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Superoleophobicity: Superoleophobic Slippery Lubricant-Infused Surfaces: Combining Two Extremes in the Same Surface (Adv. Mater. 45/2018)
Advanced Materials
Abstract
In article number 1803890, Pavel A. Levkin and co‐workers develop a slippery superoleophobic surface, which is realized by using 3D direct laser writing. The nanostructures on each micropillar stabilize the impregnated slippery lubricant, while the re‐entrant geometry of the micropillars prevents the lubricant from spreading. The slippery layer reduces the adhesion of liquid to the pillars, as proved using scanning droplet adhesion microscopy (SDAM), while the doubly re‐entrant micropillars make the surface superoleophobic.
Smart surfaces with responsive wettability are unstable, and depend upon continuous external stimuli, which limits their widespread application in switchable oil/water emulsions separations. In this study, a Ti‐based 3D porous structure (SLM‐3DTi) is printed using advanced selective laser melting (SLM) technology for the switchable separation of oil/water emulsion. With the assistance of the computer program, porous structure and re‐entrant texture can be easily designed and printed in one step. Without any continuous external stimulus, the wettability of SLM‐3DTi can be reversibly switched between underwater superoleophobicity and underoil superhydrophobicity simply by drying and washing cycles. The SLM‐3DTi achieves switchable surfactant‐stabilized oil‐in‐water emulsion (SSE(o/w)) and surfactants‐stabilized water‐in‐oil emulsion (SSE(w/o)) separation with purity above 99.8% at a flux of more than 2000 L m⁻² h⁻¹. In addition, the re‐entrant texture of the SLM‐3DTi surface is formed with the partially melting powder particles on the part contour, which has much stronger mechanical durability than any binder. Furthermore, SLM‐3DTi has excellent corrosion resistance due to the material properties of Ti. More importantly, based on the visualization analysis of the simulation, the mechanism of SLM‐3DTi emulsion separation is further elucidated. Therefore, SLM‐3DTi has broad practical application potential for high‐flux, high‐purity, and switchable oil/water emulsion separation.
Slippery liquid‐infused porous surface (SLIPS) is an emerging anti‐icing method. The anti‐icing mechanism of SLIPS is to prevent the heat transfer from liquid droplets to the surface by forming an oil film on the solid surface. However, the contact area between the droplet and the surface of the specimen increases due to the existence of the oil film, which accelerates the heat transfer and emergence of ice. Herein, in order to further improve the anti‐icing performance of SLIPS, a microscale square column array is fabricated by wire cut electrical discharge machining (WEDM) technology, on which a circular hole structure is constructed by nanosecond laser. Lubricating oil is then injected into the fluorinated surface to produce SLIPS. The maximum contact angle (CA) of the surface is 150° ± 1.2° and the contact angle hysteresis (CHA) is 1.2°. The contact mode of droplets on the surface is liquid–oil and liquid–gas, which is a multiphase mixing interface. The icing test shows that the icing time of the droplet is 4785 s at a surface temperature of −10 °C, exhibiting a good icing‐delay performance.
Devising effective methods to reduce drag forces is of great interest as these methods could prevent the wastage of fuel and decrease carbon emission and the global warming rate. Since the invention of Liquid-Infused Surfaces (LISs) in 2011, numerous investigations have been conducted to study their capability in various applications. Due to recirculation or drawing of the penetrated liquid within the surface structure, these surfaces acquire a slippiness property. The conducted investigations showed that these slippery surfaces have great potential for reducing drag forces, whether in a laminar or turbulent flow. In the present work, we first briefly elucidate common drag reduction methods, effective factors, and types of liquid-infused surfaces and their applications. In addition , we address issues such as lubricant evaporation and depletion that threaten the performance of the LISs. Then, we comprehensively focus on the drag reduction ability of these surfaces and state the criteria required to achieve stable drag reduction. Finally, future outlooks and research topics requiring further investigations are discussed.
Controlling surface wettability has important application prospects in many aspects. Both the Cassie-Baxter equation and Wenzel equation explain the effects of surface chemical composition and roughened texture on surface wettability. Recently, the third factor - reentrant surface curvature - has been proposed; nonetheless, its impact based on theoretical calculation has never been investigated. Based on the theory proposed by Marmur, herein, the overlap fraction φr was introduced to explain the effect, and three novel necessary conditions for the existence of the Cassie state were drawn. This theory allows us to quantitatively evaluate the influence of various reentrant structures on the wettability and stability of the Cassie state. In addition, the simulation results of two typical structures of paraboloid and square T-shaped were demonstrated. For the paraboloid structures, the influence law of spatial period on the apparent contact angle of the Cassie state (CAC) and GT∗ was found to be opposite. In contrast, for the square T-shaped structures, the GT∗ increased gradually, but the CAC remained constant with the increase of φr. Although the reentrant structures provide us with the possibility to construct the superamphiphobic surface, the stability of the Cassie state of the square T-shaped structure is extremely poor, and a larger φr carries a risk of structural mechanical strength reduction. Finally, three reentrant structures reported in the literature were simulated to verify the correctness of this theory. Excessive pursuit of one of the two properties results in a decrease in the combination property. These results may be potentially helpful for the design of superamphiphobic surfaces in real applications.
Slippery liquid-infused porous surfaces (SLIPSs) have been actively studied to improve the limitations of superhydrophobic (SHP) surfaces, especially the defects of non-wetting chemical coating layer and weak mechanical robustness of surface micro/nanostructures. However, SLIPSs also have several drawbacks including volatilization and leakage of lubricant caused by long-term usage. In this study, we suggest the use of icephobic, highly transparent, and self-healing solid slippery surface to overcome the limitations of both surfaces (SLIPS and SHP) by combining specific biomimetic morphology and intrinsic properties of paraffin wax. A moth-eye mimicking nanopillar structure was prepared instead of a porous structure and was coated with solid paraffin wax for water repellence. Moth-eye structures enable high surface transparency based on antireflective effect, and the paraffin layer can recover from damage due to sunlight exposure. Furthermore, the paraffin coating on the nanopillars provides an air trap, resulting in a low heat transfer rate, increasing freezing time and reducing adhesion strength between the ice droplet and the surface. The heat transfer model was also calculated to elucidate the effects of the nanopillar height and paraffin layer thickness. Anti-reflection and freezing time of the surfaces enhance with increase in nanopillar height. The paraffin layer slightly deteriorates the transmittance but enhances the icephobicity. The solar cell efficiency using a biomimetic solid slippery surface is higher than that of bare glass due to the anti-reflective effect. This integrated biomimetic solid slippery surface is multifunctional due to its self-cleaning, anti-icing, anti-reflection, and self-healing properties and may replace SLIPS and SHP surfaces.
Hydrophilic materials are easily fouled by organic contaminants owing to their high surface energy, and this oil-fouling problem severely hinders their use in practical applications. To address this challenge, herein, a hydrophilic coating with oil repellency and photocatalytic activity is developed by a spray-casting process. In the air surrounding, a water droplet spreads over the coating surface completely, while oil droplets exhibit contact angles more than 150° and moving on the coating freely. The water-wetted coating still had oil repellency, as the water layer on the coating surface can act as a lubricant to repel oil. Although methylene blue aqueous solution contaminates the coating by wetting it completely, these water-soluble organic molecules can be removed by UV illumination, due to the photocatalytic activity of the coating. Exploiting its water-attracting and oil-repelling properties, the coating deposited on a copper mesh is applied as a multiplatform for oil–water separation with high separation efficiency. This study provides a novel and efficient way to solve the oil-fouling problem of hydrophilic materials.
Development of oil-repellent coatings that are anti-fouling with water alone is highly desirable, yet still challenging. Herein, to address this challenge, we fabricate a slippery oil-repellent hydrogel coating that exhibits oil repellency when exposed in the air as well as underwater. In the air, water wets the slippery oil-repellent hydrogel coating surface completely, while organic liquid drops such as toluene can slide off the water wetted coating surface easily. When immersed in water, the slippery oil-repellent hydrogel coating surface exhibits excellent underwater superoleophobic property with all oil contact angles more than 159° and oil adhesive forces less than 1 μN. The hydrogel coating keeps its oil repellency after long-term outdoor storage, thermal treatment, knife scratching and other treatments. Exploiting its water-attracting and oil-repelling property, the slippery oil-repellent hydrogel coated copper mesh and filter paper are demonstrated as reusable membranes to separate oil–water mixtures with separation efficiency more than 97%.
Graphic abstract
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