Liu Huang

Fabrication of Long-Term Underwater Superoleophobic Al Surfaces and Application on Underwater Lossless Manipulation of Non-Polar Organic Liquids

Underwater superoleophobic surfaces have different applications in fields from oil/water separation to underwater lossless manipulation. This kind of surfaces can be easily transformed from superhydrophilic surfaces in air, which means the stability of superhydrophilicity in air determines the stability of underwater superoleophobicity. However, superhydrophilic surfaces fabricated by some existing methods easily become hydrophobic or superhydrophobic in air with time. Here, a facile method combined with electrochemical etching and boiling water immersion is developed to fabricate long-term underwater superoleophobic surfaces. The surface morphologies and chemical compositions are investigated. The results show that the electrochemically etched and boiling-water immersed Al surfaces have excellent long-term superhydrophilicity in air for over 1 year and boehmite plays an important role in maintaining long-term stability of wettability. Based on the fabricated underwater superoleophobic surfaces, a special method and device were developed to realize the underwater lossless manipulation of immiscible organic liquid droplets with a large volume. The capture and release of liquid droplets were realized by controlling the resultant force of the applied driving pressure, gravity and buoyancy. The research has potential application in research-fields such as the transfer of valuable reagents, accurate control of miniature chemical reactions, droplet-based reactors, and eliminates contamination of manipulator components.

Large-Area Fabrication of Droplet Pancake Bouncing Surface and Control of Bouncing State

Superhydrophobic pillar arrays, which can generate the droplet pancake bouncing phenomenon with reduced liquid-solid contact time, have huge application prospects in anti-icing of aircraft wings from freezing rain. However, the previously reported pillar arrays, suitable for obtaining pancake bouncing, have a diameter ≤100 μm and height-diameter ratio >10, which are difficult to fabricate over a large area. Here, we have systematically studied the influence of the dimension of the superhydrophobic pillar arrays on the bouncing dynamics of water droplets. We show that the typical pancake bouncing with 57.8% reduction in contact time with the surface was observed on the superhydrophobic pillar arrays with 1.05 mm diameter, 0.8 mm height, and 0.25 mm space. Such pillar arrays with millimeter diameter and <1 height-diameter ratio can be easily fabricated over large areas. Further, a simple replication-spraying method was developed for the large-area fabrication of the superhydrophobic pillar arrays to induce pancake bouncing. No sacrificial layer was needed to reduce the adhesion in the replication processes. Since the bouncing dynamics were rather sensitive to the space between the pillars, a method to control the contact time, bouncing shape, horizontal bouncing direction, and reversible switch between pancake bouncing and conventional bouncing was realized by adjusting the inclination angle of the shape memory polymer pillars.

Unpowered oil absorption by a wettability sponge based oil skimmer

Oil spills seriously pollute the environment and cause huge economic losses. Recently, materials with extreme wettability are widely used for oil/water separation. However, the existing methods have drawbacks including complicated and costly fabrication processes for oil/water separation materials, limited oil collecting capacity and reduced oil/water separation efficiency after being polluted by oil, etc. Therefore, an efficient and low-cost method for oil spill collection is urgently needed. We proposed a one-step immersion method to fabricate an oil spill absorption material. Superhydrophobicity and superoleophilicity were obtained by a common polyurethane sponge after being immersed into a cupric stearate solution and drying process. Water droplets can be supported as a spherical shape with a contact angle above 155.5° and can easily roll off from the surface, while hexane, hexadecane, diesel oil, peanut oil and lubricating oil can spread completely on the modified sponge. The prepared sponge can maintain water repellency after continuous oil spill collection for more than 4 h. A large-scale oil skimmer was designed to collect oil spills by employing the extreme wettability sponge and was successfully used for the collection of floating diesel oil, peanut oil, and lubricating oil on water. This method is simple, low cost and mass-producable, and the obtained oil skimmer possesses a high oil collection capacity, excellent durability and recyclability. Therefore, it has application prospects for the collection of oil spills on the sea or under other harsh conditions.

Multi-functional application of oil-infused slippery Al surface: from anti-icing to corrosion resistance

Slippery liquid-infused porous surface (SLIPS) inspired by Nepenthes pitcher has attracted considerable attention owing to its great potential applications in icy or corrosive conditions. However, observing both anti-icing property and corrosion resistance of SLIPS has rarely reported before. In this paper, a simple approach was proposed to prepare SLIPS to protect metal from freezing rain and corrosion. The Al substrate was first electrochemically etched and anodized to construct hierarchical porous structures composed of AlOOH and Al2O3. After lowering the surface energy and infusing silicone oil, the SLIPS was obtained showing a water roll-off angle of 3°, which could exclude almost all liquids at an inclining angle of 5°. Comparative tests including freezing rain and corrosion in NaCl solution, were conducted on bare Al and prepared oil-infused slippery surface, demonstrating the excellent anti-icing and corrosion resistance properties of our SLIPS. Additionally, self-cleaning test was also successfully performed. So, this slippery Al surface with multiple outstanding properties would be useful for the anti-icing and corrosion resistance applications in extreme environment.

Water strider-inspired design of a water walking robot using superhydrophobic Al surface

Abstract Aquatic microrobots, which can walk freely on water mimicking water striders, have attracted considerable interest among scientists in biomimetic area. Most of previous water strider robots adopted gear pairs as their driving mechanism, which called for high assembly precision and thereby increased the processing difficulty. Here, a novel and simple method using servos as the driving module to prepare water walking robot was proposed. We fabricated this robot by using supporting and actuating legs with excellent superhydrophobicity. Our robot weighted 27.9 g, but could float and run quickly driven by mini-type servos under remote Bluetooth control in mobile terminal. The legs were obtained on Al substrates by chemical etching, boiling-water immersion and low surface energy modification. Then, surface morphology and chemical compositions were subsequently investigated by SEM, EDS and XRD. In order to better illustrate the floating and rowing mechanism of this robot on the water surface, mechanics analysis models were proposed to analyze the lifting force and resistance force, respectively. Due to its excellent floating capacity and rowing ability, the biomimetic robot has promising application potential in water quality monitoring, aquatic exploration, and other surveillance missions. Graphical Abstract