Structure Designing And Performance Researching Of “Water Harvesting Sail” Of Fog Harvesting Device

Clean water is vital to the survival of humanity around the world.Lack of clean water is a problem not only in deserts,semideserts,coastal and alpine regions but also in other parts of the globe,in today’s modern industrial society.For a long time,how to obtain clean water from the atmosphere and seawater has been a concern and studied by scientists all over the world.In order to solve the problem of lacking clean water,especially in fog deserts,semideserts,coastal areas,and alpine areas(For example,the high mountains in the North of Vietnam),the method of fog harvesting has been studied and applied.However,the water harvesting efficiency of the fog harvesters studied for a long time is still low,and the ability to maintain the persistent water harvesting efficiency is limited.In other words,there are still a lot of difficulties and deficiencies in practical applications.In the fog harvesting field,there have been some reports of the research on water harvesting sails with different structures and materials in fog harvesters.The most common application is still the traditional mesh structure(metal mesh and polymer mesh).Recently,the application of woven fabrics in water harvesting sails has attracted the attention of scientists.In fact,most studies are limited to improving the fabric surface,or changing the surface structure of the fabric through simple weaving patterns,and have not considered the relationship between aerodynamic factors and fabric structure.This resulted in relatively low water harvesting efficiency of fabric sails.Simultaneously,there have not been many research on either improvement or preparation of three-dimensional fabric water harvesting sails for fog harvesters or improvement of water harvesting efficiency.In addition,there have been no reports on vertical mesh water harvesting sails combined from different polymer filaments.Particularly,there have been no reports on utilizing the elastic properties of textile threads to improve the water harvesting efficiency from the fog.This showed that textile threads/fabric structures and properties(flexibility and diversity)have great potential for the development of fog harvesters(and enhance water harvesting efficiency).Therefore,based on analyzing the previous research situation and existing problems,starting from the aspects of material optimization and structural design,this thesis has carried out a series of in-depth studies on preparation and modification of textile-based fog harvesters,as well as enhancing the water harvesting efficiency of water harvesting sails.This thesis,firstly,referring to the principles of geometric surface gradient,the wetting non-uniformity of the back of desert beetles,and the shedding phenomenon of water droplets on the surface of rice leaves,a series of basic research was carried out on the surface of the fabric water harvesting sails to improve its ability of transporting water droplets.A porous superhydrophilic strip composed of polyvinyl acetate(PVAc)nanofibers and polyvinyl alcohol(PVA)nanoparticles was prepared by electrospinning and electrospray technology,and a defective porous surface was created by dissolving PVA particles,which had good water absorption during fog harvesting.Subsequently,the superhydrophilic nanofiber strip was assembled on the hydrophobic modified fabric substrate.Hydrophobic modification is achieved by modifying polyester fabric with tetrabutyl titanate by sol-gel method,and then fluorinating with(3,3,3-trifluoropropyl)methyl dichlorosilane.The modified fabric surface becomes more hydrophobic and the hydrophobicity is more stable.The surface wettability difference between the hydrophobic fabric surface and the superhydrophilic nanofiber strip makes it easy for droplets to be transported from the hydrophobic region to the more hydrophilic region.Thereafter,this thesis reformed the structure of woven water harvesting sail by changing the number ratio of warp and weft yarns,so that it had different aerodynamic characteristics.Aerodynamic factors such as shadow coefficient(SC)and Stokes number(St)had great influence on the water harvesting rate of the woven fabric water harvesting sails.Among them,the structure of Warp@fabricwas mainly composed of warp yarns whose arrangement density reduced by half,which overcame the inherent weakness of poor aerodynamic characteristics of previous fog harvesting textiles due to compact structure and improved the fog collection ability.In addition,the wavy yarn structure composed of periodic convex and concave could help to form large droplets quickly.PET monofilament was modified into PET/PMMA core-shell structure monofilament by electrospray technology.This core-shell structure promoted droplet transport and migration,improved droplet shedding efficiency and avoids droplet blockage and detention through wettability change and the synergistic effect of capillary force,Laplace pressure and gravity.The droplet shedding time of PET/PMMA core-shell monofilament was about 8 times earlier than that of the original PET monofilament.Then,a vertical filament mesh water harvesting sail(VFM)was researched to further improve the water harvesting efficiency.Due to the efficient droplet capture and rapid droplet shedding,VFM exhibited higher fog harvesting efficiency than that of vertical wire mesh,and hydrophobic VFM has greater fog harvesting efficiency than that of hydrophilic VFM.The 3D PU-SA micro-bumps were formed on the surface of the filament by spraying the mixed solution of PU-SA(similar to the back of the desert beetles),which improved the water harvesting efficiency of all VFMs by about 30-80%.The maximum water collection efficiency of fourlayer PTFE 3D VFM with PU-SA micro-convex surface is 2.88 g/cm2/h.This was attributed to the improved synergistic effects of droplet capture(large droplet capture area),droplet growth,and droplet shedding.The results of the study also showed that the water harvesting efficiency depended not only on the rapid droplet shedding but also on the critical volume of the droplet(the volume of the droplet at which it started to shed off).VFM could not only effectively capture the droplets,but also ensure that the droplets fell off quickly,which avoided the blockage and retention of the droplets.Additionally,the study also discovered some new phenomena and principles related to the fog harvesting efficiency of VFM,such as the "new three-dimensional droplet capture area" on the surface of the filament,and the phenomenon of "droplet capture droplets",which improved the water harvest efficiency of VFM.Specification parameters of VFM,such as filament pitch,filament density,filament radius,filament layer number,etc.,had a great influence on aerodynamic factors and water harvesting efficiency.The reason why vertical monofilament water-collecting sail(VFM)had high-efficiency water-collecting ability was explained: the high-efficiency water harvesting ability of water harvesting sail was attributed to the comprehensive improvement of its synergistic effect of large droplet capture area,rapid droplet growth and efficient droplet shedding.The thesis also used elastic textile threads(ETT)to study the preparation of fog harvesters.Based on the excellent tensile properties of elastic yarn,a "water harvesting sail" stretchingrecovery system was designed and applied to change the yarn diameter during fog harvesting process,and thus effectively promoting the convergence of large droplets and droplet shedding.The temporary relaxation and adhesion of adjacent elastic yarns on the automatic elastic yarn water harvesting sail auto-ETTs promoted the droplet convergence and droplet shedding.The auto-ETT’s fog harvesting rate was about 60% higher than ETT’s.Especially after three kinds of elastic yarns were modified by spraying PU adhesive(auto-i@ETT),compared with ETTs,the water harvesting rate of auto-i@ETTs increased by about 80%-150%.The rough surface obtained by modifying the surface of elastic yarn with silica nanoparticles and titanium dioxide particles promoted the capture of fog drops on the surface of spandex yarn.In addition,the highly hydrophobic polymer PTFE was used to adhere silica nanoparticles to the surface of polyurethane(PU)yarn,creating a highly hydrophobic surface with convex structure.The water harvesting efficiency of three kinds of modified elastic yarn water harvesting sails(m@ETTs)was about 60%~120% higher than that of three kinds of original elastic yarn water harvesting sails(ETTs).The research results provided a new way for designing and manufacturing fog catcher with elastic yarn.Finally,this thesis studied and designed a textile three-dimensional elastic structure waterharvesting sail.Taking full advantages of the elastic properties and structure of 3D spacer fabrics,combined with the design and application of an automatic compression relaxation system,two unique and novel 3D structured water-harvesting sails "Auto 3D@SF" and "Tree3D@SF" were researched and fabricated.The inner small filament shape(rainbow filament)of Tree 3D@SF was almost similar to the branch of the Swamp Foxtail flower,while V-thread and filament structures were almost similar to the two edges of a shorebird’s beaks.This resulted in the novel Tree 3D@SF structure.In addition,the filament surfaces of Auto 3D@SF and Tree 3D@SF were modified by using a double-layered spray method and applied Rayleigh instability/Furmidge principle to create hydrophobic/high hydrophobicity simulating the back bumps of desert beetles’ surfaces.Auto 3D@SF and Tree 3D@SF combined the synergistic advantages of droplet capture,droplet growth,and droplet shedding by exploiting the elasticity of the filament and modifying the surface of the filament.The water harvesting rate for Auto 3D@SF was twice as much as that of Original 3D@SF.The water harvesting rate of Tree 3D@SFs was about 100% higher than that of the original 3D@SF.This was mainly attributed to the hydrophobic dendritic threedimensional structure and the forced droplet convergence and shedding realized by an automatic system,which solved the problems of droplet clogging retention and timely droplet shedding.An automatic compression-relaxation system was designed and applied,which could compress,squeez and relax the elastic three-dimensional spacer fabric structure,so that the state of droplets on the surface of water harvesting sail could be dynamically changed and the droplets could be effectively migrated,gathered and fall off.This was also an important discovery of this study.It could provide reference for the design and application of fog catcher.In addition,it was also found that the angles between adjacent rainbow monofilaments and between rainbow monofilaments and V-shaped filaments would lead to the change of droplet state and position in the three-dimensional structure of "Tree 3D@SF".This was an interesting new discovery in the field of fog capture and water collection.In conclusion,the findings of this thesis revealed some new phenomena and principles involving the relationship between the behavior of droplets in the fog harvesting and the structure of the fog harvester.This is the first research that was developed and analytically reported to use the elasticity of textile threads and textile structures for fog harvesting.In this thesis,a variety of fog harvesters with high feasibility and practicability have been designed and prepared,which can be widely used in the practice of fog harvesting.In comparison with the previously studied fog harvesters,the vertical filament water harvesting sails,the automatic elastic thread water harvesting sails,the three-dimensional tree structure water harvesting sails and the automatic elastic fabric water harvesting sails prepared in this thesis can excel in fog harvesting.The water harvesting rates of all fog harvesters reached a high level,which was2.88,3.35,4.29,and 4.31 g/cm2/h,respectively.The results of this thesis have theoretical and practical reference value for the research in the field of fog harvesting and the research on the new types of fog harvesters.