In nature, various plants, including the lotus leaf, exhibit the unusual phenomenon of superhydrophobicity. The surfaces of these leaves usually have binary structures on the micrometer and nanometer scales, resulting in low water sliding angles (WSAs) and high water contact angles (WCAs) up to 162°± 5°. This is because air can be trapped between the droplets and the wax crystals at the plant surface, which minimizes the real contact area.[1] Water repellency is important in daily life as well as in many industrial and biological processes, such as the reduction of frictional drag on ship hulls, antiicing and self-cleaning.[2]Artificial superhydrophobic surfaces with WCAs larger than 150°and very low WSAs have been prepared by various processing methods through controlling the surface topography of expensive hydrophobic materials, using techniques such as etching and machining. [3][4][5][6] However, these methods for creating superhydrophobic surfaces typically use expensive materials or severe conditions, limiting the application of superhydrophobic surfaces. Bionic polymer surfaces with superhydrophobicity have been prepared by a one-step casting process under ambient atmosphere. [7,8] Erbil et al.[7a] employed a very simple method involving solvent evaporation to fabricate a superhydrophobic surface, which provides a new, promising method to fabricate artificial superhydrophobic surfaces on polymer surfaces. However, it is well known that the various superhydrophobic surfaces were only fabricated on glass substrates at room (or low) temperature, and the cohesion between the polymer coatings and glass substrates was weak, allowing the films to be easily scraped off, and no longterm stability over a wide pH range was achieved in comparison with the high-temperature process. These superhydrophobic surfaces cannot withstand low-to-high temperature changes. Conventional fluorine-polymer hydrophobic coatings have been used as antisticking and antifouling surfaces and to reduce drag and flow noise for long time because of their low surface free energy. [9,10] However, the ordinary fluorine-polymer hydrophobic surfaces have WCAs of merely 110°-125°, i.e., they are not superhydrophobic.[10e]In this Communication, we demonstrate that bionic poly-(tetrafluoroethylene)/poly(phenylene sulfide) (PTFE/PPS) superhydrophobic coatings with long-term stability, high cohesional strength, and resistance to temperature change can be prepared by a simple, inexpensive, and conventional curing process. A superhydrophobic surface with a porous network, micrometer-nanometer-scale binary structure (MNBS) roughness, and the lowest surface energy hydrophobic groups (-CF 3 ) was fabricated using commercially available PTFE and PPS on stainless steel and engineering materials. The fabrication of a superhydrophobic coating by our conventional curing process is reported in this Communication for the first time to our knowledge. It is expected that this technique will make it possible to prepare superhydrophobic engineering materials with new ...

The present work reports a replication approach to imprint complex micro/nanostructures into polymeric coatings and materials, such as silicone elastomers, polyurethane, ultra-high-molecularweight polyethylene and polytetrafluoroethylene etc., with applications in engineering. Al and Al 2 O 3 molds with terraced micro/nanostructures were produced using an industrially compatible anodization method. To assist replication, the molds were coated with a water-soluble polymer layer as the sacrificial layer, to reduce the adhesion between replica and mold. Polymeric replicas with complex terraced structures were successfully obtained, which were otherwise impossible to achieve with conventional perfluorinated molds. The as-prepared replicas all exhibited superhydrophobicity without further modification with low-surface-energy coatings. Interestingly, residual hydrophilic sacrificial layer resulted in high water-adhesion without losing superhydrophobicity; these surfaces turned extremely slippery after removal of the residual sacrificial layer. A molecular mechanism is proposed to interpret the contrast adhesion. After being coated with a sticky perfluoroalkyl, containing methacrylate, the surfaces convert to superoleophobicity and low adhesion to a number of oils.

Grain refining can improve the mechanical properties and solidification-cracking resistance of the weld. Ultrasonic grain refining was conducted by dipping an ultrasonic probe in the weld pool to stir it at a distance behind the arc. This new approach produced effective grain refining in arc welds of Mg alloys AZ31 Mg and AZ91 Mg. Grain refining increased when the probe was positioned farther behind the arc. This suggests the initial crystallites or dendrite fragments generated by ultrasound in a cooler melt farther behind the arc were better able to survive. This also suggests dendrite fragmentation was more likely to occur because the probe was closer to the mushy zone. However, a probe too far behind the arc ended up being inside the mushy zone and grain refining, though highly effective, was restricted to only near the weld centerline. At the same probe position, grain refining increased with increasing ultrasound amplitude. Grains were significantly finer in AZ91 Mg welds than AZ31 Mg welds. This suggests grain refining increased with increasing constitutional supercooling caused by the higher solute content of AZ91 Mg than AZ31 Mg.

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Grain refining is known to improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction. The present study was conducted to grain refine Mg welds by arc oscillation, which has not been investigated so far. First, significant grain refining was demonstrated by transverse arc oscillation. The effects of oscillation amplitude, oscillation frequency and torch travel speed on grain refining were shown. The effect of the alloy composition on grain refining was also demonstrated. Second, by using an overlap welding procedure, the grain refining mechanism was identified as dendrite fragmentation. Third, cooling curves recorded during welding showed that transverse arc oscillation caused reheating during solidification, which has been shown in casting/solidification to cause dendrite fragmentation by melting off dendrite arms. The cooling curves also showed that transverse arc oscillation significantly reduced the temperature gradient G along the torch travel direction, which suggested constitutional supercooling was increased. Thus, transverse arc oscillation not only caused dendrite fragmentation but also increased constitutional supercooling to help dendrite fragments survive and grow into fine equiaxed grains. Dendrite fragmentation by remelting, instead of mechanical breakup, of dendrites was discussed in the context of welding.

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