Construction and corrosion behaviors of a bilayer superhydrophobic film on copper substrate

Fabricating low-cost, robust superhydrophobic coatings with re-entrant topology for self-cleaning, corrosion inhibition, and oil-water separation

HYPOTHESIS

The superhydrophobic surfaces with re-entrant microstructures are known to provide robust superhydrophobicity by enhancing the energy barrier for Cassie-Baxter to Wenzel transition. However, the fabrication of such structured surfaces often involves sophisticated techniques and expensive ingredients.

EXPERIMENTS

Herein, a multifunctional, low-cost, and fluorine-free superhydrophobic coating with re-entrant surface topology was fabricated using fly ash (FA) and room-temperature-vulcanizing silicone. A systematic study was performed to evaluate the coating properties and durability. The robustness was evaluated as a function of particle size and inter-particle spacing. The performance in self-cleaning, corrosion inhibition and oil-water separation has been presented.

FINDINGS

The synthesized coatings are substrate-versatile and demonstrate superhydrophobic behavior. The close-packed coating of re-entrant FA particles attained via vibration compaction was seen to provide high robustness. The coatings retain their superhydrophobicity after multiple cycles of tape-peeling and exposure to environmental factors including temperature, pH, and UV radiation. These coatings exhibit excellent corrosion inhibition (corrosion efficiency > 99.999%), outperforming the majority of the previously reported superhydrophobic coatings. It also displays excellent self-cleaning property and high separation efficiencies in oil-water separation (>99%). We envision that such FA-based superhydrophobic coatings can solve the issues of synthesizing cheaper, sustainable, and robust superhydrophobic surfaces while simultaneously opening new avenues for FA utilization.

Sessile Droplet Freezing on Hydrophobic Structured Surfaces under Cold Ambient Conditions

There have been conflicting reports as to whether surface wettability is effective in the freezing delay enhancement of attached water droplets. It is an important problem in the development of anti-icing surfaces needed for applications, such as aircraft wings and infrastructures. Here, we prepared precooled ambient conditions and surfaces which included smooth, microstructured, and two nanostructured surfaces with hydrophobic coatings to create an environment closer to the actual environment and to avoid frost formation, which enhances wetting transition and nucleation. Static and dynamic wetting characteristics of each surface were investigated as the fundamental properties and the freezing behavior of precooled water droplets were observed. A distinct elongation of the freezing delay time was observed for droplets on nanostructured surfaces which have static contact angles >150°, in contrast to those on smooth and microstructured surfaces. However, the difference in droplet adhesion induced by nanostructures showed a negligible impact on freezing delay. These results indicated that the reduction of the actual contact area between the solid and liquid phases restricted ice nucleus formation.

Superoleophobic textured copper surfaces fabricated by chemical etching/oxidation and surface fluorination

We report a convenient route to fabricate superoleophobic surfaces (abridged as SOS) on copper substrate by combining a two-step surface texturing process (first, the substrate is immersed in an aqueous solution of HNO3 and cetyltrimethyl ammonium bromide, and then in an aqueous solution of NaOH and (NH4)2S2O8) and succeeding surface fluorination with 1H,1H,2H,2H-perfluorodecanethiol (PFDT) or 1-decanethiol. The surface morphologies and compositions were characterized by field emission scanning electron microscopy and X-ray diffraction, respectively. The results showed that spherical micro-pits (SMP) with diameter of 50-100 μm were formed in the first step of surface texturing; in the second step, Cu(OH)2 or/and CuO with structures of nanorods/microflowers/microballs were formed thereon. The surface wettability was further assessed by optical contact angle meter by using water (surface tension of 72.1 mN m(-1) at 20°C), rapeseed oil (35.7 mN m(-1) at 20°C), and hexadecane (25.7 mN m(-1) at 20°C) as probe liquids. The results showed that, as the surface tension decreasing, stricter choosing of surface structures and surface chemistry are required to obtain SOS. Specifically, for hexadecane, which records the lowest surface tension, the ideal surface structures are a combination of densely distributed SMP and nanorods, and the surface chemistry should be tuned by grafted with low-surface-energy molecules of PFDT. Moreover, the stability of the so-fabricated sample was tested and the results showed that, under the testing conditions, superhydrophobicity and superoleophobicity may be deteriorated after wear/humidity resistance test. Such deterioration may be due to the loss of outermost PFDT layer or/and the destruction of the above-mentioned ideal surface structures. For UV and oxidation resistance, the sample remained stable for a period of 10 days.

One-step solution immersion process to fabricate superhydrophobic surfaces on light alloys

A simple and universal one-step process bas been developed to render light alloys (including AZ91D Mg alloy, 5083 Al alloy, and TC4 Ti alloy) superhydrophobic by immersing the substrates in a solution containing low-surface-energy molecules of 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS, 20 μL), ethanol (10 mL), and H2O (10 mL for Al and Mg alloy)/H2O2 (15%, 10 mL for Ti alloy). Field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and water contact angle measurements have been performed to characterize the morphological features, chemical composition, and wettability of the surfaces, respectively. The results indicate that the treated light alloys are rough-structured and covered by PFOTS molecules; consequently, the surfaces show static contact angles higher than 150° and sliding angles lower than 10°. This research reveals that it is feasible to fabricate superhydrophobic surfaces (SHS) easily and effectively without involving the traditional two-step processes. Moreover, this one-step process may find potential application in the field of industrial preparation of SHS because of its simplicity and universality.