Frost Formation on Aluminum and Hydrophobic Surfaces

Abstract

Ice and frost formation on the surfaces of car windshield, airplanes, air-conditioning duct, transportation, refrigeration and other structures is of great interest due to its negative impact in the efficiency and reliability of the system. Frost formation is a complex and fascinating phenomenon. Frequent defrosting are required to remove the ice that causes economic losses. In order to delay the freezing phenomenon, hydrophobic surfaces (Al-H) were prepared using a very simple and low cost method by dip coating of Aluminum in Teflon© and FC - 40 solution at a ratio of 2:10. Later, the samples were placed on a freezing stage in a computer controlled environmental chamber. The freezing stage was held at a constant temperature of 265 ± 0.5 K. The environmental temperature was set to 295 ± 0.5 K and the relative humidity (RH) was set to 40% and 60% respectively. The samples were observed via optical microscopy from the top and videos of the freezing dynamics were captured. The time required for the whole surface to freeze was named as ‘Freezing time’ and is determined by investigating the consecutive images. The inter-droplet freezing wave propagation was accelerated via a frozen droplet/area and then propagates through the surface very quickly. Ice bridging was also seen for the frost propagation. However, the maximum freezing front propagation velocity was found for Al surfaces at 60% RH. At 40% RH, the Al surface required approximately 10 ± 1 minutes to freeze while the Al-H surface delay freezing until 15 ± 1 minutes. This is due to a slow rate of nucleation and also increased rate of coalescence. At 60% RH, both surface froze faster than 40% RH. The Al surface required 6.5 ± 1 minutes and the Al-H surface froze after 10 ± 1 minutes. The change in freezing kinetics, freezing time, the size of droplets at freezing, and the surface area covered at freezing are all related to the rate of coalescence of droplets. Again, the added thermal resistance of the coating and less water-surface contact area of the droplet to the cooled hydrophobic surface inhibited the growth rate resulting the freezing delay.