RUAN Min, XU Junjie, FAN Shilin, CHEN Ying, WU Hang, CHANG Zhongwei,CHEN Yue, ZHAO Dongnan, LU Lilin

(1. Institute of Materials Science and Engineering, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, Hubei Polytechnic University, Huangshi 435003, China; 2. College of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430080, China)

Abstract: The effects of modifiers on the anti-wetting and anti-icing property of the prepared rough aluminum surface were investigated. The rough aluminum substrates were obtained through electrochemical oxidization with 15 wt% sulfuric acid solution as the electrolyte at the constant current of 4 mA for 3 h. And then they were modified with octadecanoic acid (C18), polyethylene (PE), polystyrene (PS), polyethylene glycol (PEG) and hexamethylenetetramine (HMTA), respectively, whose surface free energies were 27.6, 31.0,33.0, 61.6 and 70.0 mN/m, respectively. The contact angles (CA) were 154.6°, 128.4°, 127.6°, 5.0° and 0.0°,respectively, and the ice adhesion pressures were 15.9, 36.3, 55.9, 155.3 and 216.1 kPa, respectively. The ice adhesion strengths decrease along with the increasing anti-wetting property of aluminum surfaces and the decreasing of the surface energy of modifiers. These provide some new insights when designing the aluminum surface with anti-icing properties in some special applications.

Key words: modifier; aluminum; hydrophobicity; ice adhesion; surface energy

1 Introduction

Icing can bring undesirable troubles to our daily life, resulting in great economic losses[1,2]. Inspired by the Lotus Effect, superhydrophobic surfaces are considered to be a promising anti-icing strategy without any energy consumption[1,3]. Many new strategies are applied in the anti-icing field, such as slippery liquid-infused porous method (SLIP)[4,5]. McKinleyet al[6]designed self-lubricating icephobic coating by PDMS- PEG amphiphilic copolymers with low ice adhesion strength values of 50 kPa. Wanget al[7]employed lubricant PU polymer to form self-lubricating liquid water layer to decrease the ice adhesion to be 30 kPa. Jianget al[8]employed PAA grafted inside the MPS silicon wafer using PEGDA as the cross-linker to form a self-lubricating liquid water layer with the ice adhesion of about 55 kPa.

An interfacial cohesive strength, as represented by the ice-adhesion strength, is one way to describe the bonding across an interface[9]. The efficacy of these coatings has been evaluated by measuring the force,F, to debond a specified area,A, of ice, and defining an ice-adhesion strengthP=F/Aas the characteristic property for the system[10]. Golovin demonstrates that the shear strength of the interface, controls delamination when the length of the interface is relatively small[11]. Ice adhesion is treated according to the surface energy[12]or interfacial cavitation[13]. Icephobic surfaces are defined by an ice adhesion strength < 100 kPa[13,14]There are no available commercial instruments designed to measure solid-solid adhesion strengths.[10]A few groups have deposited glaze ice by spraying supercooled water droplets over test substrates and then measured the average strength of ice adhesion using a centrifuge apparatus[15,16]. A number of other groups used apparatus that involve pouring liquid water onto a substrate, freezing the water, and then measuring the average stress required to remove the ice from the surface[3,17].

There are many factors influencing icephobic property, such as the roughness of surface[18], the adsorption configuration[19]and stability[20]of modifiers coated on surface. Chenet al[18]shows that the icephobic property is closely related with the anti-wetting property of surfaces. Arianpouret al[19]states that the dense and ordered layers of SAM formations on the aluminum oxide layer may significantly decrease the values of adhesion strength. McKinley[21]suggestes that the nano-blended PVA/PAA multilayer with PEG segments has the ability to rapidly absorb molecular water by using hydrogen-bonding-assisted layer-by-layer (LbL).Zuoet alfound that the smoother silicon-doped surfaces with lower surface energy have lower ice adhesion strength[22]. When modifier forms a monolayer on surface, the surface energy of the vertical adorption is much smaller than the parrallel[23]. In order to know details of modifiers effect on the icephobicity of aluminum surface, the roughness of aluminum surface was fixed with the same electrochemical anodic oxidtation time. Modifier types with different surface energy are designed to modify the rough aluminum surfaces, and the effects of modifiers on ice are investigated experimentall and theoretically in this work. The modified aluminum surfaces are charactorizedwith the CA, SEM, ice adhesion strength and the banding energy of the modifier with ice were calculated with density functionl theory.

2 Experimental

2.1 Materials and methods

The surface morphology of the samples was observed using JSM-7610F scanning electron microscopy(SEM). The roughness of surfaces was fabricated with CS310 CS electrochemical station. The ice adhesion pressure was tested using home-made equipment.

2.1.1 Pretreatment of aluminum sheet surfaces

The aluminum sheet with 15 mm×35 mm×1 mm size was polished with 800#, 1 000# and 2 000# sandpaper, respectively until the surface is bright, and then was washed in an ultrasonic cleaning machine with acetone, anhydrous ethanol and deionized water for 10, 5 and 5 min respectively, and finally was dried in oven at 60 ℃.

2.1.2 Electrochemical oxidation of aluminum surfaces

The roughness of the aluminum surface was fabricated with electrochemical anode oxidation method.The prepared aluminum sheet oxidized with 15 wt%sulfuric acid solution was used as the electrolyte at the constant current of 4 mA for1, 2, 1.5, 2.5 and 3 h,respectively, followed by modified with 0.15 mol/L octadecane acid solution, then the sheet was dried. The oxidation time was selected according to the CA of the modified surfaces.

2.1.3 The prepared aluminum surfaces modified with different modifiers

After the aluminum sheet was oxidized with 15 wt% sulfuric acid solution at the current of 4 mA for 3 h, washed with deionized water and dried in the oven,it was modified with different types of modifiers.

The prepared rough aluminum sheet was soaked into the 2 wt% C18 ethanol solution for 30 min, and was cleaned in anhydrous ethanol for 3 s with ultrasonic.

2 wt% PE was dissolved in xylene at 110 ℃ with magnetic mixing at the speed of 400 r/min until the solution was transparent. Then the pretreated aluminum sheet was immersed in the solution for 10 s and dried at 60 ℃.

0.5 g PS was gradually dissolved into 10 mL tetrahydrofuran with magnetic stirring. The PS was separated using phase separation method with 49%(v/v) ethanol. The solution was dropped on the prepared aluminum surface followed by being dried for 10 h in atmosphere.

The prepared aluminum plates were immersed in 2 wt% PEG aqueous solutions for 30 min, respectively,and dried at 60 ℃.

The prepared aluminum sheet was immersed in the 0.5 mol/L HMTA aqueous solution for 1.5 h at 180℃, and was dried at 60 ℃ for 1 h.

2.2 Characterization

2.2.1 CA characterization

Values of CA were measured on FTA1000,First Ten Angstroms, USA following standard procedures. The measurements were performed using the Young-Laplace method that is theoretically considered as the most accurate since the distorted drop (4 µL)shape due to the liquid weight is taken into account.The CA is the average value of the five measurements.

2.2.2 Icing adhesion strength test

Fig.1 The diagram of home-made icing adhesion measurement

The icing adhesion strength of the samples was tested with a home-made icing monitoring equipment that was shown in Fig.1. This equipment consists of cooling system, mechanical system, force sensor, temperature sensor, displaying screen and sample stage.Temperature sensor and force sensor connects with a separate display screen respectively. The samples surfaces are fixed on the sample stages. A buckled colorimetric dish with internal area of 1 cm2is fixed on the sample surface, and it is filled with distilled water with 1/3 of the total volume. When the force sensor works,the screen displays the actual value of the force sensed in real time and records the minimumFvalue that need to deboned the ice from the sample surface. The water column will freeze into ice completely in the cooling system. The ice-adhesion strength of iceP=F/A=F(N) / 1×1 (cm2) =F(N) / 10-4(m2) = 10 ×F(kPa)as the characteristic property for the system.Fis the minimum force to push down the ice column from the surface, andAis the internal area of the cuvette.

3 Results and discussion

3.1 The influence of oxidation time on the hydrophobicity of surface

The rough aluminum surface was fabricated with electrochemical anode oxidation method. When it was oxidized with 15 wt % sulfuric acid solution at the constant current of 4 mA for 1, 2, 1.5, 2.5 and 3 h, respectively, and was dried, the CA is 37.2, 16.6, 17.9, 19.9 and 19.8º, respectively as shown in Fig. 2. If the prepared aluminum surface was modified with 0.15 mol/L C18 solution, the CA is 119.8, 140.1, 144.9, 150.5 and 154.6º, respectively. It can be seen that the all of the surfaces have bigger CA when they are modified with C18 compared with the surface without modification with the same roughness. The CAs of the unmodified oxidized aluminum surfaces are all smaller than 40°with the oxidation time increasing to 3 h, while the CAs of the aluminum surfaces that modified with C18 increased from 119.8º to 154.6º with the oxidation time increasing from 1 to 3 h. The aluminum surface was superhydrophobic with the CA of 154.6° due to the synergistic effect of modification and roughness.

The SEM show the morphologies of aluminum surfaces oxidized with 1 and 3 h, respectively in Fig.3.Fig.3(a) and 3(b) are for the aluminum surface with 1 h anodic oxidation with 5 K and 100 K magnification,respectively. Fig.3(c) and 3(d) are for the aluminum surface with 3 h anodic oxidation with 5 K and 100 K magnification, respectively. It can be clearly seen that the honeycomb holes of (c) and (d) distributed uniformly with nano-scale. The mastoids between the holes in (b) are much higher than in (d), resulting the CA of (b) and (d) is 119.8°and 154.6°, respectively. Anodic oxidation of 3 h was selected for constructing the roughness of aluminum surface at the constant current of 4 mA.

Fig.2 The influence of oxidation time on the hydrophobicity of aluminum surface

Fig.3 The aluminum surface morphology with different anodic oxidation time and magnification: (a) 1 h(5 K); (b)1 h(100 K); (c)3 h(5 K); (d)3 h(100 K)

3.2 The influence of modifier on the hydrophobic/icephobic properties

Fig.4 shows the surface energy, the CA and ice adhesion strength along with the types of modifiers. It can be seen that when the aluminum surface modified with C18, PE, PS, PEG and HMTA, whose surface energy is 27.6[24], 31.0[24,25], 33.0[24], 61.6[26]and 70.0[27]mN/m, respectively, the CA is 154.6°, 128.4°, 127.6°,5.0° and 0.0°, respectively and ice adhesion strength is 15.9, 36.3, 55.9, 155.3 and 216.1 kPa, respectively.

The surface energy of water and ice is 72.8 and 82[24]mN/m, respectively. The ice adhesion pressure is 15.9, 36.3, 55.9, 155.3 and 216.1 kPa for the surface modified with C18, PE, PS, PEG and HMTA respectively. It can be seen that the ice adhesion strength is closely related with the wetting and surface energy of the aluminum surface. Different with reference[22], ice adhesion strength decreased with decreasing the surface energy through changing the surface roughness. The ice adhesion strength decreased with the decreasing surface energy and the increasing anti-wetting property with the roughness of aluminum surface unchanged in this study.

Fig.4 The influence of modifier on ice adhesion strength and CA of the prepared aluminum surfaces

Fig.5 Microstructures of aluminum surface modified with different modifiers: (a) C18; (b) PE; (c) PS; (d) PEG; (e) HMTA

The SEM of the aluminum surface modifier with C18, PE, PS, PEG and HMTA are shown in Fig.5, and the molecular structures of the modifiers are listed in Fig.6. C18 is a saturated fatty acid and contains -COOH group with significant acid–base interactions that can interact strongly with metal oxide surface as shown in Fig.5(a), which resulting in significant mechanical strength[24]. The rough aluminums surface has a uniform mastoid microstructure that is similar with lotus surface from Fig.5(a). The surface energy of C18 is only 27.6 mN/m, and when it reacts with the rough aluminum surface, the surface free energy of the surface can be greatly reduced, resulting the (CH2)16CH3hydrophobic group toward the air, resulting the superhydrophobicity of the surface and the small ice adhesion strength of 15.9 kPa.

When the rough aluminum surface was modified with PE, the microstructure is rough with the shape of rope junction according to Fig.5(b). And it has the shape of ball when the surface is modified with PS as shown in Fig.5(c). The CA is 128.4° and 127.6° of the surfaces modified with PE and PS respectively, and the ice adhesion is 36.3 and 55.9 kPa respectively. It may be because of the difference surface energy of PE and PS, 31.0 and 33.0 mN/m, respectively. Furthermore, the interactions between the nonpolar modifier that can be illustrated as Fig.5(b) and 5(c) and metal oxide surface is weak due to the dispersion forces.

Fig.6 The molecular structures of modifiers

Fig.5(d) shows that the surface is micro-scale with large and uneven grooves when the rough aluminum surface is modified with PET with the CA of 5.0°.PEG is surperhydrophilic because of the large amount of ethoxy and hydroxyl groups in the molecule shown as in Fig.5(d) with the surface energy of 61.6 mN/m,which can react with aluminum oxide surface and form hydrogen bonds with H2O, reducing to the large ice adhesion strength of 155.3 kPa.

The microstructure of the aluminum surface modified with HMTA is shown in Fig.5 (e). There are many micro-scale chrysanthemum flower shape crystals forming on the aluminum substance, and the grooves between the flowers are deep and in micro-scale, reducing the superhydrophilicity of the surface with CA of 0.0°and the large ice adhesion strength of 216.3 kPa. Furthermore, HMTA shown as in Fig.5 (e) can react with aluminum through C6H12N4+4H2O→C6H16N44++4OHand 2Al+6H2O→Al(OH)3↓+3H2↑ to form Al(OH)3with high roughness and surface energy, reducing the superhydrophilicity with high ice adhesion strength.

4 Conclusions

The aluminum surface was electrochemical oxidized in the 15 wt% sulfuric acid electrolyte solution at the constant current of 4 mA for 3 h to construct the roughness. Then the surface was modified with C18,PE, PS, PEG and HMTA with the surface free energy of 27.6, 31.0, 33.0, 61.6 and 70.0 mN/m respectively. It shows that the CA of the fabricated aluminum surface is 154.6°, 128.4°, 127.6°, 5.0° and 0.0°, and the ice adhesion pressure is 15.9, 36.3, 55.9, 155.3 and 216.1 kPa when the surface is modified withC18, PE, PS, PEG and HMTA respectively. It can be seen that the ice adhesion strength is closely related with the anti-wetting property and surface energy of the aluminum surface.The ice adhesion strength decreased with the same roughness of the substrate aluminum surface when it was modified with modifiers with the decreasing surface energy and the increasing anti-wetting property.

Conflict of interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.