Anti-icing Behaviors Of Superhydrophobic Surface On Aluminum Alloy

Icing is a beautiful phenomenon in nature, which can not only brings the enjoyment of vision, but also may result in inconvenience to people’s life, even disaster. By virtue of their unique surface wettability, superhydrophobic materials show great application prospects in the field of anti-icing and the related researches have become a hot topic in recent years. In this thesis, superhydrophobic surface(coded as SHS) was fabricated on aluminum alloy substrate via a simple chemical etching and subsequent surface passivation. The anti-icing behaviors on SHS and the control samples(i.e., hydrophilic and hydrophobic surfaces on Al alloy, coded as HIS and HHS, respectively) were investigated on the self-designed experimental platform. Specifically, the freezing of sessile water droplet, the shape of the so-frozen ice droplet, the formation of frost on the so-frozen ice droplet, and vertical adhesion strength between ice and the substrates, were analyzed in detail. Main research contents and results were as follows:(1) SHS on Al alloy and control samples(i.e., hydrophilic and hydrophobic surfaces on Al alloy) were fabricated. Static contact angle(SCA) and sliding angle(SA) for SHS were measured to be 164.6° and 4°, respectively. For hydrophobic and hydrophilic samples, SCA was 116.5° and 57.1°, while the contact angle hysteresis(CAH) is 20° and 45° respectively. Surface morphology observations revealed that the surface on superhydrophobic sample was uneven with rough micro-structure; however, for hydrophobic and hydrophilic surfaces, they were much more smoother. The stability of SHS was also invetigated. The change in SCA and SA for SHS was not obvious after the sample being exposed to air conditions(25 °C) for 30 days or ultrasonicated in ethanol for 45 min.(2) The freezing of sessile water droplet on cold surface was investigated. Experimental results showed that such a freezing process could be divided into two sub-processes, i.e., homogeneous cooling and heterogeneous freezing. Parameters, such as homogeneous cooling time(coded as tc), heterogeneous freezing time(coded as tf), and heterogeneous freezing rate(coded as Vj) were proposed to characterize such a freezing process. For the homogeneous cooling sub-process, tc for SHS was longer as compared with the control samples under the cold surface temperature of-9 °C,-14 °C and-19 °C. However, as the temperature of cold surface increased to-24 °C and-29 °C, the tc difference between SHS and its control samples were not so obvious. For the heterogeneous freezing sub-process, compared with its control samples, tf for SHS was much longer and Vj for SHS was much lower.(3) The shape of the so-frozen sessile droplet on cold surface and further formation of frost thereon was investigate. Experimental results showed that, under the same cold surface temperature, the curvature at the top of the so-frozen ice droplet was greatest for SHS and smallest for HIS. The greater the curvature at the top of the so-frozen ice droplet, the faster the water condensation from air, and the faster the frost formation. The lower the cold surface temperature, the faster the frost formation. Experimental results showed that, under the same cold surface temperature(for example,-19 °C), the formation of frost on the ice droplet was fastest for SHS and lowest for HIS. As cold surface temperature decreasing from-14 °C to-19 °C and finally to-24 °C, the formation of frost on the ice droplet increased.(4) Ice vertical adhesive strength(IAS) between ice and different substrates was measured on a self-designed apparatus. Under the cold surface temperature of-9 °C and-14 °C, IAS for SHS was smallest and for HIS was greatest. However, as the cold surface temperature decreasing to-19 °C, IAS for HHS was smallest and for HIS was greatest. This suggested that the IAS was closely related with surface wettability and cold surface temperature.