Influence Of Microstructure For Superhydrophobic Surfaces On Superhydrophobicity And Its Application

The wetting of the surfaces, as an important property, is associated with therelevant physical and chemical process, i.e., absorption, lubrication, adhesion,dispersion and friction. Recently, the superhydrophobic surfaces (SHS) have attractedthe widespread attentions from both academic and industrial fields due to its potentialapplications in self–cleaning, micro–fluid system and biologic compatibility.However, up to now, two aspects of both theoretical and experimental study are faraway the level of making the SHS be of practicality. There are experimentally nopractical methods exploited to prepare the durable SHS or the special SHS that areapplied to the specific environment; and also there are theoretically no perfectconceptions used for explantion of micro–wetting mechanisms of the SHS. From boththe wetting and movement of the droplets, both Young and Cassie equations as wellas Wenzel equation are only a static description for the droplets, not involving thequantitative description for the movement and the hysteresis of the droplets, and alsonot establishing the relationships between the surface wetting properties and themicrostructures. Although the researchers have made a great effort to settle down thedescription for the movement in thermodynamics, the used methods aremathematically obscure and fall into the phenomenon description. Based on thepresent situation, we attempt to search for the methods to solve the problems on thebasis of the wetting of the SHS for the purpose of establishing the relationshipsbetween the static apparent contact angle (CA) with dynamic angle (includingadvancing, receding CAs together with sliding angles) as well as between thehydrophobic properties and the surface microstructures.Meanwhile, the SHS again shows a new application in the icing of thetransmitting lines. The study indicates that the SHS can effectively inhibit the icingfrom the droplets with macro–scale, delay the time to ice, and reduce the quantity ofthe icing and the adhesion between the icing and them. Therefore, we attempt to applythe SHS to the anti–icing of the high–voltage cables and further analyze the anti–icingmechanism of the SHS and the possible methods used for the anti–icing.Reviewing the study, we have finished the following work:Firstly, the relationships have been established between the static apparent CAand the dynamic CAs, including advancing or receding CAs, by considering the SHSas our object of our study. Based on the quantitative and qualitative experiments, we analyzed the variations of both the apparent CA and the changes in the radius of thethree–phase–contact line along with free energy among the initial, pre–advancing,advancing, pre–receding, receding states, and put forward two new conceptions ofboth changes in free energy (CFE) and free energy barrier (FEB) to explain themovement of the droplets on the SHS.Secondly, based on the conclusion that the hydrophobic properties of the SHSdepends mainly on the surface microstructures, we discussed the wetting of thesystem of both the droplets and the surfaces in the composite/non–composite wettingstates on the ground of the Cassie/Wenzel equations respectively, and established therelationships between the hydrophobic properties and the surface microstructures byboth the solid fraction and the roughness factor of a three–dimensional model, anddiscussed the variations of the properties of the SHS with the microstructures.(1) On the basis of the one–step microstructures of some artificial and naturalSHS, we proposed one–step model for simulation of the SHS, and established therelationships between the hydrophobic properties, i.e., the apparent CAs, CAH, CFE,FEB, adhesion work (Wa), the spreading coefficient (SS/L), and the width, heighttogether with spacing of a pillar according to composite/non–composite wetting state;and explained that the energy barrier, being overcome by a droplet while it moves, isindependent of the state of the solid–liquid interfaces within the three–phase contactline, and depends mainly on the materials and microstructures near the three–phasecontact line; meanwhile mathematically explained that the SHS, e.g., a lotus leafsurface, lost their hydrophobic ability to the condensed vapor.(2) Similarly, enlightened by a two–step micro–structure, we developed thetwo–step model to simulate the lotus leaf surfaces, and established the relationshipsbetween the hydrophobic properties of the lotus–simulating surfaces andmirostructures; and mathematically explained why the two–step microstructures caneffectively reduce the CAH; and discussed the transition between the composite andnoncomposite wetting states from both the energy barrier and adhesion work point ofview.(3) Then on the basis of the above work, we discussed the hydrophobicproperties of the SHS with different dimension by modeling in fractal geometry; andproposed that the transition between composite and non–composite wetting isindependence of the fractal dimension and only determined by the microscopicroughness; and found that the critical roughness factor for the transition is1.8or so, which is in agreement with the one from the changes in CAH. By comparing, wefurther found that designing the SHS with two–step microstructure is enough tosatisfy the requirement of the hydrophobicity; higher dimensional surfaces, forexample, three–dimensional surfaces are unnecessary; meanwhile, the SHS withone–step microstructures may be an ideal selection, if our considering both thedurability and the surface formation.Thirdly, based on the study of the SHS, the thermodynamic mechanism for theanti–icing was also discussed. In view of the inconvenience of the measurement of theadhesion force and the existing measurement not being uniform, the anti–icingproperty of the SHS was analyzed only from the adhesion work point of view in thisstudy. The study indicates that the anti–icing property of the SHS depends mainly onboth the surface materials and microstructures as demonstrated for the hydrophobicproperty；when the system of both the surfaces and the droplets is in the compositewetting state, the SHS surely have the ability to anti–icing due to the main role of themicrostructures; conversely, the anti–icing property was related to the materials in thenon–composite wetting state, both the hydrophilic and hydrophobic materials showingthe icing and anti–icing respectively.Fourthly, considering both the complexity of the anti–icing process and theimperfect techniques for the superhydrophobicity, we have designed a new–type ofhigh–voltage cable with anti–icing property based on the compressed steel–corealuminum alloy twisted lines used in the heavy icing area, integrating theelectricity–heating techniques. The cable was designed to anti–ice with two lines:those which the iron–chrome–aluminum lines inserted into the cable are to heat withthe action of the current; the others which the superhydrophobic coatings are towaterproof and anti–ice. With the iron–chrome–aluminum lines connected to thepower, the ice directly contact with cable surface is melted firstly; and thecorresponding water is repelled by the hydrophobic coatings. When the ice is meltedto some extent, the ice will slide off the cables with its gravitation. The two lines canwork cooperatively or independently as well.