Water-repellent Nature Research Articles

Fabrication of CP-Ti structure with controllable wettability using powder bed fusion and eco-friendly post-process

Hydrophobic surfaces have a wide range of applications, such as water harvesting, self-cleaning, and anti-biofouling. However, traditional methods of achieving hydrophobicity often involve the use of toxic materials such as fluoropolymers. This study aims to create controllable wettability surfaces with a three-dimensional geometry using a laser base powder bed fusion (PBF) process with commercially pure titanium (CP-Ti) and silicone oil as non-toxic materials. The optimal PBF process parameters for fabricating micropillar structures, which are critical for obtaining the surface roughness necessary for achieving hydrophobic properties, were investigated experimentally. After fabricating the micropillar structures using PBF, their surface energy was reduced by treatment with silicone oil. Silicone oil provides a low-surface-energy coating that contributes to the water-repellent nature of hydrophobic surfaces. The wettability of the treated CP-Ti surfaces was evaluated based on the diameter of the pillars and the space between them. The structure with the optimal diameter and spacing of micropillars exhibited a high contact angle (156.15°). A pronounced petal effect (sliding angle of 25.9°) was achieved because of the morphology of the pillars, indicating the controllability of wetting. The micropillar diameter, spacing, and silicone oil played crucial roles in determining the water contact and sliding angle, which are key metrics for surface wettability.

Long Term Durability of a Lubricant-Infused Surface for Dew Harvesting.

Hydrophobic, lubricant-infused surfaces offer enhanced potential for dew harvesting compared to bare metal substrates due to their water repellent nature. Most of the studies to date examine the condensation effectiveness of the nonwetting surfaces over a short duration and have not considered the durability or performance of the surfaces over extended periods. To address this limitation, the present study experimentally investigates the long-term performance of a lubricant-infused surface subject to dew condensation for 96 h. Condensation rates as well as sliding and contact angles are measured periodically to examine the surface properties and water harvesting potential over time. Due to the narrow time window in which dew harvesting can be conducted in application, the additional collection time gained by shedding droplets at lower nucleation times is explored. It is shown that three phases occur in lubricant drainage, which affect performance metrics relevant to dew harvesting. The first 24 h of condensation induces drainage that has little effect on the adhesion of droplets to the surface and on the additional collection time. The next phase, from about 24 to 72 h, showed steady drainage and a steady decrease in performance. The final 24 h, from about 72 to 96 h of operation, was seen to have little added effect on drainage and therefore on the performance metrics. The study bears significance in the design of surfaces for long-term use in practical water harvesters.

Value of dehydrated food waste fertiliser products in increasing soil health and crop productivity

Dehydration of food waste is a technique in which food waste is dewatered to form a low moisture product. This research characterised the physicochemical properties of different dehydrated food waste products and examined their value in improving physical, biological, and chemical properties of soils. Dehydrated food waste products were slightly acidic (4.7–5.1) with high levels of electrical conductivity (EC) (4.83–7.64 mS cm−1). The products were composed of complex carbohydrates, polysaccharides, alcohols, phenols, carboxylic acid, lipids, and fats and contained high levels of total and available nutrients. Dehydrated food wastes slightly impacted the soil pH; however, they significantly increased soil EC, which may cause soil salinity when applied repeatedly. The food waste products also increased macro-nutrients (N, P, and K) for plants across different soil types. Carbon and nutrients in dehydrated food waste increased microbial activity, measured by basal respiration. Delayed germination and reduced plant growth of corn (Zea mays) and wheat (Triticum aestivum) plants were observed at high application rates of dehydrated food waste. This may have resulted from a combination of phytotoxins, anoxic conditions, salinity as well as the water-repellent nature of dehydrated food waste. However, release of nutrients increased nutrient uptake and plant biomass in corn and wheat plants at low levels of food waste application. The dehydrated food waste products may require composting prior to soil application or incorporation into soil for a long duration prior to planting. These processes will overcome the limitations of phytotoxins, anoxic conditions, salinity, and water repellence. Further work is required to optimise the levels of dehydrated food waste application to improve soil health and crop productivity.

Durable and Regenerable Superhydrophobic Coating on Steel Surface for Corrosion Protection

Corrosion limits the usage of steel and leads to the tremendous loss of the economy. Superhydrophobic coating is proven an effective way to prevent corrosion due to its excellent water repellent nature. Herein, we describe the successful preparation of superhydrophobic coating on steel surface which is synthesized by using chemical etching technique with mixture of hydrochloric and nitric acid followed by treatment with lauric acid. By optimizing the etching process parameters, superhydrophobicity with water static contact angle of 170° and tilt angle of 5° is successfully achieved. Coated steel is found to be thermal, chemical, and mechanical stable. Besides, coating has also good regenerability. Furthermore, coating exhibits the excellent self-cleaning and anti-fogging properties. The electrochemical study is revealed the enhancement of corrosion resistance of steel after coating. This facile strategy inspires the large-scale fabrication of multifunctional steel surfaces for practical applications in self-cleaning, anti-fogging, and corrosion protection.

Development of a microbial coating for cellulosic surface using aloe vera and silane

The highly contagious nature of SARS-CoV-2 (COVID-19) virus has created a havoc effect all over the world in a very short period. The most effective method for precaution of this virus as suggested by WHO is use of PPEs (triple layered face mask, body suits) and social distancing. However, the frontline doctors and medical staff have high risk of exposure to the virus during treatments and removal and discarding of the PPEs. Also the PPEs are of the onetime use and wearing these PPEs in hot and humid seasons is very uncomfortable. A possible solution of this problem is if clothes are anti-bacterial and anti-viral in nature, one single-layered will be sufficient and the spread of virus will also be minimized. Considering this, we have designed a facile and durable anti-wash antimicrobial coating on cloth by aloe vera and hexadecyltrimethoxysilane (HDTMS). In lab scale study, present coating shows good chemical and thermal stability making it reusable multiple times even after repeated washing. The coated cloth reveals the excellent anti-dirt and stain resistance properties leading to complete non-adherence of dirt and stain (e.g. Color, food, ink) on it. Anti-bacterial and anti-fungi properties of the coated cloth were confirmed by doing E. coli (bacteria) and A. Niger (fungus) culture studies, respectively. This coating is imbibed with well-known anti-viral agent aloe vera which inhibits the attachment of the virus on the surface. The water-repellent nature of the coating combined with the use anti-viral agent, aloe vera makes it a potential anti-COVID-19 coating.

Promote anti- /de- frosting by suppressing directional ice bridging

Condensation frost is one of the most common types of icing encountered in nature and industrial applications, while its accretion induces multiple non-negligible negative impacts. Many passive anti-/de- frosting approaches by tailoring surface topology and chemistry have been reported recently. To date, however, reliable engineered surfaced that can be compatible with heat exchangers from both mechanical and economic perspectives are limited. In this paper, we present a hierarchically structured surface that can profoundly reduce the frost coverage, and meanwhile promote the rapid removal of frost melts. This surface consists of microgroove pattern and nanoblade structure, where the former provides preferential sites for the growth of condensed droplets, and the latter promises a robust water repellent nature. During frosting, the ice phase spreads only among droplets on groove edges while condensed droplets in valleys are completely evaporated off, yielding a typical pattern that ice stripes distribute discretely in the dry background. During defrosting, frost melts coalesce into large droplets that spontaneously depart the substrate surface. Our results open up a new avenue for surface engineering, particularly on the design of economic and scalable anti-/de- frosting surfaces.

Antiwetting and low-surface-energy behavior of cardanol-based polybenzoxazine-coated cotton fabrics for oil–water separation

In the present work, the surface behavior of cotton fabrics coated with a series of bio-based polybenzoxazines is explored. Herein, we report the design and synthesis of bio-based benzoxazine monomers (C-x) using cardanol (C) and seven different amines (x = ba, ha, dda, oda, ddm, jef, and fa). The molecular structures of the benzoxazine monomers have been confirmed using FTIR and NMR analyses. The microstructure of the polybenzoxazine-coated cotton fabrics observed from FE-SEM reveals that formation of rough nanostructure is influenced by molecular structure of monomers. Further, surface analysis shows that poly(C-dda)-coated cotton fabric offers superior water contact angle (WCA = 155° ± 3) with low sliding angle (6°). Also, poly(C-dda)-coated cotton fabric delivers the lowest surface energy (14.1 mN/m) and high resistance against acidic and alkaline media. Subsequently, oil–water separation investigation shows that the poly(C-dda)-coated cotton fabric yields 99% of separation efficiency with flux value of 7200 L/m2h. Thus, the cardanol-based polybenzoxazine-coated cotton fabrics prepared in the present work can find application in the field of oil–water separation due to their superior water-repellent nature.

A New Silicon Dioxide-Coated MALDI-ToF Sample Plate for Peptide Analysis.

In this report, we describe the development and testing of a new coated plate which improves the sensitivity and accuracy in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS). The coated plate was covered with a thin layer of hydrophobic silicon dioxide, which enabled sample enrichment due to the water repellent nature of the silicon dioxide surface. Sensitivity and required laser strengths were tested using peptide standards, with the results that these coated plates required lower laser power and showed increased sensitivity than that of common plates. Accuracy was tested using bacteria, saliva, and serum samples. The coated plates showed significantly increased degrees of accuracy through their capacity to reduce mass shift. The importance and necessity of accuracy analysis in the assessment of new sample plates, which is rarely described in other papers, is also discussed.

Hygric behavior of hydrophobized brick and mortar samples

Moisture that penetrates into porous building materials is a major reason for their deterioration and the subsequent failure of the whole structure. Water repellency treatment could prevent serious damages to materials and components. Hydrophobization is a method with a long tradition for protecting buildings from wind-driven-rain-induced moisture absorption. Nonetheless, it remains an ambiguous practice. To understand the nature of water repellency it is important to examine how hydrophobization works, how the existing products differ from one another, and how the hygric response of the substrate changes after treatment. The aim of this article is to characterize the impact of water repellent agents on the properties of building materials mostly used in old facades: clay brick and lime mortar.The resulting open porosity and pore size distribution, determined with vacuum saturation and mercury intrusion testing respectively, reveal only minimal change in the overall pore structure after impregnation. Our findings also show that hydrophobic treatment is nearly impermeable to liquid water, by evaluating the samples with capillary absorption tests, but still permeable to water vapour, by testing the samples with cup tests. Moreover, the water impermeability grows after exposure to water. In addition, the water repellent agent appears to spread progressively in the material for a long time after the hydrophobic treatment, yielding high final impregnation depths. These findings confirm that water repellent agents successfully hydrophobize the tested materials, with a water-tight but vapour-open hydrophobic layer that goes deep into the material, without notably changing its pore size distribution though.

Durability of superhydrophobic laser-treated metal surfaces under icing conditions

Ice accretion on external surfaces of aircraft due to the impingement of supercooled liquid water droplets can be tackled by the implementation of icephobic surfaces. Among these, superhydrophobic surfaces represent a promising solution, due to their water repellent nature. In the last decade, short/ultra-short pulsed laser technologies have been proposed as a one-step process to manufacture superhydrophobic surfaces. However, the effectiveness and durability of such surfaces in operational icing conditions has not yet been validated. In this work, we investigate ice adhesion strength and the chemical stability of metal alloys textured with a UV nanosecond laser via Direct Laser Writing.

One-step ultrasound fabrication of corrosion resistant, self-cleaning and anti-icing coatings on aluminium

This study aimed to fabricate a highly hydrophobic aluminium surface in a one-step ultrasound process in a sodium hydroxide solution containing various alkoxysilanes. The latter were based on the trimethoxysilane of various type and length of the introducing group at the silyl bond (SiC): Ra-Si(OMe)3 and Rf-(CH2)2-Si(OMe)3, where Ra and Rf are alkyl and perfluoroalkyl introducing groups, respectively, and OMe is the methoxy group. The effect of methoxy and ethoxy groups at the siloxy bond was additionally investigated for perfluorodecyl silanes.The corrosion performance was studied using potentiodynamic electrochemical techniques. The wettability, morphology, surface topography and surface composition of treated aluminium surfaces were characterised using an optical tensiometer, a scanning electron microscope coupled with an energy dispersive X-ray spectrometry system, a contact profilometer and a Fourier transform infrared spectroscopy system.Micro-/nanostructures formed on the treated aluminium surfaces lead to an increase in the water contact angle from 71° for ground surface, up to 115° for the surface treated with alkyl silanes and up to 155° for that treated with fluoroalkyl silanes with a long chain. The superhydrophobic coatings show corrosion-resistant behaviour in chloride solution, while the durability in slightly acidic/alkaline solutions indicates the excellent water-repellent nature of the coatings, the self-cleaning ability for different types of pollutants and improved anti-icing properties with freezing and melting delay.

Super Water-Repellent Cellulose Acetate Mats

A single-step synthesis of super-water-repellent oil sorbents based on cellulose acetate (CA) mats is reported in this paper. Key phenomenological mechanisms involving roughness and changes in chemistry are used to describe the change in hydrophobic behavior of the CA mats. Contact angle calculations followed by Cassie’s model apparent contact angle prediction have shown roughness alone is not capable of producing the super-hydrophobicity exhibited by as-spun mats. Fourier transform infrared spectroscopy of spin coated and electrospun mats shows a significant difference in the stretching of the hydroxyl bonds of the two materials. As it is this hydroxyl group which adds to the overall polarity of surface thus hydrophilicity of the material, we propose that the electrospinning process not only creates a rougher surface but also alters the chemistry of the electrospun cellulose acetate mats which ultimately gives rise to the reported hydrophobicity. Finally, due to their water repellent nature, and oleophilicity of the as-spun mats were tested as oil sorbent mats. The as-spun mats were capable of absorbing thirty times their weight in oil demonstrating their application for oil-water remediation.

Repellency-induced runoff from New Zealand hill country under pasture: A plot study

Soil water repellency is a common phenomenon which develops when surface soils become dry in summer and autumn. It is claimed that repellency is likely to result in a lower infiltration rate and a concomitant increase in surface runoff, particularly on slopes. This study quantifies the effect of water repellency on runoff from a series of small plots on a range of slopes (20° and 30°) and aspects (N, S and E) in a hilly landscape in the south-east of the North Island of New Zealand. The plots (1 m wide and 2 m long) were set up to capture runoff via a slotted PVC pipe and measure it using tipping bucket apparatus: at each of the slope/aspect locations there were duplicate plots. A meteorological station was also established at the site along with TDR probes to measure soil moisture down to 300 mm depth. When moist, the soil at the site had a very high infiltrability (>1.5 mm/min). On nine occasions, runoff was measured (ranging from 1 to 59% of rainfall) when the soil surface was dry and rainfall was intense (greater than 0.1 mm/min). However, during the two-year study period, this repellency-induced runoff equated to only 5% of the total rainfall. Furthermore, the infiltration rate of initially dry, repellent soil (ranging from 0.2 to 0.6 mm/min) partly recovered over a ten-minute period (0.6–1.0 mm/min) and, with sufficient rainfall, repellency completely disappeared within two days. The transitory nature of water repellency was confirmed in an experiment on large soil slabs conducted in the laboratory where repellency-induced runoff was observed to largely disappear over a period of 30 min. Overall, it is concluded that soil water repellency does not play a major role in the soil water balance of the hill country at the study site.

Ureteric stents: The past, present and future

Ureteric stents are fundamental to modern-day urological practice. This article aims to chronicle their development over the last century and the key individuals whose efforts have made their development possible. Early stents were ureteric catheters that were exteriorised outside the body and were associated with complications including migration, infection and encrustation. The use of polyethylene stents in humans was first reported by Tulloch in 1952. Polyethylene was thought to be a promising material due to its durability and water-repellent nature. It would, however, suffer the problems that would become associated with stents over the following decades mentioned above. The first silicone ureteric stents were developed in the 1960s by Zimskind and provided prolonged, efficient drainage but were complicated with stent migration. Collars, wings, flanges and barbs were developed to help prevent migration. Finney developed a double ‘pig-tail’ stent in the 1970s which helped to prevent both proximal and distal migration and the modern-day ureteric stent was born. Modern polymers have been developed such as polyurethane or styrene ethylene-butylene (C-flex®). Metal stents have also been used over the last three decades including the Wallstent™, Resonance® and Memokath™ stents. They have shown promising results particularly when long-term relief of ureteric obstruction is needed. Various strategies are available for removal including cystoscopically, stents with a metallic end that can be removed by using a catheter with a magnet at the proximal end, tethered stents that can be removed noninvasively either by the urologist, nurse or even by the patient and dissolvable stents which are in development.

Fabrication of durable porous and non-porous superhydrophobic LLDPE/SiO2 nanoparticles coatings with excellent self-cleaning property

In this current work, SiO2 nanoparticles embedded linear low density polyethylene (LLDPE) superhydrophobic coatings on glass substrates were fabricated using dip-coating technique. Porosity of SiO2 nanoparticles coupled LLDPE superhydrophobic coatings was also varied using non-solvent (ethanol) by employing phase separation method. Surface morphology, water contact angle, self-cleaning and water-repellency of coatings were characterized. Furthermore, the stability of these coatings was also evaluated by conducting thermal, chemical and mechanical stability tests at perturbation conditions. Along with, comparative studies of porous and non-porous LLDPE/SiO2 coatings on their performances were presented in the current work. By creating porosity and/or embedding SiO2 nanoparticles in to LLDPE matrix, superhydrophobicity is achieved with water contact angle of 170° and sliding angle of 3.8°. Coatings exhibit the excellent self-cleaning property. Their superhydrophobic property is also maintained after annealing in temperature range from 40 to 120°C. Water jet impact test reveals the excellent water repellent nature of coatings. Non-porous coatings exhibit more stability in pH range from 2 to 9 than porous coatings. By abrasion test with micro-fiber cloth and tape peeling tests, it is observed that porous coatings are more durable than non-porous coatings. The aforesaid coatings have great industrial applications.

Effect of biochar on crust formation, penetration resistance and hydraulic properties of two coarse-textured tropical soils

Biochar (BC) has been reported to improve a number of soil structural and hydraulic properties but detailed studies are scant on how BC affects crust formation, penetration resistance, water repellency and saturated hydraulic conductivity (Ksat). The objective of this study was to quantify the effect of maize cob BC of three different particle sizes on soil crusting (penetration resistance), water repellency, and Ksat of loamy fine sand and sandy loam in Zambia. The BC particle sizes were <0.5 and 1–5mm applied at 17.5 and 35tha−1 in the two soils and intermediate size of 0.5–1mm applied at lower rates (17.5 and 28tha−1 in the loamy fine sand and 13.3 and 26.7tha−1 in the sandy loam). Water repellency included both water drop penetration time (WDPT) and minimum molarity of the ethanol droplet at which rapid infiltration into the soil occurs. The BC was produced by slow pyrolysis of corn cobs at a temperature of 350°C. Biochar, added homogeneously to the upper 7cm of the soil, reduced the penetration resistance of surface soil of sandy loam with both the crust intact (−2.1±0.6Ncm−2 per percent BC added; p=0.001 in March 2015 and slightly smaller in October 2014) and the crust removed (−2.9±0.6Ncm−2 per percent BC added; p=0.0001). This effect occurred irrespective of particle size of BC (p>0.05). No effect of BC on penetration resistance was found in the loamy fine sand (p>0.05). In dry sandy loam with moisture content <1% v/v, the proportion of wettable crusted surface was significantly smaller (25%) than in moist soil (98%) with moisture content of ∼ 10% v/v. Only fine BC of <0.5mm increased WDPT of the crusted surface of sandy loam (p<0.05), reducing the proportion of wettable surface from 98 to 80% in moist soil and from 25 to 18% in dry soil. Coarser BCs, instead, increased the proportion of wettable crusted surface from 25% to 45% and 90% for 3% 0.5–1mm BC and 4% 1–5mm BC addition, respectively, in dry soil. Biochar significantly reduced Ksat (p<0.05) in sandy loam below the crust by 0.17±0.07cmh−1 per percent BC added. However, no effect was found in loamy fine sand. Since BC amended sandy loam below the crust showed no water repellency, reduction in Ksat cannot be explained by water-repellent nature of BC. Instead, this may be due to clogging of soil pores by BC or to collapse of soil structure near water saturation.

Ag/C:F Antibacterial and hydrophobic nanocomposite coatings

Silver-based nanomaterials that exhibit antibacterial character are intensively studied as they represent promising weapon against multi-drug resistant bacteria. Equally important class of materials represent coatings that have highly water repellent nature. Such materials may be used for fabrication of anti-fogging or self-cleaning surfaces. The aim of this study is to combine both of these valuable material characteristics. Antibacterial and highly hydrophobic Ag/C:F nanocomposite films were fabricated by means of gas aggregation source of Ag nanoparticles and sputter deposition of C:F matrix. The nanocomposite coatings had three-layer structure C:F base layer/Ag nanoparticles/C:F top layer. It is shown that the increasing number of Ag nanoparticles in produced coatings leads not only in enhancement of their antibacterial activity, but also causes substantial increase of their hydrophobicity. Under optimized conditions, the coatings are super-hydrophobic with water contact angle equal to 165[Formula: see text] and are capable to induce 6-log reduction of bacteria presented in solution within 4[Formula: see text]h.

Fabrication of Mechanically Stable Superhydrophobic Aluminium Surface with Excellent Self-Cleaning and Anti-Fogging Properties.

The development of a self-cleaning and anti-fogging superhydrophobic coating for aluminium surfaces that is durable in aggressive conditions has raised tremendous interest in materials science. In this work, a superhydrophobic Al surface was synthesized by employing chemical etching technique with a mixture of hydrochloric and nitric acids, followed by passivation with lauric acid. The surface morphology analysis revealed the presence of rough microstructures on the coated Al surface. Superhydrophobicity with water contact angle of 170 ± 3.9° and sliding angle of 4 ± 0.5° was achieved. The surface bounced off the high-speed water jet, indicating the excellent water-repellent nature of the coating. It also continuously floated on a water surface for four weeks, showing its excellent buoyancy. Additionally, the coating maintained its superhydrophobicity after undergoing 100 cycles of adhesive tape peeling test. Its superhydrophobic nature withstood 90° and 180° bending and repeated folding and de-folding. The coating exhibits an excellent self-cleaning property. In a low temperature condensation test, almost no accumulation of water drops on the surface showed the excellent anti-fogging property of the coating. This approach can be applied to any size and shape of Al surface, and hence has great industrial applications.

Superhydrophobic coatings for aluminium surfaces synthesized by chemical etching process

ABSTRACTIn this paper, the superhydrophobic coatings on aluminium surfaces were prepared by two-step (chemical etching followed by coating) and one-step (chemical etching and coating in a single step) processes using potassium hydroxide and lauric acid. Besides, surface immersion time in solutions was varied in both processes. Wettability and surface morphologies of treated aluminium surfaces were characterized using contact angle measurement technique and scanning electron microscopy, respectively. Microstructures are formed on the treated aluminium surfaces which lead to increase in contact angle of the surface (>150°). Also on increasing immersion time, contact angle further increases due to increase in size and depth of microstructures. Additionally, these superhydrophobic coatings show excellent self-cleaning and corrosion-resistant behavior. Water jet impact, floatation on water surface, and low temperature condensation tests assert the excellent water-repellent nature of coatings. Further, coatings are to be found mechanically, thermally, and ultraviolet stable. Along with, these coatings are found to be excellent regeneration ability as verified experimentally. Although aforesaid both processes generate durable and regenerable superhydrophobic aluminium surfaces with excellent self-cleaning, corrosion-resistant, and water-repellent characteristics, but one-step process is proved more efficient and less time consuming than two-step process and promises to produce superhydrophobic coatings for industrial applications.

Is it Possible to Change Wettability of Hydrophilic Surface by Changing Its Roughness?

Wetting behavior of model rough surfaces made of hydrophilic square pillars is investigated. The hydrophilic pillars are equally spaced on hydrophilic surface. The surface roughnesses are altered by varying the pillar width and interpillar spacing. Wetting to dewetting transition is observed for these surfaces. This is one of the first accounts of observation from molecular simulations where hydrophilic surface converts into hydrophobic by changing its roughness. The extent of hydrophilicities are also changed to gain more insightful observations. Energies of the wetting to dewetting transitions are analyzed by calculating the contribution from water–water and water–surface energy components. A correlation between energy and the wetting to dewetting transition has been established, which rationally explains the observed water repellent nature of hydrophilic surface as a function of roughness.