Prevention of moisture content on aircraft structure surface based on lotus effect.
Solid surfaces are classified in terms of hydrophilicity or hydrophobicity that describes the wettability by water. A surface of hydrophobicity is easily wettable by water and hydrophobic surface is unwettable. The above behaviors can be correlated with surface energy of both water and solid material. The factors that influence the wetting behavior of surfaces are surface structures such as porosity, roughness, chemical heterogeneity or reactivity.
A super hydrophobic coating is a nanoscopic surface layer that repels water. Droplets hitting this kind of coating can fully rebound in the shape of column or pancake. The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing" and it describes the segregation of water and nonpolar substances, which allows maximizing hydrogen bonding between molecules of water and minimizing the area of contact between water and nonpolar molecules. A super hydrophobic coating is a nanoscopic surface layer that repels water. Droplets hitting this kind of coating can fully rebound in the shape of column or pancake.
The contact angle (CA) can be defined as the angle between liquid-gap interface and liquid-solid interface when a liquid droplet is placed on a solid surface. Hydrophobicity is usually determined by measuring the CA of water droplets on a surface as shown in Figure 1 (a). The contact angle hysteresis (CAH) is the difference between the advancing CA, [theta] and the receding CA, [[theta].sub.rec], as shown in Figure 1(b) are an important parameter to characterize the wetting phenomena on any surface. A super hydrophobic surface should have a low CAH (<[10.sup.0]) in addition to high CA (>150[degrees]).
While non-wetting means that the contact angle is greater than 90[degrees]. Generally, when the water contact angle is less than 90[degrees] the surface is called hydrophilic; when the contact angle is greater than 90[degrees] the surface is hydrophobic. A surface having a water contact angle greater than 150[degrees] is usually classified as ultra hydrophobic, i.e., a water-repellent surface.
Rajendra P. Bansod, Dr. Ashish and B. Deoghare (2013) reported on surface roughness analysis. Electrochemical polishing is a process of improving micro smoothness, micro topology and material brightness. Electro polishing streamlines the microscopic surface of a metal object by removing metal from the object's surface through an electrochemical process. In Electro polishing the anode surface is smoothened and brightened by the use of appropriate electrolyte under optimum conditions of current density and temperature.
Wen Li et al.  prepared super hydrophobic surfaces on Al substrates using a simple etching method. The aluminium is etched by mixture of HCl and water at various times. The etched Al plates were modified with 5 weight % ethanol Lauric acid solution for 1.5 hours and dried in air at room temperature for 2 hours. The CA is measured by using Goniometer. The microstructures of the super hydrophobic surfaces were observed using a SEM and EDX.
Markus Susoff et al.  evaluated the ice phobic coatings on aluminium substrate. The sol-gel coatings were synthesized by using silica precursors consisting of tetraethylorthosilicate (TEOS) and (3 -glycidylpropyl) trimethoxysilane (GPTMS) in different ratios. Diluted hydrochloric acid was used as a catalyst. The synthesized silica sol was fluorinated with flouro-alkalisilane (FAS). The fluorinated silica sol was applied on aluminium by dip coating. After dip-coating the substrates (aluminium), the coatings were cured for 1 hour at 1200 C.
A. Selecting a Material:
Material selection is the far most important criteria in the current scenario and it must be in accordance with the aircraft needs. Aluminium alloys are alloys in which Aluminium (Al) is the predominant metal. The typical alloying elements are Copper, Magnesium, Manganese, Silicon, Tin and Zinc. Based on the availability of material on market we have selected Aluminium 7075 T6 material. Dimensions of material: 3.5x19x16.5cm.
The material properties of Aluminium 7075 T6 material showed in Table 1.
B. Preparation of Material:
1. CUTTING: The material is cut into number of required pieces with specified dimensions.
2. SAND PAPER: The sand paper is used to clean the surface of the aluminium alloy to remove the impurities present.
3. BLUFFING OR POLISHING: This method is used to reduce the surface roughness created while using sand paper to remove impurities. It makes the surface smooth.
II. Experimental Investigation:
A. Preparation Of Substrate:
Purpose of cleaning is to degrease the metal surface and to remove any surface contamination and oxide layer. Initially the aluminium substrate is rinsed with distilled water and acetone. The rinsed aluminium substrate is immersed in sonicator for 20 minutes which contains distilled water. The ultrasonic sound waves produced in the sonicator cleans the surface and removes the contaminants.
B. Ultrasonic Cleaning:
Ultrasonic cleaning method is used to clean soft aluminium substrate. The industrial grade aluminium 7075 T6 plates were first surface-polished with fine metallographic sand paper to remove the surface oxidation layer and other impurities to achieve uniform roughness on the deposition surface. Purpose of cleaning is to degrease the metal surface and to remove any surface contamination and oxide layer.
C. Chemical Etching:
Chemical etching is a simple and cost effective method to produce the micro scale roughness. Chemical Etching is unique in that the process does not change the internal structure or properties of the metal. The grain structure, hardness and ductility of the materials are not affected.
This specimen was immersed in 20% of HCl solution for 2, 4, 6, 8 and 10 minutes respectively.
D. Hot Water Treatment:
This etched specimens were treated with hot water at 880C for 2 minutes. After this process it rubbed with cotton cloth. This purpose is to remove the residual acids.
E. Modification Process:
To obtain an appropriate Micro/Nano structure and lower the surface energy is based on Solution immersion method. After hot water treatment, it immersed in a dimethylformamide (DMF) / water and stearic acid ethanol solution (mass fraction of 1%) for 3 hours, before being taken out from the solution.The Super hydrophobic aluminium alloy sheets were finally obtained.
The experimental results and values of wettability, Roughness, Self cleaning ability and Ice delaying property are discussed below.
The figure 3 shows the water droplets on super hydrophobic treated Al 7075 alloy substrate, which obviously shows the change of wettability.
G. Roughness Test:
The roughness values were compared untreated and etched specimens. The following figures represents the roughness values of the specimen, when treated with HCL for various timings.
A super hydrophobic aluminium alloy surface is fabricated by treating in the acid bath, boiling water and immersing in STA-ethanol and DMF-[H.sub.2]O solution. After the chemical modification the wettability of the surface was changed from super hydrophilic to super hydrophobic with high CA as [158.sup.0] and a low sliding angles. Meanwhile, the etching time and STA-ethanol and DMF-[H.sub.2]O solution play important roles on the super hydrophobicity of the aluminium alloy, while the super hydrophobic aluminium alloy surface with water contact angle of [158.sup.0].
It shows a very simple and environment-friendly method for the fabrication of super hydrophobic surfaces overall sheets.
We would like to express our warmth filled thanks to our Department faculty members for the support to us in helping as to successfully complete this project.
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(1) A.Anandha Raja, (2) M.Sasi Kumar, (3) K.Nehru, (4) S.N.Logu, (5) C.Gayathri
(1,2,3,4) Assistant Professor, Dept.of Aeronautical Engineering, SNS College of Technology, Saravanampatti (PO), Coimbatore--35. (5) UG Scholar, Dept.of Aeronautical Engineering, SNS College of Technology, Saravanampatti (PO), Coimbatore--35.
Received 2 September 2016; Accepted 2 December 2016; Published 31 December 2016
Address For Correspondence:
A. Anandha Raja, Assistant Professor, Dept. of Aeronautical Engineering, SNS College of Technology, Saravanampatti (PO), Coimbatore--35.
Caption: Fig. 1: Schematics of a Liquid Drop on a (a) Rough Solid Surface and (b) Tilted Surface Showing Advancing and Receding Contact Angles.
Caption: Fig. 2: Represents prepared material for the further progress
Caption: Fig. 3: Super hydrophobic Al 7075 alloy substrate
Table 1: Material properties of Aluminium 7075 T6 material Al Alloy Grade Aluminium Other Ultimate Yield (%) Materials Tensile Strength (%) Strength (MPa) (MPa) 7075 Mg--2.4 T6 Mn--0.3 572 503 91.4 Cr--0.18 Cu--1.2 T7 Fe--0.5 505 435 Si--0.4 Ti--0.2 Zn--5.1 Fig. 4.1:2 minutes roughness test Roughness comparison 1 2 Specimen 1 0.27 1.79 Specimen 2 0.26 1.82 Note: Table made from bar graph Fig. 4.2:4 minutes roughness test Roughness comparison 1 2 Specimen 1 0.28 2.38 Specimen 2 0.27 2.45 Note: Table made from bar graph Fig. 4.3:6 minutes roughness test Roughness comparison 1 2 Specimen 1 0.27 2.94 Specimen 2 0.26 3.02 Note: Table made from bar graph Fig. 4.4:8 minutes roughness test Roughness comparison 1 2 Specimen 1 0.24 3.68 Specimen 2 0.26 3.72 Note: Table made from bar graph Fig. 4.5:10 minutes roughness test Roughness comparison 1 2 Specimen 1 0.26 4.21 Specimen 2 0.27 4.18 Note: Table made from bar graph
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|Author:||Raja, A. Anandha; Kumar, M. Sasi; Nehru, K.; Logu, S.N.; Gayathri, C.|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Dec 1, 2016|
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