New Technology: The Performance Of New Liquid Repellent Surface Is Upgraded Again

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Although the application prospect of solid and durable repellent surface is bright, some problems need to be solved before its large-scale promotion. It is of great significance to solve the problem of reasonable selection and unification of durability evaluation methods, optimize the strategies for improving durability, and develop excellent processing methods for liquid repellent surfaces to promote the industrial application of solid and durable liquid repellent surfaces.

Lotus leaves "come out of the mud without staining"; The pitcher grass "slides" the insects contacting it into the cage; Collembola can float on the surface of water or oil; When fish are in the water, oil cannot adhere to their body surfaces... The reason why these animals and plants have "super power" is that they have liquid repellent surfaces.

Liquid repellent surfaces have shown great application prospects in waterproof, self-cleaning, anti icing, anti-corrosion, fluid manipulation, biomedicine and other aspects, and have received extensive attention from scholars in bionic manufacturing, microfluidic and other fields. However, the durability of most liquid repellent surfaces is poor, and they are easy to be damaged and lose their liquid repellency in the real environment, which greatly limits their practical application.

Scientific researchers have carried out a lot of research work on the durability of the liquid repellent surface, and achieved many breakthrough results. Recently, Chen Faze, a lecturer from the School of Machinery of Tianjin University, published a review on the international journal Review of Chemical Society, summarizing the research status of solid and durable repellent surfaces in recent years, and was selected as the current cover paper.

Learn from biology, uncover the "super power" of the surface of the repellent

The lotus leaf has excellent self-cleaning performance because its surface is super hydrophobic. The water drops on the lotus leaf surface are spherical and easy to roll off and take away the attached dust; The inner wall of pitcher trap is a typical liquid filled super slippery surface; Collembola can survive in humid environment due to the super oil repellency of its epidermis; Because the fish scale surface firmly adsorbs the water layer, the oil cannot directly contact the surface, thus showing the super oil repellency under water... Although the performance is different, the surface with super hydrophobic, super oil repellency and other properties belongs to the liquid repellent surface.

"Liquid repellent surface refers to the surface that is difficult to be wetted by liquid and on which liquid drops are easy to roll off or slip off, mainly including superhydrophobic surface, superhydrophobic surface and superhydrophobic surface filled with liquid." Chen Faze said.

In recent years, inspired by these organisms, researchers have carried out extensive and in-depth research on the liquid repellent surface, which has greatly enriched the development of liquid repellent surface in bionic manufacturing, micro nano processing, surface engineering, interface chemistry and other fields.

"The micro nano rough structure and special chemical composition of the liquid repellent surface make it liquid repellent." Chen Faze explained that the superhydrophobic and superhydrophobic surfaces in the air have the characteristics of micro nano scale rough structure and low surface energy chemical composition. When this kind of surface contacts with the test liquid, an air layer (also called an air cushion) will be formed between the solid and liquid, which will prevent the liquid from penetrating into the rough structure, and the liquid gas contact area is far larger than the solid liquid contact area, making it impossible for the liquid to wet this kind of surface, so it shows superhydrophobicity or superhydrophobicity. For the underwater super oil repellent surface and the liquid filled ultra slippery surface, its micro nano rough structure can lock the water or injected lubricating liquid firmly, forming a stable liquid layer on its surface. When the test liquid that is not miscible with the liquid layer contacts with this surface, the liquid layer can effectively block the direct contact between the surface and the test liquid, so that it has underwater super oil repellency or super sliding characteristics.

Therefore, the key to machining the surface of repelling liquid is to build a suitable micro nano rough structure and prepare suitable special chemical components to ensure that there is an air layer or an immiscible solution layer between such surface and the test liquid when they are in contact.

Thanks to the excellent liquid repellency and the extremely small solid-liquid contact area, the liquid repellent surface has great application prospects in waterproof, self-cleaning, fluid drag reduction, anti icing, anti-corrosion, fluid manipulation, oil-water separation, anti biological deposition, enhanced heat transfer, highly sensitive chemical and medical detection, fluid energy conversion power generation and other scenarios.

Replenish the short board to improve the solid durability of the liquid repellent surface

Although great progress has been made in the research on the surface of the liquid repellent in recent years, most of the liquid repellent surfaces have low strength, poor durability, and are easy to be damaged and lose the liquid repellency in the real environment. The lack of ruggedness and durability greatly limits the large-scale promotion and industrial application of the surface of the repellent.

Chen Faze said that, taking waterproof umbrellas as an example, the super hydrophobic surface imitating lotus leaf has been applied to the production of waterproof umbrellas, but the scenes easily encountered in practical applications such as high-speed raindrop impact, folding bending, friction, etc. may make the super hydrophobic surface invalid, affecting the performance and durability of waterproof umbrellas, leading to the difficulty of large-scale popularization of such waterproof umbrellas. Taking marine engineering equipment as an example, super hydrophobic surface or super slippery surface can reduce resistance of the hull and prevent biological deposition, but the above surfaces are easy to be corroded and penetrated when immersed in seawater, and the liquid repellency cannot be maintained for a long time, so it is difficult to be widely used in the field of marine engineering equipment. Therefore, it is of great significance to improve the durability of the liquid repellent surface and solve a series of problems that limit the application of the liquid repellent surface to promote its practical application.

"The durability of liquid repellent surface mainly includes thermodynamic durability, mechanical durability and chemical durability." Chen Faze said. Thermodynamic durability refers to the stability of the air layer or liquid layer on the surface of the repellent liquid in the thermodynamic process of diffusion, evaporation, condensation, etc. It can be generally evaluated by increasing water hammer pressure, immersion, steam condensation, droplet evaporation, gravity drainage, etc; Mechanical durability refers to the ability of the repellent surface to maintain its wettability under external force, which can be generally tested by tangential wear, dynamic impact, tape stripping, matrix deformation and other methods; Chemical durability refers to the stability of the surface of the repellent under various chemical factors, which can generally be tested by acid, alkali and salt corrosion, ultraviolet light irradiation, active particle treatment, organic solvent immersion, high and low temperature treatment and other methods.

In recent years, researchers have proposed four kinds of strategies to improve the surface durability of repellent based on the regulation of material surface structure and composition.

The first type is to effectively improve the mechanical durability and chemical durability of the liquid repellent surface by optimizing the matrix material or surface chemical composition and selecting the matrix material or surface modifier with good durability.

The second is to design special micro nano structures. For example, optimize the geometric shape and size parameters of the microstructure to improve the mechanical strength of the microstructure and improve the mechanical durability of the liquid repellent surface; The concave angle structure or nanostructure is designed to improve the stability of the air layer or liquid layer on the surface of the liquid repellent and improve its thermodynamic stability; Through self similar micro nano structure, the surface of the repellent will expose rough structure and chemical composition similar to the original surface after mechanical wear or chemical corrosion, so as to maintain its liquid repellency; In the micro nano composite structure, the micro structure with relatively high mechanical strength plays a similar role as "armor" to protect the fragile nano structure, thus improving the mechanical durability of the liquid repellent surface.

The third type is to enhance the connection strength between the repellent coating and the substrate by means of glue layer assistance, so as to significantly improve the mechanical durability and chemical durability of this type of repellent surface.

The fourth type is to prepare a self healing liquid repellent surface, so that the liquid repellent surface can recover itself after losing its liquid repellency, avoiding permanent failure of the liquid repellent surface.

The prospect is bright, and large-scale promotion still needs to solve multiple problems

With the continuous development of technologies related to solid and durable liquid repellent surfaces, we are expected to use the "super power" of animals and plants to apply liquid repellent surfaces to the fields of daily life, machinery, energy, chemicals, biology, materials, etc. For example, waterproof and self-cleaning fabrics and buildings, drag reducing and corrosion resistant underwater vehicles and fluid transport pipelines, anti icing and easy deicing aircraft wings, self-cleaning solar cell covers, high-sensitivity array sensors, etc.

"Although the prospect of solid and durable liquid repellent surface is very bright, some problems need to be solved before its large-scale promotion." Chen Faze thinks.

First, the problem of reasonable selection and unification of durability evaluation methods for solid and durable liquid repellent surfaces should be solved. The solution of this problem will play an important role in promoting the standardized development in the field of solid and durable liquid repellent surfaces. "The evaluation method for the durability of the surface of solid and durable liquid repellent shall be reasonably selected according to the actual application scenarios. For example, the durability of the roof of super hydrophobic buildings in terms of raindrop impact, acid rain corrosion, ultraviolet light irradiation, etc. shall be considered first." Chen Faze explained.

Secondly, we should optimize the strategies to improve the surface durability of solid and durable repellent. Existing strategies have limited improvement on such surface durability, and most of them only focus on a single aspect of thermodynamic durability, mechanical durability or chemical durability. If the existing methods can be ingeniously optimized and the synergy of existing strategies can be brought into play, it will be expected to further improve the durability of the liquid repellent surface and promote the practical application of the solid and durable liquid repellent surface.

Finally, we need to develop an excellent processing method for solid and durable liquid repellent surface with low cost, green and large area. At present, a large number of processing methods for solid and durable liquid repellent surfaces have many problems, such as complex process, high cost and serious environmental pollution. These problems limit the large area processing and practical application of the solid and durable liquid repellent surface.

"The development of a simple, low-cost, green and large-scale processing method for liquid repellent surfaces is of great significance to promote the industrial application of solid and durable liquid repellent surfaces." Chen Faze said.