Application of Surface Wettability to Control Spreading of an Impacting Droplet.

While the simplest outcome of a normal impact on a flat stationary solid surface is radially symmetric spreading, it is important to note that asymmetric spreading can intrinsically occur with a tangential velocity along the surface. However, no previous attempt has been made to restore the symmetry of a lamella that intrinsically spreads asymmetrically. Adjusting the lamella's asymmetric shape to a symmetric one is achieved in this work by varying wettability to affect the receding velocity of the contact line, according to the Taylor-Culick theory. Here we theoretically and practically show how restoring the symmetry can be achieved. Theoretically we built a framework to map the needed receding velocity at every given point of the contact line to allow for symmetry to be restored, and then this framework was applied to generate a wetting map that shows how at each local the wettability of the surface needs to be defined. Simulated results confirmed the effectiveness of our framework and identified the envelope of its applicability. Next, to apply the idea experimentally, the wetting map was transformed to a single wettability contrast area dubbed the "patch". Experimental results showed the effectiveness of the patch design in correcting the asymmetric spreading lamella for water droplets impacting a surface for the following Weber number conditions: Wen ≤ 300, Wet ≤ 300, and 0.51 ≤ Wen/Wet ≤ 2.04.

Innovative Solid Slippery Coating: Uniting Mechanical Durability, Optical Transparency, Anti-Icing, and Anti-Graffiti Traits.

Slippery coatings, such as the slippery liquid-infused porous surface (SLIPS), have gained significant attention for their potential applications in anti-icing and anti-fouling. However, they lack durability when subjected to mechanical impact. In this study, we have developed a robust slippery coating by blending polyurethane acrylate (PUA) with methyltriethoxysilane (MTES) and perfluoropolyether (PFPE) in the solvent of butyl acetate. The resulting mixture is homogeneous and allows for uniform coating on various substrates using a drop coating process followed by drying at 160 °C for 3 h. The cured coating exhibits excellent water repellency (contact angle of ~108° and sliding angle of ~8°), high transparency (average visible transmittance of ~90%), exceptional adherence to the substrate (5B rating according to ASTMD 3359), and remarkable hardness (4H on the pencil hardness scale). Moreover, the coating is quite flexible and can be folded without affecting its wettability. The robustness of the coating is evident in its ability to maintain a sliding angle below 25° even when subjected to abrasion, water jetting, high temperature, and UV irradiation. Due to its excellent nonwetting properties, the coating can be employed in anti-icing, anti-graffiti, and anti-sticking applications. It effectively reduces ice adhesion on aluminum substrates from approximately 217 kPa to 12 kPa. Even after 20 cycles of icing and de-icing, there is only a slight increase in ice adhesion, stabilizing at 40 kPa. The coating can resist graffiti for up to 400 cycles of writing with an oily marker pen and erasing with a tissue. Additionally, the coating allows for easy removal of 3M tape thereon without leaving any residue.

A harsh environment resistant robust Co(OH)2@stearic acid nanocellulose-based membrane for oil-water separation and wastewater purification.

Traditional membranes are inefficient in treating highly toxic organic pollutants and oily wastewater in harsh environments, which is difficult to meet the growing demand for green development. Herein, the Co(OH)2@stearic acid nanocellulose-based membrane was prepared by depositing Co(OH)2 on the nanocellulose-based membrane (NBM) through chemical soaking method, which enables efficient oil/water mixtures separation and degradation of pollutants by photocatalysis in harsh environments. The Co(OH)2@stearic acid nanocellulose-based membrane (Co(OH)2@stearic acid NBM) shows good photocatalytic degradation performance for methylene blue pollutants in harsh environment, and has significant degradation rate (93.66%). At the same time, the Co(OH)2@stearic acid NBM with superhydrophobicity and superoleophilicity also exhibits respectable oil/water mixtures separation performance (n-Hexane, dimethyl carbonate, chloroform and toluene) under harsh environment (strong acid/strong alkali), which has an excellent oil-water mixtures separation flux of 87 L·m-2·h-1 (n-Hexane/water) and oil-water mixture separation efficiency of over 93% (n-Hexane/water). In addition, this robust Co(OH)2@stearic acid NBM shows good self-cleaning and recycling performance. Even though seven oil-water separation tests have been carried out under harsh environment, it can still maintain respectable oil-water mixture separation rate and flux. The multifunctional membrane has excellent resistance to harsh environments, oil-water separation and pollutant degradation can be performed even in harsh environments, which provides a convenient way to treat sewage under harsh conditions efficiently and has great potential in practical application.

Experimental Investigation of Dropwise Condensation Shedding by Shearing Airflow in Microgravity Using Different Surface Coatings.

The shedding kinematics of water droplets in a condensation environment when exposed to aerodynamic forces in microgravity was studied. Understanding the shedding of droplets from a surface is a critical part of the dropwise condensation process for improving heat transfer. Because gravity as a droplet removal technique is not available in space, the use of airflow to shed droplets is considered for condensing heat exchangers in environmental control and life support systems. Surface coatings affect drop adhesion, and here, four different surfaces (PMMA, PS, PTFE, and SHS) and various droplet sizes (80, 60, and 40 µL) were used to understand the above phenomenon. It was found that the critical velocity to shed a droplet in microgravity was up to 8% lower than that in normal gravity. Also, the effect of the droplet size was investigated for both microgravity and normal gravity; the shedding velocity was lower for microgravity, and it decreased as droplet size increased. Increasing the hydrophobicity of the coating decreased the critical velocity for shedding. Finally, the droplet was found to detach from superhydrophobic surfaces in microgravity. The detachment of droplets from the substrate will hamper the condensation process that can produce a larger fresh area; also, detachment of droplets and entrainment in airflow counter the concept of removing moisture from the air in a dehumidification process.

Folding characteristics of membranes in capillary origami.

A study was conducted to understand the effects of membrane shape, thickness, contact angle, surface tension and large deflection on capillary origami. For experiments, square and triangular membranes made of PDMS with various thicknesses and sizes were used to encapsulate different liquids. Models for membranes under pure bending were developed using the energy balance between interfacial energies (liquid-vapor, solid-liquid and solid-vapor energies) and bending energy evaluated by a small-deflection and a large-deflection assumptions. This paper is the first study to consider the large deflection for membranes as well as to include the terms for the wettability of the membrane and its shape. The developed models evaluated an important characteristic length, i.e., elasto-capillary length (LEC), which is proportional to the critical length (Lcritical) below which membranes cannot be closed to encapsulate liquid. The experimental results showed that the large-deflection model can estimate Lcritical more accurately in terms of membrane shape, thickness, contact angle and surface tension for liquids with similar properties to water than the small-deflection model. The developed models should be further improved to extend the applicability to liquids with low surface tension and low contact angle.

Perturbative vibration of the coupled hydrogen-bond (O:H-O) in water.

Perturbation Raman spectroscopy has underscored the hydrogen bond (O:H-O or HB) cooperativity and polarizability (HBCP) for water, which offers a proper parameter space for the performance of the HB and electrons in the energy-space-time domains. The OO repulsive coupling drives the O:H-O segmental length and energy to relax cooperatively upon perturbation. Mechanical compression shortens and stiffens the O:H nonbond while lengthens and softens the HO bond associated with polarization. However, electrification by an electric field or charge injection, or molecular undercoordination at a surface, relaxes the O:H-O in a contrasting way to the compression with derivation of the supersolid phase that is viscoelastic, less dense, thermally diffusive, and mechanically and thermally more stable. The HO bond exhibits negative thermal expansivity in the liquid and the ice-I phase while its length responds in proportional to temperature in the quasisolid phase. The O:H-O relaxation modifies the mass densities, phase boundaries, critical temperatures and the polarization endows the slipperiness of ice and superfluidity of water at the nanometer scale. Protons injection by acid solvation creates the H↔H anti-HB and introduction of electron lone pairs derives the O:⇔:O super-HB into the solutions of base or H2O2 hydrogen-peroxide. The repulsive H↔H and O:⇔:O interactions lengthen the solvent HO bond while the solute HO bond contracts because its bond order loss. Differential phonon spectroscopy quantifies the abundance, structure order, and stiffness of the bonds transiting from the mode of pristine water to the perturbed states. The HBCP and the perturbative spectroscopy have enabled the dynamic potentials for the relaxing O:H-O bond. Findings not only amplified the power of the Raman spectroscopy but also substantiated the understanding of anomalies of water subjecting to perturbation.

Fabrication of polydimethylsiloxane-attached solid slippery surface with high underwater transparency towards the antifouling of optical window for marine instruments.

Marine optical instruments are commonly suffering serious biofouling problem caused by the adhesion of marine microorganisms, which severely affects the instruments to monitor the marine environment. Herein, we developed a robust solid slippery surface (SSS) by fabricated a covalently attached polydimethylsiloxane (PDMS) layer on glass substrate to solve the biofouling problem of marine optical instrument windows. The SSS could effectively inhibit the settlements of marine microorganism (bacteria and alga) in various environmental conditions, resulting from the high flexibility of PDMS molecular chains, and thus could maintain its high underwater-transparency. The antifouling mechanism of SSS was results from the weak nonspecific electrostatic Lifshitz-van der Waals forces and less specific hydrogen bonds between SSS and microorganism, which was been confirmed via both single bacterial force spectroscopy measurement and molecular dynamics simulation. Compared with the traditional slippery lubricant-infused porous surface (SLIPS), the SSS exhibited a better robust mechanical stability than that of the SLIPS. In addition, our study provides a valuable method to fabricated the SSS with reliable underwater-transmittance and antifouling properties, which is promised for the applications for the antifouling of marine optical instruments.

Improving the curing and flame retardancy of epoxy resin composites by multifunctional Si-containing cyclophosphazene derivatives.

Novel star-like molecules containing P, N and Si with dual functions of flame retardance and curing promotion (abridged as HCCP-KH540) were successfully synthesized through the nucleophilic substitution reaction of hexachlorocyclotriphosphazene (HCCP) and 3-aminopropyltrimethoxysilane (KH540). HCCP-KH540 was incorporated with the matrix of epoxy resin (EP) to form a flame retardant composite abridged as E-HK. The activation energy of the curing reaction of the E-HK composite was reduced but the curing reaction rate was accelerated by HCCP-KH540. The E-HK composite with 30 phr content of HCCP-KH540 exhibited excellent flame retardancy with limiting oxygen index of 29.6% and V-1 rating in the vertical burning test as well as excellent thermal stability with a char yield of 23.77% at 700 °C, compared with only 8.64% for pure EP.

Novel SLIPS based on the photo-thermal MOFs with enhanced anti-icing/de-icing properties.

A photo-thermal anti-icing/de-icing SLIPS coating is designed based on porous light-responsive MOFs. Due to the strong light absorption and high light-thermal conversion, the as-synthetic SCMOFs exhibited prolonged freezing delay time and depressed water crystallization point under light irradiation. Meantime, the SCMOFs exhibit good deicing properties. With the irradiation, the half-melted ice slips off quickly.

A multifunctional and environmentally safe superhydrophobic membrane with superior oil/water separation, photocatalytic degradation and anti-biofouling performance.

Wastewater is typically complicated with spilled oil, water soluble toxic dyes and microorganisms, making it hard to be processed and causing a significant threat to the environmental safety and human health. In this paper, we demonstrate a simple solution immersion method to obtain a multifunctional cellulose-based membrane (CBM) that possesses both superhydrophobicity with a water contact angle of 163° and superior functionalities including self-cleaning, oil-water separation, anti-biofouling, and photocatalytic degradation capabilities. The achievement of separation efficiency (96%), comparatively high flux (141 L·m-2·h-1) and recyclable (7 times) oil/water separation performance is attributed to the robust superhydrophobicity enabled by the synergy of metal oxide (i.e., CuO) nanostructure coating and stearic acid (SA) modification. The superhydrophobic CBM also preferentially adsorbs organic dyes in aqueous solution, e.g., methylene blue (MB), promoting their efficient decomposition (about 70.3% of MB decomposed in 3 h) with high recyclability under UV irradiation. Most remarkably, the CBM exhibits superior anti-biofouling capability and persistently resists the algae adhesion in long duration (over 20 days), as a result of the self-cleaning ability as well as the antimicrobial property of CuO nanoparticles. Our finding here paves the way to use simple, cost-effective, environmentally safe, and reliable method to fabricate multifunctional materials for wastewater treatment in complex environments.

Porous Ultrahigh Molecular Weight Polyethylene/Functionalized Activated Nanocarbon Composites with Improved Biocompatibility.

Ultrahigh molecular weight polyethylene (UHMWPE) materials have been prevalent joint replacement materials for more than 45 years because of their excellent biocompatibility and wear resistance. In this study, functionalized activated nanocarbon (FANC) was prepared by grafting maleic anhydride polyethylene onto acid-treated activated nanocarbon. A novel porous UHMWPE composite was prepared by incorporating the appropriate amount of FANC and pore-forming agents during the hot-pressing process for medical UHMWPE powder. The experimental results showed that the best prepared porous UHMWPE/FANC exhibited appropriate tensile strength, porosity, and excellent hydrophilicity, with a contact angle of 65.9°. In vitro experiments showed that the porous UHMWPE/FANC had excellent biocompatibility, which is due to its porous structure and hydrophilicity caused by FANC. This study demonstrates the potential viability for our porous UHMWPE/FANC to be used as cartilage replacement material for biomedical applications.

Stable, superfast and self-healing fluid coating with active corrosion resistance.

Fluid material can recover from damage rapidly with no demand of external triggering in contrast with the traditional self-healing material which presents low healing efficiency and demands external triggering, such as heat, light, moisture, electricity, etc. However, due to its low viscosity, fluid material is easy to flow away from the surface and thus it is difficult to form a stable coating on the surface to provide practical corrosion resistance to the substrate. Herein, a stable and superfast self-healing coating on steel substrate has been obtained by incorporating carbon nanotube (CNT) into the fluid matrix of epoxy resin (EP) or silicone oil (OIL). To further achieve the active corrosion resistance, 1H, 1H, 2H, 2H- perfluorooctyltriethoxysilane (PTES) which can react with the water inside the coating is added. The coating possesses superfast (tens of seconds) self-healing properties against millimeter-scale scratch repeatedly and excellent corrosion resistance in the aqueous solution of HCl (1 M) and NaOH (1 M). In-situ self-healing and electrochemical behavior in scanning vibrating electrode technique (SVET) measurement indicate the fluid coating possesses infinite self-healing capacity theoretically. Due to its excellent durability and infinite self-healing capacity with short responding time, the optimized fluid coating can be a smart corrosion barrier coating for metals.

Durable Superhydrophobic Wood via One-Step Immersion in Composite Silane Solution.

A superhydrophobic coating endows pristine hydrophilic wood with excellent water/moisture repellency and thus prolongs its service life. Generally, the superhydrophobic coating on wood is fabricated by a two-step process in which the nanoparticles are first introduced onto the surface and then modified by low-surface-energy molecules. Herein, for the first time, we have fabricated the superhydrophobic wood via a one-step process free of nanoparticles by immersing the pristine hydrophilic wood, such as pine, balsawood, and basswood, into a composite silane solution of hexadecyltrimethoxysilane and methyltrimethoxysilane. The wood remains superhydrophobic or highly hydrophobic after long-term exposure to mechanical damage (such as abrading, knife-cutting, and tape-peeling), chemical damage (such as immersion in acid, alkali, or ethanol), and environmental impacting (such as UV irradiation and low/high-temperature exposure).

Biochar Nanocomposite Derived from Watermelon Peels for Electrocatalytic Hydrogen Production.

Water splitting is the most potential method to produce hydrogen energy, however, the conventional electrocatalysts encounter the hindrances of high overpotential and low hydrogen production efficiency. Herein, we report a carbon-based nanocomposite (denoted as CCW-x, x stands for the calcination temperature) derived from watermelon peels and CoCl2, and the as-synthesized CCW-x is used as the electrocatalyst. The overpotential and the Tafel slope of CCW-700 for oxygen evolution reaction (OER) is 237 mV at 10 mA cm-2 and 69.8 mV dec-1, respectively, both of which are lower than those of commercial RuO2. For hydrogen evolution reaction (HER), the overpotential of CCW-700 (111 mV) is higher than that of the widely studied Pt/C (73 mV) but still lower than those of lots of carbon-based nanomaterials (122-177 mV). In the light of CCW-700 is highly active for both OER and HER, we assembled a water-splitting electrocatalyst by employing nickel foam loaded with CCW-700 as the anode and cathode in 1 M KOH. The water-splitting voltage is only 1.54 V for the CCW-700//CCW-700 electrodes and 1.62 V for the RuO2//Pt/C ones. Therefore, the so-denoted CCW-x powder possesses good electrocatalytic hydrogen production efficiency.

Water and mildew proof SiO2 & ZnO/silica sol superhydrophobic composite coating on a circuit board.

In order to improve the waterproof and mildew resistance of electronic equipment, a superhydrophobic coating was prepared on a circuit board. First, hexadecyl trimethoxysilane was used to modify the nano silica and nano zinc oxide particles, and then the modified nanoparticles were mixed with the silica sol. Then the superhydrophobic coating was prepared on the surface of the printed circuit board by a spraying process. The preparation technology and physical and chemical properties of the coating were studied. The contact angle of the final sample can reach 169.47°, the sliding angle can reach 1.2°, it has good acid and alkali corrosion resistance, resistance to NaCl, self-cleaning performance and antimildew performance.

Spray-On Nanocomposite Coatings: Wettability and Conductivity.

Nanocomposite coatings, i.e., a combination of nanocompounds, and a polymer matrix together with suitable additives and solvents is a very versatile method for producing multifunctional coatings. Some of the most desired coating properties have a high repellency to liquids (e.g., superhydrophobic and/or superoleophobic) and electrical and thermal conductivities. From a practical perspective, coatings that can be sprayed are very suitable for large-scale production, conformity, and reduced time and cost. Carbon-based, metallic, and ceramic are the three groups of nanocompounds commonly used to formulate spray-on nanocomposite coatings. In this invited feature article, we discuss the applications, advantages, and challenges of using such nanocompounds to produce coatings with good water repellency or/and elevated electrical or/and thermal conductivities. We also discuss the role of additives and solvents briefly in relation to the properties of the coatings. Important spraying parameters, such as stand-off distance and its influence on the final coating properties, will also be examined. Our overall aim is to provide a guideline for the production of practical multifunctional nanocomposites utilizing carbon-based, metallic, or ceramic nanoparticles or nanofibers that covers both aspects of in-air wettability and conductivity under one umbrella.

Functionalized cellulose nanocrystals as the performance regulators of poly(ß-hydroxybutyrate-co-valerate) biocomposites.

To investigate the relationship between functional groups on cellulose nanocrystals (CNC) and the performance of poly(ß-hydroxybutyrate-co-valerate) (PHBV), the surface of CNC was modified by surface graft modification and PHBV/CNC biocomposites were prepared by melt blending. To demonstrate the interfacial adhesion difference between hydrophobic PHBV and hydrophilic CNC, palmitoyl chloride and ε-caprolactone had been used to tailor the oleophilic property of CNC. Results showed that CNC had heterogeneous nucleation effect on the crystallization process of PHBV, while the entanglement of molecular chains weakened the promoting functions of CNC-g-C16 (CNC grafted with palmitoyl chloride) and CNC-g-CL (CNC grafted with ε-caprolactone). Furthermore, CNC-g-CL exhibited better interfacial adhesion with PHBV when compared with CNC-g-C16. And 1 wt% CNC-g-CL improved the tensile strength of PHBV biocomposite to 38.09 MPa, which is 26.25% higher than PHBV.

Mechanically Robust and Repairable Superhydrophobic Zinc Coating via a Fast and Facile Method for Corrosion Resisting.

Zinc coatings and superhydrophobic surfaces have their own characteristics in terms of metal corrosion resistance. Herein, we have prepared a robust and repairable superhydrophobic zinc coating (SZC) based on a widely commercially available cold galvanized paint via a fast (within 10 min) and facile process for corrosion resistance. Specifically, the cold galvanized paint was sprayed onto the iron substrate, followed by acetic acid (HAc) etching and stearic acid (STA) hydrophobizing. The as-obtained sample was coded as Fe-Zn-HAc-STA and possessed an apparent contact angle of 168.4 ± 1.5° as well as a sliding angle of 3.5 ± 1.2°. The Fe-Zn-HAc-STA sample was mechanically durable and easily repairable. After being ultrasonicated in ethanol for 100 min, the superhydrophobicity was still retained. The Fe-Zn-HAc-STA sample lost its superhydrophobicity after being abraded against sandpaper with a load of 100 g and regained its superhydrophobicity after HAc etching and subsequent STA hydrophobizing. The corrosion resistance of the SZC was investigated by immersing the Fe-Zn-HAc-STA sample into the static or dynamic aqueous solution of NaCl (3.5 wt.%) and the lasting life of the entrapped underwater air layer (EUAL) was roughly determined by the turning point at the variation curve of surface wettability against immersion time. The lasting life of the EUAL iwas 8 to 10 days for the SZC in the static NaCl solution and it decreased sharply to 12 h in a dynamic one with the flow rate of 2 and 4 m/s. This suggests that the superhydrophobic surface provided extra corrosion protection of 8 to 10 days or 12 h to the zinc coating. We hope that the SZC may find its practical application due to the facile and fast fabrication procedure, the good mechanical durability, the easy repairability, and the good corrosion protection.

Graphene-Wrapped Ni(OH)2 Hollow Spheres as Novel Electrode Material for Supercapacitors.

Graphene-wrapped Ni(OH)2 hollow spheres were prepared via electrostatic interaction between poly(diallyldimethylammonium chloride) (PDDA) modified Ni(OH)2 and graphene oxide (GO) in an aqueous dispersion, followed by the reduction of GO. Morphological and structural analysis by field-emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis confirmed the successful coating of graphene on Ni(OH)2 hollow spheres with a content of 3.8 wt%. And then its application as electrode material for supercapacitor has been investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge tests. Results show that the sample displays a high capacitance of 1368 F g(-1) at a current density of 1 A g(-1), much better than that of pure Ni(OH)2, illustrating that such composite is a promising candidate as electrode material for supercapacitors.