Experimental And Theoretical Investigations On The Al-based Superhydrophobic Materials And Their Anti-Icing Activity

Ice accretion or accumulation on high-voltage transmission lines and aircrafts is one of the most powerful natural hazards. Various de-icing techniques including the thermal and mechanical methods are kinds of rather passive approaches. One active solution has been suggested to use the superhydrophobic (SHP) materials. However, at present the SHP materials are still subject to the poor mechanical strength, the complicated preparation approaches, and the high cost. Meanwhile, few experimental instruments are available for the in situ characterization of the icing process. Therefore, the studies of the SHP materials in the field of ice accretion are less extensive than its wide application in other techniques.As an attempt to explore the efficiency of SHP materials for anti-icing, extensive experimental and theoretical investigations on the Al-and alloy-based SHP materials have been carried out systematically in this thesis. Firstly, the methods for the preparation of the SHP materials were screened to find the experimental protocols which are simple, reliable, and inexpensive. Secondly, a two-stage refrigerating apparatus was built-up for the first time to characterize quantitatively the in-situ anti-icing behavior of the fabricated SHP materials. Moreover, the microscopic structures and anti-icing mechanisms of the SHP materials have been revealed at the atomic and molecular levels using quantum chemical calculations and molecular dynamics simulations.1. A new methodology for the preparation of Al-based SHP materials.It was found that the SHP aluminum surface could be produced by using the cheaper Lauric acid as fabrication reagent. The good performance of SHP material has been demonstrated by the relatively large contact angle of167.5°and the low contact angle hysteresis of only2.3°. The experimental conditions are as follows:the aluminum plate was etched in Beck’s dislocation for15seconds and then modified for1.5hours with5wt%Lauric acid-ethanol solution. As indicated by the SEM、EDS、 FTIR analysis, the Al-based SHP materials involve the hydrophobic structures similar to lotus leaves. 2. A modified methodology for the preparation of Al-alloy-based SHP materials.In the consideration of the efficiency, the chemical etching method is generally more affordable than the anodic oxidation method. It is worth noting that the one-step etching procedure is even more effective than the regular two-step method. More interestingly, the experimental condition could be even milder if the etching reagent is the mixture of aqueous FeCl3and HCl. The procedure is as follows:the Al-alloy substrate was etched with20wt%FeCl3,3wt%HCl, and5wt%Lauric acid solution in40℃water bath for around20minutes. The contact angle and the contact angle hysteresis of such SHP materials are159.1°and4.0°, respectively.3. An integrated real-time and in situ ice accretion experimental apparatus.On the basis of the two-stage refrigerating system, an integrated real-time data collecting and analysis apparatus has been designed to characterize the anti-icing performance of the SHP materials. The present micro system is capable of providing the wide ranges of icing temperatures (-10～2℃), wind speeds (0-30grades adjustable), droplets speeds (0-20grades adjustable), and humidity for the typical sizes (1.5cm×1.5cm) of the samples. The anti-icing properties of the fabricated Al and Al-alloy SHP materials have been characterized quantitatively. Compared with the normal Al plates, the icing time of the SHP surfaces could be delayed by60%and the icing temperature could be lowered by up to220%. For instance, water droplets start to freeze at-2.2℃on the pre-conditioned Al-alloy surface in406seconds. In contrast, the fabricated SHP surface does not involve any icing until-6.1℃after676seconds. For the tilted surfaces, the icing temperatures are-3.9℃and below-8.0℃for the normal Al-alloy surface and the SHP surface, respectively. In conclusion, the Lauric acid fabricated Al-and Al-alloy SHP materials exhibit generally good anti-icing performance.4. Microscopic mechanisms for the adsorption and dissociation of the acid fabrication reagent on the alumina substrate.The fabrication reagent is supposed to be attached on the outer oxidation layers of Al and Al-alloy. In order to reveal the molecular basis of the SHP nature after acid fabrication, formic acid, a prototype fabrication reagent, and the most stable α-Al2O3(0001) surface, have been employed to investigate the chemical adsorption mechanisms using the density functional theory. Two adsorption pathways were revealed for the first time, namely, carbonyl and hydroxyl adsorption, forming the partially protonated surfaces with the ROCO-A1bonding structures with an exothermicity of about50kcal/mol. The chemically modified surfaces were confirmed by the reasonable agreement between the theoretical and experimental infrared spectra. In view of the Laurie acid, it should be bonded tightly to the alumina surface via the carbonyl group and the nonpolar hydrophobic hydrocarbon tail is outward the surface. In addition, the dehydrogenation and dehydration mechanisms for the catalyzed decomposition of formic acid were studied as well. The corresponding dissociation barriers are both lowered by about30kcal/mol in comparison with those in the gas phase.5. Molecular picture for the hydrophobic nature of the acid fabricated surfaces.Intermolecular interactions between water droplets and fabrication acids on the alumina surfaces were investigated using the molecular dynamics simulation with the ab initio COMPASS force filed. It is revealed that the hydrophobic surface could not be produced until the concentrations of Laurie acid are high enough to form the dense monolayer. However, if the concentrations of Laurie acid increase beyond the critical values, the hydrophobic ability decreases because of the direct solvation of acid molecules in water starts to play an important role. In contrary to dodecanethiol, the Laurie acid molecules prefer to be adsorbed perpendicularly to the surface monolayer. Moreover, the longer the nonpolar hydrocarbon tails, the better ordered the monolayer. The molecular dynamics simulations provide a qualitative understanding of the superhydrophobic nature of the acid fabricated materials.