Analysis Of Droplet Phase Transition Processes On Special Wettability Surface And Their Application

In recent years,surfaces with special wettability derived from bionics have been investigated and applied in anti-corrosion,anti-icing,anti-fouling,heat and mass transfer enhancement,droplets manipulation,desalination and water collection.Fields and processes mentioned above basically involve the participation of water,especially the phase transition processes of water droplets.The characteristics of water droplets on surfaces with special wettability have become a research hotspot.At present,single phase transition process such as evaporation,condensation and icing of water droplet on the surface has received more attention and investigation.However,phase transition processes in actual situation are often more complicated.Several phase transition processes exist successively or simultaneously.In addition,there are some components in water droplets instead of pure water.Comprehensive research on the phase transition processes of droplets on surfaces with special wettability will be of greater practical engineering significance.Based on the engineering application,surfaces with different wettability were fabricated and characterized.Besides,the phase transition processes of droplets on these surfaces were observed to obtain the specific data of the effects of the surfaces.Finally,the application potential of the special wettability surfaces in water collection and anti-icing was analyzed.The main findings of this paper were concluded as follows:(1)The titanium was anodized and photocatalyzed to fabricate superhydrophilic,hydrophilic,hydrophobic,superhydrophobic and patterned surfaces.In order to improve durability of surface,polydimethylsiloxane was used as repair agent and tubular fluorinated halloysite and spherical fluorinated titanium dioxide were used as hydrophobic particles.Three types of self-healing superhydrophobic surfaces with different morphologies were prepared by adjusting the content and distribution of two kinds of hydrophobic particles.(2)The liquid-gas phase transition process(evaporation)of the droplets on surfaces with different wettability was investigated.At 40??60?,the substrate temperature increases by10? each time would reduce the evaporation time of pure water droplet and Na Cl droplet by at least 40% and 30% respectively.The increase in surface hydrophilicity was conducive to evaporation,but the adhesion area of the particle layer deposited by the Na Cl droplets became larger.Droplet with higher Na Cl concentration facilitated crystallization,but had no effects on the evaporation mode.When the droplets evaporated on the patterned surface,they would shrink perpendicular to the outline of the pattern.The stripe patterned surface showed the fastest evaporation rate and conducive to the separation of Na Cl.(3)The gas-liquid phase transition process(condensation)of droplets on surfaces with different wettability was investigated.The coverage rate of droplets on the hydrophilic surfaces was significantly greater than that on the hydrophobic surfaces.The droplets on the hydrophobic areas of the patterned surfaces tended to move to the hydrophilic areas.While the limited droplets capacity of the hydrophilic area made the water patterns in adjacent hydrophilic areas present a tendency to merge.Reasonable design of hydrophilic area pattern,higher proportion of the hydrophilic areas and smaller distance between the hydrophilic areas might be helpful for the merge of condensed water but they were not necessarily beneficial for the whole condensation process.(4)The liquid-solid phase transition process(icing)of droplets on surfaces with different wettability was investigated.Superhydrophobic surfaces displayed obvious effect of delaying freezing.The surfaces with the same microstructure and less degradation of hydrophobic substances possessed better icing delay performance and the droplet profile parameters change more obviously,which resulted from the ability of trapping more air during the cooling process.When designing anti-icing surfaces,it was necessary to take into account the ability of the surface microstructure to capture air and trap air during the cooling process.In addition,materials with lower thermal conductivity were helpful for delaying icing.(5)The water collection and anti-icing performances of surfaces with different wettability were investigated.Surface with more hydrophilic wettability possessed lower water collection efficiency.Surface patterning was helpful for water collection.On square patterned surfaces with the same ratio of hydrophilic-hydrophobic area,longer length of the grid was more helpful for water collection.However,the increasment of the proportion of hydrophilic area did not necessarily benefit water collection.The water collection efficiency was: stripe>square?hexagonal.Compared with the hydrophobic surface and the hydrophilic surface,the superhydrophobic surface possessed better anti-icing performance,which could delay the freezing time of droplets by about 68% and 374%,and reduce the ice adhesion strength by about 84% and 89%,respectively.