Fabrications And Properties Of Biomimetic Fog Harvesting Surfaces Inspired By Desert Beetles

Over the past few decades,global shortage of fresh water has threatened sustainable development and ecological security all the world.It has become an argent problem for scientist to acquire fresh water in a facile and efficient way.Fog,consisting of a large number of tiny water droplets suspended in air,dominates as a water resource that cannot be ignored,especially in arid and semi-arid regions.Therefore,the collection of fog from the air is of great significance to alleviate freshwater crisis in these areas.In nature,many flora and fauna have evolved special surface structures and chemistries to harvest fog for their survival,for instance,alternate hydrophobic and hydrophilic pumps on the back of beetle,periodic spindle-knot structure on spider silk,the cactus conical spines and so on.Considerable researches reveal that anisotropic wettability and asymmetric conical structure could produce a wettability gradient and Laplace pressure difference,which are favorable to overcome capillary adhesion or contact angle hysteresis of surface,thereby realizing efficient transportation and collection of droplets.Inspired by these creatures,researchers have prepared a large of biomimetic fog harvesting materials,among which the hydrophobic/hydrophilic alternating surfaces inspired by beetles attract broad attention.There are currently two main strategies for preparing such surfaces,by randomly or precisely dispersing the hydrophilic regions on hydrophobic surface.The former has the advantage of being simple and easy to operate,while the latter usually involves complex techniques such as photolithography approach or inkjet printing technology.In the paper,the fog harvesting behaviors of fauna and flora in nature are firstly introduced,and the corresponding water collection mechanisms are explained.Then,related concepts and theories of surface wettability are elaborated.Afterwards,recent advances in bionic Namib desert beetle materials are systematically summarized.On the basis of these,two water collection surfaces inspired by beetles are prepared via two simple methods and the surface topography,chemical composition,wetting and fog harvesting performance of the samples are investigated.Firstly,a layer of Zn O nanorods was grown on stainless-steel mesh substrate by a hydrothermal method.Zn O nanorods were coated with Fe₂O₃ nanoparticles by a dip-coating and annealing process,which was subsequently modified with 1-octadecanethiol（ODT）/ethanol solution.The surface topography and chemical composition were characterized by SEM,XPS and XRD.ODT could selectively react with Zn O but not with Fe₂O₃,thus forming a hydrophobic/hydrophilic alternating surface similar to the back of beetles.The surface showed superhydrophobicity in air and underwater superoleophobicity.Interesting,the surface wettability could be tuned by changing the dip coating time,thereby affecting the water collection rate.When the dip time was 60 s,the highest water collection rate with a value of977.0 mg cm^-2 h^-1 could be achieved.Moreover,the surface presented an excellent reusability.Then,Cu（OH）₂ nanoribbons structure was grown on copper mesh by ammonia corrosion method,which was subsequently modified by two mixed thiols(HS（CH₂）₁₁CH₃ and HS（CH₂）₁₁OH)/ethanol solution.The surface topography and chemical composition were characterized by SEM,XPS and XRD.The methyl-terminated thiol was hydrophobic,while hydroxyl-terminated thiol was hydrophilic,resulting in a hydrophobic/hydrophilic alternating patterned surface at nanoscale.By increasing the molar fraction of methyl-terminated thiol in the mixed solution,surface with controllable wettability from superhydrophilic to superhydrophobic could be achieved.When molar fraction of methyl-terminated thiol was 0.6,the patterned surface showed the highest collection efficiency with the value of 1375.3 mg cm^-2 h^-1.