Stable Contact Angle Research Articles

Developing Super-Hydrophobic and Abrasion-Resistant Wool Fabrics Using Low-Pressure Hexafluoroethane Plasma Treatment.

The growing interest in wool fibres as an eco-friendly and sustainable material for diverse industrial applications requires an enhancement of their functional performance. To address this, wool fabrics were treated in the present research with low-pressure hexafluoroethane (C2F6) plasma to impart superhydrophobicity and improve their abrasion resistance. Unscoured and scoured wool fabrics were treated with C2F6 while varying plasma power (80 W and 150 W), gas flow rate (12 sccm and 50 sccm) and treatment time (6 min and 20 min), and the effect of plasma parameters on the abrasion resistance, water contact angle and dyeing behaviour of the wool fabrics was studied. Martindale abrasion testing showed that the surface abrasion of the wool fabrics increased with the number of abrasion cycles, and the samples treated with 150 W, 20 min, 12 sccm showed superior abrasion resistance. The scoured wool fabrics showed a contact angle of ~124°, which was stable for only 4 min 40 s, whereas the plasma-treated samples showed a stable contact angle of over 150°, exhibiting a stable superhydrophobic behaviour. The C2F6 plasma treatment also significantly reduced the exhaustion of an acid dye by wool fabrics. The EDX study confirmed the deposition of fluorine-containing elements on the wool fabrics significantly altering their properties.

Wetting Behavior of the Ag-5CuO Brazing Alloy on ZTA Composite Ceramic with/without CuO Coating in Air

The wetting of Ag-5 wt.% CuO (Ag-5CuO) alloy on initial/CuO-coated zirconia toughened alumina (ZTA) composite ceramic in air was studied in detail. The results showed that the contact angle of the Ag-5CuO/ZTA system rapidly decreased from 81° at 970 °C to 45° at 990 °C during the heating process, however, moderate reductions in contact angle were observed in the subsequent heating and temperature holding stages. In comparison with the contact angle of pure Al2O3, an increment of about 4° of the stable contact angle of Ag-5CuO alloy on the heterogeneous ZTA was observed. The reaction between Al2O3 and CuO can reduce the damage of the CuO-rich liquid to ZrO2 in the ZTA substrate. Both oxygen and CuO were helpful in reducing the contact angle of Ag on ZTA and enhancing the bonding of the Ag/ZTA interface. The continuous CuO coating on ZTA and the monotectic liquid containing more CuO in the region near the triple line induced reductions of more than 40° and about 10° in the contact angle, respectively, between the initial and the CuO coating-improved wetting systems.

Droplet Evaporation on a Hydrophobic Photothermal Conversion Substrate

Use of the photothermal effect to control droplet evaporation shows great promise in various applications. Droplet evaporation initiated by direct photothermal conversion between light and fluid is limited by the interaction between the wavelength and fluid type. Here, we report laser-induced sessile droplet evaporation on a hydrophobic photothermal conversion substrate formed by embedding Ti₂O₃ particles into polydimethylsiloxane. Our results indicated that the photothermal conversion substrate could efficiently convert light energy into heat to enable droplet evaporation on it. Unlike droplet evaporation actuated by direct photothermal conversion, in which higher interface temperature was located at the laser projection region, the droplet evaporation on the photothermal conversion substrate showed higher temperature near the triple-phase contact line because of heat transfer from the substrate to the droplet and small heat transfer resistance resulting from small height at the edge. Due to the increased substrate temperature, the droplet evaporation on the photothermal conversion substrate could exhibit a relatively stable and long-term constant contact angle mode even under the localized heating effect. Moreover, by controlling the laser beam position, droplet evaporation could exhibit a directional long-term constant contact angle mode. The obtained results will be helpful for the design and operation of droplet microfluidics based on the photothermal conversion substrate.

Hydrophobization of cellulose oxalate using oleic acid in a catalyst-free esterification suitable for preparing reinforcement in polymeric composites

It is common practice to use cellulose as reinforcement and fatty acid as compatibilizer in the preparation of polymeric composites. However, the used catalysts (e.g., pyridine) are usually toxic and should be avoided. In this study, a new type of microcellulose – cellulose oxalate (COX) was chosen as reinforcement to be reacted with oleic acid to prepare hydrophobic fillers in a catalyst-free esterification for different times. For comparison, microcrystalline cellulose (MCC) was also selected. The success of esterification of COX and oleic acid was confirmed but little esterification occurred when MCC was used. After reacting COX with oleic acid for 18 and 48 h, the products showed stable water contact angles of about 130°. Composites of polypropylene with COX or MCC were prepared. Tensile tests showed that for a given reaction time, the COX-based composites exhibited higher values of both Young’s modulus and tensile strength than those of MCC-based composites.

Parylene micropillars coated with thermally grown SiO2

The modification of surface properties frequently requires the binding of suitable compounds to the original surface. Silanes or thiols can be directly covalently bonded to either Si-based materials or Au, thus ruling out polymers. Here, we show the utilization of a layer of SiO2 with a thickness of a few nanometers that serves as a cross-linker between polymers and silanes providing covalent bonding to the surface. We deposited a polymer onto a thermally oxidized microstructured Si surface followed by subsequent Si removal. We demonstrated a Si-based nanotechnology fabrication method that can be generally used to modify the surface properties of practically any polymer via SiO2 cross-linking. This can produce any topology, including microstructures, nanostructures, or composite microstructure/nanostructures terminating in different shapes, since all the steps involving polymer deposition are conducted at room temperature after the Si surface has been thermally oxidized. This technique opens a broad field of new applications for polymers in microstructures and nanostructures that have stable water surface contact angle values with the contact angle set by demand for gecko-mimicking structures or lotus leaf inspired surfaces.

Robust and durable surperhydrophobic F-DLC coating for anti-icing in aircrafts engineering

The nose cones of plane engines adjust the airflow direction and guarantee steady operation but they are vulnerable to icing at high altitude leading to potential hazards and accidents. Superhydrophobicity is effective against icing but most organic superhydrophobic materials are not suitable for engineering applications. Herein, a mechanically robust superhydrophobic coating is prepared on stainless steel by incorporating laser-textured micro/nano structures into the plasma-coated multilayered structure of Cr/CrC/DLC/F-DLC. The multilayered coating shows a stable surface contact angle of >150° in addition of robust mechanical properties even in harsh environments such as prolonged ultraviolet light irradiation, wear, and corrosion. Tests conducted under simulated working conditions reveal significantly reduced ice accumulation on the coating because of easy roll-off of droplets consequently giving rise to excellent anti-icing properties.

Superhydrophobic coatings on wood made of plant oil and natural wax

Protection against wetting is a central issue in the finishing of wood, since water promotes degradation and reduced service life of the material. Ideally, wood finishes should provide efficient protection from water while at the same time preserving the overall bio-based character of the material. In order to meet this challenge, we propose a facile way to achieve superhydrophobic coatings on wood by using environmentally benign and even food-safe materials only. Spruce wood samples were dip-coated in a mixture of tung oil and natural beeswax, followed by the deposition of micronized sodium chloride particles. After a washing procedure, EDX confirmed that no sodium chloride remained at the surface. SEM-imaging illustrated the successful formation of a micro-structured topography endowing the surface with water repellence as indicated by stable static water contact angles above 160°. It is thus demonstrated that fully bio-based superhydrophobic wood coatings may be achieved using templating with water-soluble salt.

Surface Tension/Contact Angle Characters of Aprotic Binary Borneol-Dimethyl Sulphoxide Mixture

Borneol and dimethyl sulphoxide (DMSO) have been used as the skin penetration enhancers. Different concentrated borneol solutions in aprotic DMSO were prepared as the binary mixtures and determined their surface tension and contact angle behaviors using goniometer. Low borneol concentration minimized the surface tension and there was nearly stable contact angle for 30-70% borneol solutions. The trend contact angle value was decreased on glass slide, acrylic and LDPE whereas increasing on agarose gel and porcine tissues including buccal mucosa, gum and tongue with borneol concentration dependence. However, the borneol-DMSO binary mixture exhibited high wettability owing to its contact angle on glass surface was less than 90°. The solvent exchange between DMSO of borneol solutions and aqueous phase form agarose gel and porcine tissues including tongue, gum and buccal mucosa initiated the phase transformation from solution into matrix-like and promoted the higher contact angle with borneol concentration dependence. The understanding of surface tension and contact angle behaviors of aprotic borneol-DMSO binary mixture is useful for developing them into in situ forming gel for drug delivery such as in the oral cavity.

Silver-Polymethylhydrosiloxane Nanocomposite Coating on Anodized Aluminum with Superhydrophobic and Antibacterial Properties.

Biofilm formation on both animate and inanimate surfaces serves as an ideal bacterial reservoir for the spread of nosocomial infections. Designing surfaces with both superhydrophobic and antibacterial properties can help reduce initial bacterial attachment and subsequent biofilm formation. In the present study, a two-step approach is deployed to fabricate silver-polymethylhydrosiloxane (Ag-PMHS) nanocomposites, followed by a simple dip-coating deposition on anodized Al. Ag-nanoparticles (Ag-NPs) are synthesized in situ within a PMHS polymeric matrix. Morphological features of Ag-PMHS coating observed by scanning electron microscopy shows heterogeneous micro-nano-structures. The chemical compositions of these coatings were characterized using X-ray diffraction and attenuated total reflection-Fourier transform infrared spectroscopy, which indicate the presence of a low-energy PMHS polymer. The as-synthesized Ag-PMHS nanocomposite demonstrated excellent antibacterial properties against clinically relevant planktonic bacteria with zone of inhibition values of 25.3 ± 0.5, 24.8 ± 0.5, and 23.3 ± 3.6 mm for Pseudomonas aeruginosa (P.A) (Gram -ve), Escherichia coli (E. coli) (Gram -ve), and Staphylococcus aureus (S.A) (Gram +ve), respectively. The Ag-PMHS nanocomposite coating on anodized Al provides an anti-biofouling property with an adhesion reduction of 99.0, 99.5, and 99.3% for Pseudomomas aeruginosa (P.A), E. coli, and S. aureus (S.A), respectively. Interestingly, the coating maintained a stable contact angle of 158° after 90 days of immersion in saline water (3.5 wt % NaCl, pH 7.4). The Ag-PMHS nanocomposite coating on anodized Al described herein demonstrates excellent antibacterial and anti-biofouling properties owing to its inherent superhydrophobic property.

The Role of Ti in the Preparation of SiC/7075 Aluminum Matrix Composite

SiC/7075 aluminum matrix composites were prepared by a liquid stirring method. The role of Ti facilitating the preparation of SiC/7075 aluminum matrix were studied by means of direct-reading spectrometer, scanning electron microscope, energy dispersive spectrometer, X-ray diffraction and the sessile drop method. The results show that the SiC content in the SiC/7075 composite increases with an increase of Ti addition. The addition of Ti can significantly improve the wettability of SiC/Al system, there is a critical value of above 0.5% of Ti content in improving the wettability of the Al/SiC system at 1 173 K. The temperature of the “non wetting-wetting” transition for the (Al-2Ti)/SiC system is about 1 123 K, the contact angle decreases to 88 ° at 200 seconds and reaches a stable contact angle of 28 ° at 2 100 seconds.

Fabrication of pH-Sensitive Superhydrophilic/Underwater Superoleophobic Poly(vinylidene fluoride)-graft-(SiO2 Nanoparticles and PAMAM Dendrimers) Membranes for Oil-Water Separation.

The efficient treatment of oil-water emulsions under acidic condition remains a widespread concern. Poly(amidoamine) (PAMAM) dendrimer with hyperbranched structures and a large amount of primary and tertiary amino groups has exhibited advantages to solve this issue. Here, a novel poly(vinylidene fluoride)-graft-(SiO2 nanoparticles and PAMAM dendrimers) (PVDF-g-SiO2 NPs/PAMAM) membrane was fabricated using a surface-grafting strategy. SiO2 NPs were immobilized on PVDF-g-poly(acrylic acid) (PAA) membranes for improving the surface roughness, and PAMAM dendrimers were further immobilized on the membrane surface by interfacial polymerization (IP) for improving the surface energy. The obtained membrane demonstrated a water contact angle and a stable underwater-oil contact angle of 0° and >150°, respectively. These characteristics endowed the membrane with excellent water permeability [>3100 L/(m2·h) at 0.9 bar] and separation efficiency (>99%) during oil-water separation. Furthermore, the PAMAM chain will extend from a collapsed state into a fully extension state because of the protonation of amine groups under acidic condition, thus achieving a low underwater oil-adhesion property, fouling resistance, desirable stability, and recyclability (over 12 cycles) during usage. This work shows a promising prospect for the treatment of corrosive emulsions under acidic condition.

Spreading and adhesion forces for water droplets on methylated glass surfaces

Measurements of the spreading and capillary forces for liquids deposited on solid surfaces of varying surface chemistry and topography are a prerequisite to better understanding and quantification of the wetting mechanisms. In this study, glass slide surfaces were modified with trimethylchlorosilane to fabricate surfaces of varying hydrophobicity. Then, the forces of spreading and adhesion were measured between water droplets and glass surfaces using a high-sensitivity microelectronic balance. Integrated with a digital camera, the experimental set-up recorded forces and water droplet deformations during water droplet attachment, spreading, compression and retraction processes. It was confirmed that the spreading force increases with decreasing advancing contact angle for methylated glass, following a similar correlation as observed for smooth and rough polymers. However, the spreading force values for methylated glass were a few times lower than reported for polymers in spite of a similar roughness characteristic. It was also confirmed that the maximum adhesion force between water droplet and methylated glass increases with decreasing value of the most stable contact angle, a correlation that is similar to that reported for smooth polymers.

Migration characteristics of droplet condensation on end surface of single-finger microgripper

Liquid droplet is a prerequisite for micro-robot based on liquid medium. The investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper is of significance for obtaining stable droplets. The principle of flexible operation for micro-components using droplet condensation is analyzed first. The liquid bridge force acting on a microsphere is derived. A growth model of condensed droplet on the tip of a single-finger microgripper is established, including single-droplet growth, droplet coalesce, droplet movement, and pining effect. Condensation process on the tip of single-finger microgripper with a diameter of 130－400 μm is observed experimentally. Small droplets are formed by directly growing with a big growth rate in the initial stage, then the droplet growth is determined by droplet coalesce. The experimental results show that a single droplet is formed on the end surface after direct growth and droplets coalesce. The maximum droplet volume of 5.5 nL appears on the tip of a single-finger actuator with a diameter of 400 μm under the conditions of surface temperature of –5 °C, room temperature of 24 °C and humidity of 37%. The stability of the formed droplets is dominated by temperature gradients and edge effects during growth process. The distribution of condensed droplets is asymmetric while the microgripper is placed on a cooling surface with temperature gradient. A big growth rate is shown in a low temperature range. A single asymmetric droplet with an offset of 13 μm with respect to the axis of the actuator is formed, which is caused by the temperature gradient. A stable contact angle of 112° is obtained on the end surface of a single-finger microgripper with a diameter of 137 μm because of edge effect using the ambient temperature of 24 °C and humidity of 42%. Condensed droplets located on the end surface of hydrophobic microgripper are more stable than the untreated microgripper. Compared with the droplet formation (0.3 nL) on an untreated microgripper with a diameter of 150 μm, a lager stable droplet of 0.4 nL is obtained on the end face of a small microgripper with a diameter of 130 μm because of the hydrophobic action. The validity of theoretical analysis is verified by experimental results. The experimental investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper shows that the droplet shape can be changed by adjusting the temperature gradient and hydrophilic/hydrophobic performance, which plays an important role in achieving a stable droplet on the end surface.

High stable self-cleaning surface developed by monolithic hierarchical roughness

ABSTRACTMany hierarchical structures are used to create self-cleaning capability. The stability and durability of superhydrophobic surfaces are still questionable in various applications. In this article, the effect of annealing process and wax-coating approach was considered to create nanostructures on micro-rice-like patterns. The morphology of resultant structures was analysed by FE-SEM and AFM instruments. The presence of nanostructures significantly increased contact angles on the hydrophobic surfaces (from 138° to 158° in the annealed process and to 161° in the wax-coating method). The decreased sliding angle from 60° in smooth surface to 10° and 6° in both superhydrophobic surfaces demonstrated a clear correlation between hierarchical structures and self-cleaning capability. From comparison, the monolithic pattern showed higher stable contact angle during 6 months period at room temperature, higher durable self-cleaning capability, good brushing and tapping resistance. Hence this approach would offer additional insights for developing self-cleaning characteristics.

Stabilization of the Morphology and Crystal Phase in Ensembles of Self‐Catalyzed GaAs Nanowires

Recent experimental data reveal the two stable contact angles of gallium droplets promoting the vapor–liquid–solid growth of GaAs nanowires, around 90° and 130°, with the critical angle of developing the truncated growth interface being smaller, but very close to 130°. Herein, a model is presented for the time evolution of the nanowire radii and preferred crystal structures in large ensembles of self‐catalyzed GaAs nanowires. The model qualitatively explains earlier experimental observations and even contradictory results. It is shown that all nanowires stabilize to a stationary radius and pure zincblende phase within the optimal range of V/III flux ratios. Lower V/III flux ratios yield pure zincblende phase without any radius stabilization, whereas higher V/III ratios lead to a limited focusing of the radius, polytypism, or even pure wurtzite phase of GaAs nanowires.

Classification of the Morphologies and Related Crystal Phases of III–V Nanowires Based on the Surface Energy Analysis

Zincblende-wurtzite polytypism in III–V nanowires has been of great interest for a long time. However, full understanding and control over the crystal phase is still lacking. Here, we propose a model for the morphologies and related crystal phases of vapor–liquid–solid III–V nanowires by considering the minimum surface energies of different side wall facets as a function of the droplet contact angle. On the basis of the recent experimental data and earlier theoretical calculations of the surface energies, a simple classification of the possible growth modes is presented. It shows that the wurtzite phase forms in vertical nanowires at intermediate contact angles whereas the zincblende phase is predominant for vertical or inverse-tapered nanowires with a truncated top at larger contact angles. The small stable contact angle corresponds to faulted wurtzite–zincblende intermix. Pure zincblende phase is possible for even smaller contact angles only in the kinetic growth regimes. The wurtzite and zincblende dom...

Fabrication of Flexible and Superhydrophobic Melamine Sponge with Aligned Copper Nanoparticle Coating for Self-Cleaning and Dual Thermal Management Properties

In this work, a three-dimensional (3D) flexible copper coated melamine sponge (Cu-MES) with superhydrophobic properties capable of active heating and infrared insulation is fabricated via deposition of copper nanoparticles on the MES surfaces for personal thermal management applications. The results showed that the Cu nanoparticles were tightly anchored on the surface of the melamine sponge, forming a low infrared emissivity coating, resulting in high thermal insulation properties of the multifunctional Cu-MES. The promising electrical conductivity grants the superior Joule heating for extra warmth of 40 °C using a low supply voltage around 7 V. Besides, obtained Cu-MES material shows a self-cleaning property with a stable water contact angle of 157.5° and a sliding angle of 15°. These promising results including low infrared emissivity for high thermal insulation and high conductivity for heat collection make the Cu-MES promising candidates for applications in personal thermal management, energy regulation, and other facilities.

Thermodynamic spreading behaviors of oil on rough surfaces of stainless steel 316L

Thermodynamic spreading behaviors of oil were especially important for normal operation of equipment in the state of lubricating oil starvation, which were investigated experimentally and theoretically in this manuscript. Three rough surfaces of stainless steel 316L were obtained by grinding with sandpaper 600 mesh, 1000 mesh, 1500 mesh respectively. Spreading behaviors of lubricating oil 60N were studied on these rough surfaces with uniform temperature and temperature gradient separately, which were achieved by self-assembled apparatus, and a comparison for oil spreading performance was conducted between the two temperature conditions. In order to analyze the problem theoretically, a model for rough surfaces was built by assuming the profile of any rough peak and valley to be a parabola and distributed uniformly. It was found that the higher the surface temperature and the rougher the surface, the larger the oil spreading distance and velocity for surfaces with uniform temperature. For surfaces with a temperature gradient, directional motion of oil occurred when the temperature gradient was larger than 1.61 °C/mm. Although the oil droplet had the same starting temperature, however, the spreading velocity was larger on surfaces with smaller roughness, which was opposite with that with uniform surface temperature. In addition, the theoretical model could be used to describe the stable phase after oil droplet forming a relatively stable contact angle by comparing the computational results with experimental ones under temperature gradient conditions.

Study on the Fabrication of Super-Hydrophobic Surface on Inconel Alloy via Nanosecond Laser Ablation.

Nanosecond laser ablated metallic surfaces showed initial super-hydrophilicity, and then experienced gradual wettability conversion to super-hydrophobicity with the increase of exposing time to ambient air. Due to the presence of hierarchical structures and change of surface chemistry, the laser-induced Inconel alloy surfaces showed a stable apparent contact angle beyond 150° over 30-day air exposure. The wetting states were proposed to elucidate the initial super-hydrophilicity and the final super-hydrophobicity. The basic fundaments behind the wettability conversion was explored by analyzing surface chemistry using X-ray photoelectron spectroscopy (XPS). The results indicated that the origins of super-hydrophobicity were identified as the increase of carbon content and the dominance of C–C(H) functional group. The C–C(H) bond with excellent nonpolarity derived from the chemisorbed airborne hydrocarbons, which resulted in dramatic reduction of surface-free-energy. This study confirmed that the surface chemistry is not the only factor to determine surface super-hydrophobicity. The laser-induced super-hydrophobicity was attributed to the synergistic effect of surface topography and surface chemical compositions. In this work, the corresponding chemical reaction was particularly described to discuss how the airborne hydrocarbons were attached onto the laser ablated surfaces, which reveals the generation mechanism of air-exposed super-hydrophobic surfaces.

Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water.

Desalination of oil-contaminated saline water using membrane distillation requires hydrophobic membranes with underwater superoleophobic surfaces. For designing such membranes, the chemistry and morphology of the interfacial domains in contact with the contaminated water need to be adjusted such that a stable water layer, adhering to the surface, prevents oil droplets from wetting the membrane. In this article, we present an approach that relies on the controlled functionalization of the surface of polyvinylidene fluoride (PVDF) membranes; we adjust the surface topography of the membranes and introduce chemical heterogeneity to them. We show that the morphology of the PVDF surface can be altered by adjusting the composition of the nonsolvent bath used for the phase inversion process. Also, we render the surface of the membranes hydrophilic by using an alkaline chemical bath solution. The membrane morphology and effectiveness of our chemical treatment were confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), and zeta potential measurements. A stable underwater contact angle, higher than 150°, was observed for both canola oil (ρ ≈ 0.913 g cm-3, γ ≈ 31.5 mN m-1) and hexane (ρ ≈ 0.655 g cm-3, γ ≈ 18 mN m-1). We evaluated the performance of both pristine and functionalized membranes in a laboratory-scale direct contact membrane distillation (DCMD) setup and desalinated a saline solution contaminated with 500 ppm canola oil. Our results show that oil does not wet the functionalized membrane during the desalination process. The average permeate flux and salt rejection values for the functionalized membranes were 45 ± 5 Lm-2h-1 ( Tfeed = 70 °C, Tdistillate = 20 °C) and 99.99%, respectively.