Preparation And Properties Of Metal Foams-based Asymmetric Wetting Materials

Water is an important resource for human society.However,water scarcity has existed in different degrees around the world in recent years.So it is urgent to obtain fresh water resource for coping with the global water crisis.Inspired by water harvesting organisms in nature,scholars have conducted extensive research on asymmetric wetting materials.However,drawbacks such as high cost,low mechanical stability,environmental unfriendliness,and poor collection performance still exist and they limit the practical application of water harvesting materials.Three-dimensional（3D）porous metal foams have advantages of light weight,high strength,good thermal stability,durability,and so on.So they are ideal substrates for building fog collection materials.At present,the common preparation of metal foams-based asymmetric wetting materials for fog collection is divided into two steps:firstly,obtaining a superhydrophilic metal foam substrate.Secondly,modifying low surface energy substances on one side to obtain an asymmetric wetting material.However,problems such as insufficient collection performance,weak stability and durability exist for these materials.In order to solve problems above,herein,metal foams-based asymmetric wetting materials were prepared by combining alkali etching method and boiling water etching method with phase boundary modification,respectively,using metal foams（copper foam,aluminum foam）as substrates.The water directional transportation,fog water collection performance,durability and stability of the prepared asymmetric wetting metal foams were studied deeply.Conclusion with important reference value were obtained.Especially,the fog water collection rate was enhanced effectively by different low surface energy substances in regulating the topological structure of hydrophobic surfaces in the phase boundary modification process.The research has great innovation,and new ideas have been proposed for improving fog water collection rate by adjusting and controlling surface structure and composition.The main research contents and conclusions are as follows:（1）Preparation of asymmetric wettable copper foams by combing alkali etching method with phase interface modification:Asymmetric wetting copper foams were prepared on copper foams by alkali etching combined with oil-water phase interface modification.Three low surface energy substances,namely octadecylamine（ODA）,n-octadecylmercaptan（ODT）,and hexadecyltrimethoxysilane（HDTMS）,were used to construct the hydrophobic surfaces in oil-water phase boundary modification process.The resultant foams exhibited asymmetric wetting property,and they could realize unidirectional water transport.Among them,the asymmetric wetting copper foam modified by HDTMS had a higher droplet transport speed.At the same time,the optimal preparation conditions were obtained by optimizing process parameters.In addition,ODA,ODT and HDTMS were used to regulate the topological structure of hydrophobic surfaces.ODA-modified hydrophobic surface revealed a micro-nano composite structure similar to durian epidermis.ODT-modified hydrophobic surface had a micro-nano double-layer structure similar to kiwifruit surface.Meanwhile,HDTMS-modified hydrophobic surface presented a high-density micro-nano binary composite structure similar to cactus thorn.Correspondingly,the asymmetric wetting copper foam modified by HDTMS revealed the highest fog water collection capability of about 3718 mg·cm^-2·h^-1.In addition,it had good durability and stability as well.For example,it could collect fog for 20 times continuously and stably.It still exhibited very good water collection capacity even after being placed in the outdoor environment for 5 months.Besides,it showed appreciative stability upon exposed to cold and hot cycling test,ultraviolet irradiation and sandpaper grinding test.More importantly,it revealed good thermal repair performance.（2）Preparation of asymmetric wetting aluminum foams by boiling water etching combined with phase boundary modification:Four asymmetric wetting aluminum foams modified with ODT,ODA,HDTMS,and SA unilaterally were fabricated by boiling water etching combined with oil-water phase boundary modification using aluminum foams as the substrates.All of them exhibited the"liquid diode"phenomenon,especially the SA-modified aluminum foam.It had a higher unidirectional droplets transport rate.The microscopic morphology was observed by scanning electron microscopy.The hydrophobic surfaces of those foams gradually formed a higher spatial density of micro-nano-bilayer composite structure after ODT,ODA,HDTMS,and SA unilateral modification.This corresponded to their fog water collection rate.For instance,the SA-modified asymmetric aluminum foam had more efficient fog water collection rate of 5123 mg·cm^-2·h^-1.In this study,process parameters were also optimized using the control variable method.In addition,the SA-modified asymmetric wetting aluminum foam had excellent repeatable fog collection efficiency and environmental durability.It maintained high hydrophobicity and fog water collection capability after multiple cycles of high temperature treatment at 120°C and low temperature cooling at 20°C.And it had good fog collection performance after multiple cycles of UV irradiation for 36 h,sandpaper rubbing for 480 cm,or plasma bombardment combined with thermal repair.