Preparation and anti-frosting performance of super-hydrophobic surface based on copper foil
Introduction
Frost formation is a well-known and undesirable phenomenon in cryogenics, refrigeration, air conditioning, and aerospace industries. The continuous and uncontrolled frost layer formed on the heat transfer surfaces will adversely affect the performance of the refrigeration system due to the additional pressure drop and thermal resistance. Thus, defrosting is required to remove frost layer periodically in most applications. This of course will increase both operation cost and energy consumption. As the development of refrigeration, cryogenics and aerospace industries, investigation concerning the frost formation has received great attention in recent years [1], [2], [3]. Numerous researchers have focused themselves on the investigation of frost formation mechanism and try to find an effective defrosting method. They discussed the factors that influence the frost formation such as the ambient conditions, cold surface temperature and surface characteristics [4], [5], [6]. Lee et al. [7] and Liu et al. [8] have investigated the effect of surface energy on frost formation under free convection. They found that surface hydrophilicity is one of the more advanced and attractive methods to reduce frost formation on cold surfaces. Furthermore, some researchers tried to retard frost formation by means of additional force such as electric field [9], magnetic field [10], ultrasonic [11] and mechanical oscillation [12]. However, their experimental results demonstrate that these methods have no remarkable influence on the frost layer full growth period as the increasing of frost layer thickness and density.
Recently, super-hydrophobic surfaces with water contact angle higher than 150° have attracted special attention due to their unique properties and many potential applications [13]. A perfect example of super-hydrophobic surface from nature is the lotus leaf. When water falls on the lotus leaves, it formed nearly perfect spheres that readily roll off the surface, collecting particulates along the way. This phenomenon was firstly put forward by Barthlott and Neinhuis [14] as the Lotus Effect. Such surface shows a double roughness along with a waxy coating, resulting in high contact angle of water on it [15]. Presently, there is an intense interest in mimicking the lotus leaf to produce artificial super-hydrophobic surface for a wide range of potential applications [16], [17], [18], [19], [20]. Although great successes have been made in the fabrication of such surfaces, most of these methods were subject to certain limitations, such as severe conditions, multistep processes, expensive materials, poor durability, etc. Therefore, simple and cost effective approaches are highly desirable for the fabrication of biomimetic super-hydrophobic surfaces. On another hand, inspired by this kind of binary micro/nano structures and low energy surface, experimental research about the dropwise condensation and frost formation on super-hydrophobic were carried out by some researchers [21], [22]. Wu et al. [23] investigated the possibility of frost release from a cold surface; they found that the frost pattern on the hydrophobic surface was non-uniform and “pock-marked”. However, their experiments results were unsuccessfully in removing frost from a hydrophobic surface by mechanical vibration. Liu et al. [24] observed the frost deposition on a cold super-hydrophobic surface with water contact angle of 162°. The frost structure on the super-hydrophobic surface is looser, thin and shows a cluster of chrysanthemum-like pattern. The frost deposition on the surface was delayed for 55 min compared with the plain copper surface under natural conditions. The above research results revealed that increasing the contact angle by surface treating can restrain crystal nucleation and growth and thus frost deposition. However, applications of these manmade super-hydrophobic surfaces to refrigeration systems are still at the premature stage, mainly due to the lack of facile ways for large scale fabrication and insufficient durability of the prepared super-hydrophobic structures. Hence, further research work on anti-frosting performance is required if we want to obtain practical application of the super-hydrophobic surface to refrigeration systems.
In the present work, a kind of flower-like super-hydrophobic surface on the copper foil substrate was fabricated by means of a facile method adopting from Hou et al. [16]. The surface microstructure and the contact angle were investigated by the Scanning Electron Microscope (SEM) and Dynamic Contact Angle (DCA), respectively. A series of micro-observations of the water droplets formation, initial frost crystals growth and frost layer melting process were carried out on both the super-hydrophobic surface and the plain copper foil surface. The stability of anti-frosting performance was also test after the super-hydrophobic surface exposed to the air for one month. The mechanism of surface hydrophobicity influence on frost formation was analyzed theoretically based on the microstructure and wettability of the solid surface.
Section snippets
Experimental apparatus
Fig. 1 illustrates the experimental apparatus. The experimental setup mainly consists of cooling setting, microscopic image system, data acquisition system, a digital camera, optical fiber luminescence, air-conditioning system and humidity controller. The cooling system was basically a thermoelectric cooler that can provide a temperature as low as −26 °C with a relative uncertainty of ±0.1 K. A copper plate of 150 mm × 52 mm × 6 mm was mounted on the cooling unit and the plate was polished by 2000#
Results and discussion
In our previous work, the experimental results indicated that increasing the contact angle can increase the potential barrier and restrain crystal nucleation and thus frost deposition, it was also found that the formation of water droplets and frost crystals on the super-hydrophobic surface shows remarkable differences to that on a plain copper surface [8], [24]. In this paper, a closed observation was made of the initial frost crystals growth on both the flower-like super-hydrophobic surface
Conclusions
In summary, a flower-like super-hydrophobic surface on the copper foil substrate was successfully fabricated by means of a combination of chemical etch and fluorination modification. This kind of surface has double-scaled roughness structures and low surface energy as the lotus leaf when observed by SEM. Furthermore, the water contact angle is as high as 156.2° and the water droplet on the surface presents an ideal spherical shape. A series of micro-observations of the water droplets formation,
Acknowledgements
This work is supported by the Beijing Natural Science Foundation Project (No. 3073014), Beijing Outstanding Scholar Program (No.20061D0501500186) and Beijing Science and Technology Plan Project of Beijing Science and Technology Commission (No.Z07020600290793).
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