Jinlong Song

Robust self-cleaning surfaces that function when exposed to either air or oil

A robust paintlike repellent coating Superhydrophobic materials often depend on a particular surface patterning or an applied coating. However, these surfaces can be damaged by wear or fouled by oily materials. Lu et al. devised a suspension of coated titanium dioxide nanoparticles that can be spray-painted or dipcoated onto a range of hard and soft surfaces, including paper, cloth, and glass. The coatings resisted rubbing, scratching, and surface contamination. Science, this issue p. 1132 Robust, coated self-cleaning surfaces function after either abrasion or oil contamination. Superhydrophobic self-cleaning surfaces are based on the surface micro/nanomorphologies; however, such surfaces are mechanically weak and stop functioning when exposed to oil. We have created an ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticles that forms a paint that can be sprayed, dipped, or extruded onto both hard and soft materials to create a self-cleaning surface that functions even upon emersion in oil. Commercial adhesives were used to bond the paint to various substrates and promote robustness. These surfaces maintained their water repellency after finger-wipe, knife-scratch, and even 40 abrasion cycles with sandpaper. The formulations developed can be used on clothes, paper, glass, and steel for a myriad of self-cleaning applications.

Rapid fabrication of large-area, corrosion-resistant superhydrophobic Mg alloy surfaces.

A superhydrophobic magnesium (Mg) alloy surface was successfully fabricated via a facile electrochemical machining process, and subsequently covered with a fluoroalkylsilane (FAS) film. The surface morphologies and chemical compositions were investigated using a scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) and a Fourier-transform infrared spectrophotometer (FTIR). The results show hierarchal rough structures and an FAS film with a low surface energy on the Mg alloy surfaces, which confers good superhydrophobicity with a water contact angle of 165.2° and a water tilting angle of approximately 2°. The processing conditions, such as the processing time and removal rate per unit area at a constant removal mass per unit area, were investigated to determine their effects on the superhydrophobicity. Interestingly, when the removal mass per unit area is constant at approximately 11.10 mg/cm(2), the superhydrophobicity does not change with the removal rate per unit area. Therefore, a superhydrophobic Mg alloy surface can be rapidly fabricated based on this property. A large-area superhydrophobic Mg alloy surface was also fabricated for the first time using a small-area moving cathode. The corrosion resistance and durability of the superhydrophobic surfaces were also examined.

Self-Driven One-Step Oil Removal from Oil Spill on Water via Selective-Wettability Steel Mesh

Marine oil spills seriously endanger sea ecosystems and coastal environments, resulting in a loss of energy resources. Environmental and economic demands emphasize the need for new methods of effectively separating oil-water mixtures, while collecting oil content at the same time. A new surface-tension-driven, gravity-assisted, one-step, oil-water separation method is presented for sustained filtration and collection of oil from a floating spill. A benchtop prototype oil collection device uses selective-wettability (superhydrophobic and superoleophilic) stainless steel mesh that attracts the floating oil, simultaneously separating it from water and collecting it in a container, requiring no preseparation pumping or pouring. The collection efficiencies for oils with wide ranging kinematic viscosities (0.32-70.4 cSt at 40 °C) are above 94%, including motor oil and heavy mineral oil. The prototype device showed high stability and functionality over repeated use, and can be easily scaled for efficient cleanup of large oil spills on seawater. In addition, a brief consolidation of separation requirements for oil-water mixtures of various oil densities is presented to demonstrate the versatility of the material system developed herein.

Creating superhydrophobic mild steel surfaces for water proofing and oil-water separation

A simple and inexpensive two-step immersion method is reported to make mild steel superhydrophobic. Micro–nano-scale roughness and surface chemistry modifications were created via immersing mild steel into a salt solution followed by treatment with a low surface-energy polymer. The fabricated mild steel has water contact angles greater than 150° and remarkable water bouncing properties. This method was also used to treat a mild steel mesh for oil–water separation. In this paper, a new, facile and reusable gravity-induced separation system is proposed to collect floating oil, the oil collection rate can reach >96%.

One-step electrochemical machining of superhydrophobic surfaces on aluminum substrates

A superhydrophobic surface on an aluminum substrate was fabricated by one-step electrochemical machining using the sodium chloride (NaCl) aqueous solution containing fluoroalkylsilane as the electrolyte. The resulting superhydrophobic surfaces showed a static water contact angle of 166° and a tilting angle of about 1°. The morphological features and chemical compositions were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron probe micro-analyzer (EPMA), and Fourier-transform infrared spectrometer (FTIR). It shows that the binary micrometer–nanometer-scale rough structures and the low surface energy coating were present on the aluminum surfaces. The resulting surfaces have good properties of anti-adhesion and self-cleaning. The durability of the superhydrophobic surfaces on aluminum substrates was also investigated. This preparation method is advantageous as it does not require acid electrolyte or a separate process to lower the surface energy, uses simple steps, and is environmental friendly and highly efficient.

Creating robust superamphiphobic coatings for both hard and soft materials

Most superhydrophobic surfaces lose their water-repellency when either contaminated by oily liquids or by being mechanically damaged. Superamphiphobic surfaces are ones that repel both oil and water. However, to date such surfaces are hampered by being mechanically weak. Robust superamphiphobic surfaces with highly water and oil repellent properties are desired for a wide range of environments. Reported herein is a superamphiphobic coatings fabricated by a facile deposition method and followed by a low surface energy materials modification. These coatings can be applied on both hard and soft materials to repel water, glycerol, peanut-oil droplets and some organic solvents. Falling sand abrasion, UV irradiation and aqueous media immersion were used to test the mechanical robustness and durability of the superamphiphobic coatings. A multi-cycle stretch/release test was developed to characterize the robustness of the self-cleaning soft materials. A coated rubber-bond retained both water and oil repellency even after 50 stretch/release cycles. These tests show that the superamphiphobic coatings have remarkable mechanical robustness and UV/aqueous media resistance and can be readily applied to a wide variety of materials to form self-cleaning surfaces that are extremely robust and durable even under intense strains.

Water droplets bouncing on superhydrophobic soft porous materials

Creating superhydrophobic soft porous materials, such as cotton and cloths is an important area of research as these materials have important practical applications such as water repellent clothing. In this paper, a generic method is reported to fabricate superhydrophobic surfaces on soft porous substrates, which were treated with crystalline copper chloride-hydroxide, followed by a fluorinated polymer. Water droplets can be supported as a perfect sphere and even bounce on the prepared surfaces, which demonstrate superior superhydrophobicity. For many soft porous materials, it is difficult to identify superhydrophobicity using water droplet contact angles. This is because water droplets may be trapped in the concave structures, making the necessary contact line to define the contact angle unobtainable. To solve this problem, water bouncing was used as a sufficient and necessary identification for superhydrophobic soft porous materials, and a model was also made to discuss how a water droplet can bounce on soft materials.

Underwater Spontaneous Pumpless Transportation of Nonpolar Organic Liquids on Extreme Wettability Patterns.

Spontaneous pumpless transportation (SPT) of liquids has generated tremendous demands in microfluidic systems and advanced devices. However, the transportation of nonpolar organic liquids on open platforms underwater remains a challenge because most existing SPT systems are only designed for use in air. Here, we report a surface-tension-driven SPT system to transport various nonpolar organic liquids using underwater extreme wettability patterns. The patterns were fabricated with a wedge-shaped superoleophilic track on a superoleophobic background by combining CuCl2 etching, stearic acid modification, and mask-based nitrogen cold plasma treatment. Three types of underwater SPT processes-horizontal transport, tilted transport, and directional transport-were studied experimentally and theoretically. For horizontal SPT and tilted SPT, the capillary force was the main driving force, which depended on the wedge angle of the superoleophilic track. The excellent transportation ability of horizontal SPT of underwater liquid droplets was obtained at a wedge angle of 3-5°. The maximum moving height of organic liquids on the tilted SPT transport was obtained at an angle of 8°. For directional SPT, organic liquids did not drop off in the moving process because of the constraint imposed by surface tension, resulting in the sustained directional transport with long distances and complex trajectories.

Fabrication of superoleophobic surfaces on Al substrates

An easy method of fabricating superoleophobic surfaces on Al substrates by constructing reentrant structures is reported. The reentrant micro/nanometer-scale structures comprise micrometer-scale, rectangular-shaped, and step-like Al structures obtained by electrochemical etching and nanometer-scale Ag grains resulting from immersion in [Ag(NH3)2]+ solution. Surface energy is reduced by perfluorooctanoic acid (PFOA) containing –CF3 and –CF2– groups. The PFOA-modified micro/nanometer-scale rough structures enable the formation of a composite solid–liquid–air interface with peanut oil. These structures show good superoleophobicity with a peanut oil contact angle of 160.0 ± 2° and a sliding angle of 8°. Nanometer-scale structures can effectively transform the micrometer-scale non-reentrant structures into reentrant structures. With the aid of suitable low surface energy materials such as PFOA, fabricating superoleophobic surfaces on Al substrates can be easier.

Controlling the Adhesion of Superhydrophobic Surfaces Using Electrolyte Jet Machining Techniques.

Patterns with controllable adhesion on superhydrophobic areas have various biomedical and chemical applications. Electrolyte jet machining technique (EJM), an electrochemical machining method, was firstly exploited in constructing dimples with various profiles on the superhydrophobic Al alloy surface using different processing parameters. Sliding angles of water droplets on those dimples firstly increased and then stabilized at a certain value with the increase of the processing time or the applied voltages of the EJM, indicating that surfaces with different adhesion force could be obtained by regulating the processing parameters. The contact angle hysteresis and the adhesion force that restricts the droplet from sliding off were investigated through experiments. The results show that the adhesion force could be well described using the classical Furmidge equation. On account of this controllable adhesion force, water droplets could either be firmly pinned to the surface, forming various patterns or slide off at designed tilting angles at specified positions on a superhydrophobic surface. Such dimples on superhydrophopbic surfaces can be applied in water harvesting, biochemical analysis and lab-on-chip devices.