The Condensation Property Of Super-wettability Material Surfaces And The Application In Fog-harvesting

It is well known that condensation is a common phenomenon in our daily life.When air encounters the substrate at low temperature,it is easy to condense and form droplets.At present,the main modes of condensation are filmwise condensation and dropwise condensation.Herein,dropwise condensation is much higher than film condensation in terms of phase change heat transfer.The main shedding mode is gravity-induced shedding and microdroplet coalesce-induced self-propelling.For filmwise condensation,droplets often pin to the substrate surface with the mode of filmwise,which can not roll down in time,which greatly reduces the service life and application performance of the material surfaces.Therefore,the research on the condensation property of the super-wettability material surfaces in this paper mainly focuses on the study and application of dropwise condensation.The surface condensation performance of the super-wettability material plays an important role in the application of heat transfer coefficient enhancement,self-cleaning,anti-icing and fog-harvesting.In nature,many creature surfaces have the super-wettability property,that can capture water from the environment under bad conditions,such as desert beetles,cactus spines,spider silk,pitcher plants,etc.,all of which have special structures on their surfaces.How to skillfully utilize their own unique structures to combine with super-wettability surface is important for the application of fog-harvesting.From the view of biomimic,this paper mainly studied the condensation properties of several super-wettability material surfaces and their corresponding research about the fog-harvesting performance.The specific research is as follows:(1)The preparation of TiO2 micro-nano composite structured substrate and their research about the condensate micro-droplet self-propelling performance.Based on the titanium substrate,TiO2 nanostructured samples with different structures were prepared,then the condensate micro-droplet(diameter<50 ?m)self-propelling performance was studied on three designed structured surfaces.Through analyzing and comparing the coalescence process diagram of condensate micro-droplets on different surfaces,the relationship between the TiO2 nanostructures and the condensate microdroplet self-propelling performance was investigated,that nano hierarchical played the most excellent condensate micro-droplet self-propelling performance because of its largest roughness,lowest adhesion force,more hydrophilic nucleation sites and more hydrophobic regions.(2)The preparation of alumina rod-capped nanopore structured substrate and their research about the condensate microdroplet self-propelling performance.Using one-step anodic oxidation with variable voltage method,the alumina rod-capped nanopore structure was constructed on the Al alloy surface.By adjusting the electrolyte concentration,temperature and reaction time,the controllable structure preparation was realized.We discussed the effects of surface morphology and wettability on the condensate microdroplet self-propelling performance through analyzing the coalescence process of condensate microdroplet on different surfaces.Analysis revealed that alumina rod-capped nanopore structure played an excellent condensate microdroplet self-propelling performance due to the collimation structure,more hydrophilic nucleation sites and superhydrophobic characteristics with low viscosity.(3)The slippery“fog-harvesting brush" preparation of alumina micro-needle structured substrate and their enhancement of the fog-harvesting performance.By using cutting method,“fog-harvesting brush" with different sizes were prepared.Then hydrothermal method was used to construct alumina microneedle on Al foil surface.By adjusting the reaction time and temperature,the controllable structure preparation was realized.Then polymethylhydrosiloxanes was sprayed on material surfaces to achieve surfaces with slippery coating.We further investigated the influence of wettability and pattern size on the fog-harvesting performance through fog-harvesting experiments.The results showed that the droplet fast transportation could be realized on the slippery surfaces with alumina microneedle structure.Comparing the performance of the "fog-harvesting brush" with the other sizes,the sample with small size could realize the efficient fog-harvesting performance.(4)The preparation of groove structure on the PET surface coated with aluminum and their enhancement of the fog-harvesting performance.By using cutting method,the groove structures with different space were prepared on the surfaces.Then a layer of hydroxyl was grafted onto the surface by UV/ozone treatment.Using spin coating method,polymethylhydrosiloxanes could fully contact with the hydroxyl on the surface,and then slippery film with lower hysteresis was generated on the surface.We further studied the influence of wettability and groove structure on the fog-harvesting performance through fog?harvesting experiments.The results showed that droplet can transport directionally on the samples with groove structure.Compared with the other grooved samples,the sample with small groove space could achieve rapid droplet transportation with the highest fog-harvesting efficiency because the droplet coalescence probability between adjacent micro-grooves was greatly improved.(5)The preparation of Kirigami structure on the PET surface coated with aluminum and their enhancement of the fog-harvesting performance.Based on the slippery coating of the PET surface coated with aluminum,the Kirigami structure was introduced by using cutting method.Then the samples with different wettabilities,tilting angles,shapes and sizes were designed and applied in the fog-harvesting test.Through analysis and comparison,we found that the hydrophobic-hydrophilic design was advantageous to droplet condensation and transportation;the tilting angle with 900 was advantageous to the fog flow,which was beneficial for the contact with the back surface,and further strengthened fog-harvesting efficiency;the triangle shape was advantageous to the water droplets on the pattern surface move from the top to the bottom area,combined with water droplets on both sides,and then rolled down from the surface;small size(width=2.8 mm)could increase the probability of combining with water droplets on both sides,thus improved the fog-harvesting efficiency.