCF) with superhydrophobicity is designed for highly efficient and stable water harvesting. It indicates that the HPCF film has a large water contact angle (WCA) of 155.50° and ultralow sliding angle (SA) of 2°, exhibiting the self-cleaning function. Significantly, it is demonstrated that the water collection efficiency of HPCF can achieve 1.13 g·cm-2·h-1, which is much higher than the value of the blank sample, as well as most of the reported values. It is attributed to the hierarchical porous structure with the ZIF-8 crystals enhancing the surface roughness and endowing the film with the hydrophilic/superhydrophobic regions. This design promotes an optimal balance between droplet nucleation and shedding, significantly enhancing the water harvesting efficiency. Consequently, this work introduces an effective approach for water collection materials suitable for fog/mist conditions and provides an effective solution for the foggy area with water scarcity, demonstrating significance for advancing research aimed at mitigating the worldwide water shortage.

A superhydrophobic PVDF-based membrane containing Metal-organic framework for efficient Oil-water separation

The discharge of oily wastewater has caused heavy burden to the environment, so it is necessary to solve this problem. Membrane separation, as a simple and efficient separation technology, has attracted widespread attentions in the field of oil–water separation. In this work, a superhydrophobic PVDF-based membrane containing MOFs with small size (ZJU-64-CH3-S) was prepared. ZJU-64-CH3-S with different amounts (200 mg, 400 mg and 600 mg) were added into PVDF to obtain ZJU-64-CH3-S/PVDF membranes, which named as M1, M2 and M3. They displayed ideal hydrophobicity according to the large water contact angle (about 130°). And composite membranes showed good oil adsorption and excellent oil–water separation efficiency. In particularly, the oil flux of M2 reached to 333.33 L/(m2·h) and its oil–water separation efficiency was as high as 99 %. The composite membrane was expected to be used in the field of oil–water separation.

Novel fluorine-functionalized Ti3C2Tx/TiO2 hybrid coatings with enhanced weatherability, antifouling, and interfacial anticorrosion performances

As a valuable symbol of human cultural heritage, bronze inevitably endures varying degrees of corrosion due to environmental change. Thus, developing an efficient protective coating for bronze is crucial to ensure bronze relics receive more protection. Herein, a multifunctional organic-inorganic hybrid composite coating of 1 H,1 H,2 H,2 H-Perfluorodecyltriethoxysilane (PFDTES), Ti3C2Tx nanosheets and TiO2 nanoparticles (PFDTES@Ti3C2Tx/TiO2) are successfully prepared and coated the bronze, which exhibits excellent anti-corrosion, weathering stability and superhydrophobicity. The superhydrophobicity of coating is derived from introducing a low surface energy C-F bond. Furthermore, the corrosion protection ability can be improved by the barrier effect of Ti3C2Tx nanosheets, and TiO2 nanoparticles can enhance the weathering stability of coatings by providing exceptional abrasion resistance and ultraviolet (UV) resistance capabilities. Morphological examination, X-ray photoelectron spectroscopy, an accelerated aging test, and electrochemical measurements, among other methods, were used to research the protective effect and anticorrosion performance of organic-inorganic hybrid coatings. It can be found that the composite coatings have a large water contact angle (153°) and form a superhydrophobic surface. Furthermore, electrochemical results show that the superhydrophobic PFDTES@Ti3C2Tx/TiO2 coating has a higher low-frequency impedance modulus and lower current density than the uncoated substrate, indicating enhanced corrosion resistance. Based on solid and liquid pollutant tests, the PFDTES@Ti3C2Tx/TiO2 coatings also showed good self-cleaning and antifouling properties. And coating maintained good transparency without changing the bronze color and appearance. In conclusion, the superhydrophobic PFDTES@Ti3C2Tx/TiO2 composite coating has potential applications in the field of bronze protection.

Effect of solvent acids on the microstructure and corrosion resistance of chitosan films on MAO-treated AZ31B magnesium alloy

This study evaluated the effect of solvent acids on the structure and corrosion resistance performance of chitosan (CS) film on MAO-treated AZ31B magnesium (Mg) alloy. Initially, CS solutions were prepared in four solvent acids: acetic acid (HAc), lactic acid (LA), hydrochloric acid (HCl), and citric acid (CA). The CS films were subsequently deposited on MAO-treated AZ31B Mg alloy via a dip-coating technique. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), Fourier-transform infrared spectroscopy (FT-IR), contact angle measurement, and atomic force microscopy (AFM) were employed to characterize the surface and cross-sectional morphology as well as chemical composition. Furthermore, the samples were subjected to potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests to assess their resistance against corrosion in simulated body fluid (SBF). These results indicated that the CS film prepared with LA exhibited the lowest surface roughness (Ra = 31.2 nm), the largest contact angle (CA = 98.50°), and the thickest coating (36 μm). Additionally, it demonstrated superior corrosion protection performance, with the lowest corrosion current density (Icorr = 3.343 × 10−7 A/cm2), highest corrosion potential (Ecorr = −1.49 V), and highest polarization resistance (Rp = 5.914 × 104 Ω·cm2) in SBF. These results indicated that solvent acid types significantly influenced their interactions with CS. Thus, the structure and corrosion protection performance of CS films can be optimized by selecting an appropriate solvent acid.

Droplet impact and rebound dynamics on superhydrophobic surfaces

The impact and rebound dynamics of droplets on superhydrophobic surfaces were investigated through numerical analysis employing the phase field method. The influences of contact angle, impact velocity, surface tension, and dynamic viscosity on the fields of pressure and velocity as well as the spreading factor and central height were described comprehensively. The results indicate that there are a series of stages of impingement, spreading, transition, retraction, and rebound in order throughout the life cycle of a droplet. The droplet exhibits distinct pressure and velocity profiles upon impingement stage, with the maximum pressure at the lower center and higher velocities at the upper periphery, spreading around. Velocities are predominantly upward, peaking at the bottom of the droplet during the rebound stage. A larger contact angle, viscosity, surface tension, and lower impact velocity contribute to a reduced maximum spreading factor. Deposition is more likely to occur when the impact velocity, surface tension is lower, and the viscosity is larger. Droplets tend to rebound when the contact angle, impact velocity, and surface tension are larger. Thresholds for impact velocity, surface tension, and viscosity were delineated for droplet rebound. Furthermore, a correlation for predicting the maximum spreading factor of droplets on superhydrophobic surfaces was proposed.

Highly Selective Protic-Solvent-Mediated Organic-Inorganic Hybrid Cuprous Bromides Achieving Structural Transformation.

The potential application of stimuli-responsive hybrid copper halides in information storage and switch devices has generated significant interest. However, their transformation mechanism needs to be further studied deeply. Herein, two zero-dimensional (0D) organic-inorganic hybrids, namely, (TBA)CuBr2 (1) with linear [CuBr2]- units and (TBA)2Cu4Br6 (2) with [Cu4Br6]2- clusters (TBA+ = (C4H9)4N+), are synthesized using simple solvent evaporation approaches. Interestingly, upon exposure to distinct protic solvents, such as methanol, ethanol, ethylene glycol, or hot water, 1 undergoes a transformation into 2 with varying degrees of transition, accompanied by a change in luminescence color from cyan to orange (or mixed color) under high-energy emission (e.g., 254 nm) excitation. Hot water can trigger 1 to completely transform into 2 because of its large contact angle difference in the solvents. Furthermore, 2 can be converted back to 1 through a simple solid-state mechanochemical reaction. Additionally, the structure of 2 remains unchanged even after immersion in 80 °C H2O for 168 h due to the dense organic framework. This study provides valuable insights for exploring reversible structural transformation materials in the 0D metal halide system.

Wetting transitions in adhesive surfaces of polystyrene: The petal effect

HypothesisThe petal effect is a well-known natural phenomenon in surface science and has served as inspiration for the creation of several materials with superhydrophobic qualities and high adhesion. As surface roughness has a crucial role in these properties, being able to modulate it could help us design materials at will. Capillary penetration frustrates diffusion and promotes large contact angles as well as high adhesion. ExperimentsPolystyrene surfaces were created using the spin-coating technique. By varying the polymer concentration, the surface roughness was modified. To determine the roughness parameters, atomic force microscopy was used. We recorded advancing and receding contact angles using water and glycerol as test liquids to study contact angle hysteresis, the work of adhesion and the apparent surface energy, which was determined with the Chibowski and Perea–Carpio method. For the purpose of elucidating the wetting states, captive bubble experiments were conducted. FindingsUsing an easy method, we create polystyrene surfaces with both superhydrophobicity and strong adhesion, where the roughness area factor regulates wetting transitions from Cassie–Baxter to Wenzel. The receding contact angle suggests capillary penetration, which we demonstrate by captive bubble experiments. In addition, a link was found between the surface roughness and apparent surface energy.

Sulfobetaine Zwitterions with Embedded Fluorocarbons: Synthesis and Interfacial Properties.

We describe the preparation of a new set of fluorinated sulfobetaine (FSB) zwitterionic polymers in which fluorocarbon moieties are attached directly to the zwitterionic components. An efficient two-step modification to the conventional sulfobetaine methacrylate monomer synthesis gave access to a series of polymer zwitterions containing varying extents of fluorocarbon character. FSB methacrylates proved amenable to homo- and copolymerizations using reversible addition-fragmentation chain transfer (RAFT) conditions, affording polymers with molecular weights ranging from 5 to 20 kDa and with low molecular weight distributions. Thin films of FSB homopolymers on glass proved stable to aqueous environments and exhibited increasing hydrophobicity with fluorocarbon content, as well as remarkably large water contact angle hysteresis values that enable pinning of water droplets on hydrophobic surfaces, reminiscent of the "petal effect" found in nature. FSB-containing copolymers in aqueous media demonstrated markedly reduced oil-water interfacial tension values, even with moderate (20-50 mol %) FSB incorporation.

Fragmentation and reorganization of agglomerates in catalyst inks: Understanding the time-scale and temperature-dependent effects on the dispersion process

The time and temperature of the dispersive shear process of catalyst inks have a great influence on the agglomerate structure of the ink and the rheological properties of the ink. In this paper, three different catalyst inks were prepared at 20 °C, 30 °C and 40 °C, and tested using characterization such as rheological techniques, zeta (ξ) potential, and particle size measurements. As the temperature rises, the median particle size inside the ink at the end of shear was 400, 308.05 and 157.3 nm, and the zeta potential was −37, −33 and −25 mV, with a decrease in viscosity and a deterioration in thixotropy. The catalyst layers prepared from inks dispersed at different temperatures had the smallest cracks and the flattest surface at 30 °C, and the largest cracks, the largest contact angle, the roughest surface, and the poorest polarization properties at 40 °C. Through the comprehensive analysis of the experimental data, the fragmentation and reorganization processes of agglomerates during the internal dispersion of catalyst ink were revealed from the perspectives of time scale and temperature variation, respectively. This study provides an experimental basis for a deeper understanding of the dispersion performance of catalyst ink at different temperatures.

Surface modified molybdenum disulfide nanosheets for corrosion resistance improvement on polyurethane coatings

Abstract Organic–inorganic hybrid coating has been applied on metallic corrosion protection effectively. Molybdenum disulfide (MoS2) nanosheets with graphene-like two-dimensional lamellar structure were an anticorrosion inorganic additive, rendering the organic coating better corrosion resistant. However, the aggregation and poor solubility are still current issues that should be addressed. Functionalization MoS2 nanosheets with surface modified by polydopamine (PDA) and silane coupling agent (KH560) were prepared in polyurethane (PU) composite coatings to obtain dense and intact multilayer network structure coatings for corrosion protection. KH560-PDA-MoS2/PU coating with crosslinked polymer network structure has a high impedance modulus, large contact angle, and strong hydrophobicity. The coating meets the national technical standards for salt spray testing and nitric acid (HNO3) titration testing, demonstrating excellent corrosion resistance.

A numerical study of heterogeneous nucleation of ice crystals and frost layer growth on horizontal cold surfaces

In engineering applications, the formation and growth of frost layers can significantly affect the normal operation of equipment, leading to undesirable influences. Therefore, it is imperative to understand the frost formation mechanisms as a basis for developing effective anti-frosting and defrosting methodologies. This study proposes a model for the growth of horizontal cold surface frost layers using the classical nucleation theory and the Eulerian multiphase flow model. Compared to numerous models that investigate the frost formation issues using the computational fluid dynamics (CFD) method, our proposed model takes into account the crystal growth period (the heterogeneous nucleation process of ice). The predicted results of the proposed model were found to be in good agreement with the data collected from five related experimental studies. In this regard, the maximum frost layer thickness error was 23.9 % with a mean error of 5.1 %, and the maximum density error was 22.8 % with a mean error of 8.1 %. Compared to existing models in the literature, the proposed model can reduce the maximum thickness and density uncertainties by 17.4 % and 10.2 %, respectively. Additionally, the effects of the surface contact angle and environmental parameters on the nucleation process were analyzed using the established model. It was found that a larger contact angle, higher cold surface temperature, and lower air temperature, humidity, and velocity were unfavorable conditions for supporting the formation and growth of ice crystals. Moreover, the nucleation completion time was shortened with the increase in the moist air supersaturation degree, varying as an exponential function. The findings of this study provide an important theoretical basis for the development of optimizing strategies for anti-frosting and defrosting.

Effect of molecular branching and surface wettability on solid-liquid surface tension and line-tension of liquid alkane surface nanodroplets

HypothesisSurface nanodroplets have important technological applications. Previous experiments and simulations have shown that their contact angle deviates from Young's equation. A modified version of Young's equation considering the three-phase line tension (τ) has been widely used in literature, and a wide range of values for τ are reported. We have recently shown that molecular branching affects the liquid–vapour surface tension γlv of liquid alkanes. Therefore, the wetting behaviour of surface nanodroplets should be affected by molecular branching. This study conducted molecular dynamics (MD) simulations to gain insight into the wetting behaviour of linear and branched alkane nanodroplets on oleophilic and oleophobic surfaces. We aim to examine the Young equation's validity and branching's effect on fundamental properties, including solid–liquid surface tension γsl and line tension τ. SimulationsThe simulations were performed on a linear alkane, triacontane (C30H62), as well as four of its branched isomers: 2,6,13,17-tetrapropyloctadecane,2,6,9,10,13,17-hexaethyloctadecane, 2,5,7,8,11,12,15-heptaethylhexadecane and 2,3,6,7,10,11-hexapropyldodecane. Nanodroplets with a diameter of approximately 15 nm were released onto the surfaces, and their contact angles were measured. Additionally, using a novel approach, the solid–liquid surface tension (γsl), the validity of Young's equation and line tension for all alkane and surface combinations are determined. FindingsIt was discovered that the calculated γsl, deviated from the theoretical γsl,Young predicted from Young's equation for all alkanes on oleophilic surfaces. However, this deviation was minimal for branched alkanes on the oleophobic surfaces but more significant for the linear alkane. The findings indicated that γsl < 0 for oleophilic surfaces and γsl > 0 for oleophobic surfaces. Moreover, it was observed that |γsl| was lower for branched molecules and decreased as branching increased. Line tension values were then determined through a novel method, showing τ was positive for oleophilic surfaces ranging from 1.30 × 10-10 to 6.27 × 10-11N. On an oleophobic surface, linear alkane shows a negative line tension of −1.15 × 10-10N and branched alkanes up to two orders of magnitude lower values ranging from −2.09 × 10-12 to 2.43 × 10-11N. Line tension values between −1.15 × 10-10 and + 1.1 × 10-10N are calculated for various linear alkane and surface combinations. These findings show the dependence of line tension on the contact angle and branching, demonstrating that for linear alkanes, τ is significant, whereas, for branched alkanes, line tension is smaller or negligible for large contact angles.

Fabrication of robust superhydrophobic surface on TC4 alloy by wire electrical discharge machining

Superhydrophobic metal surfaces have significant applications in various areas such as surface self-cleaning, resistance reduction, anti-corrosion, etc. However, inexpensive preparation methods for superhydrophobic surfaces with a good mechanical stability are still to be developed. In this study, microarray structures on TC4 alloy were readily fabricated using a wire electrical discharge machining technology. Following the treatment with a fluorinating reagent, the microarray surface exhibited excellent superhydrophobic properties, where the largest contact angle and the smallest sliding angle reached 159° and 3°, respectively. Influences of the dimensions of microarray structure on the TC4 alloy surface wettability were also investigated in this study. Experiments show that this superhydrophobic surface possesses favorable self-cleaning, good mechanical stability, and promising anti-corrosion properties. In addition, by controlling the dimensions of the microstructure a superhydrophobic surface with a gradient wettability was also fabricated, which exhibits the property of doing nondestructive transportation of liquids.

Modelling and preparation of composite wrinkled structures on polyester fabric via plasma treatment

Micro/nanostructured surfaces with extreme wettability, such as superhydrophobic surfaces, are useful for various applications. Herein, we used Ar plasma to form polydimethylsiloxane films while immobilising SiO2 nanoparticles, which were prepared on polyester fabric surfaces under mechanical stretch–release to obtain micro/nanostructures. The treated fabrics exhibited a large contact angle of 169.06° with small sliding angle of 5°. We developed corresponding micro/nanostructure models to establish the relationships between the structural parameters and the hydrostatic pressure and water-droplet bouncing. The model is useful for guiding the regulation of water pressure.