Full Length ArticleStability of frozen water droplets on highly hydrophobic porous surfaces: Temperature effects
Graphical abstract
Introduction
Freezing of water droplets on solid surfaces has been studied with an increasing interest for both fundamental research [1], [2], [3], [4], [5] and a broad range of practical applications such as aircraft, wind turbines and marine structures [6], [7]. Many of such applications are faced with the hazardous effect of ice accumulation in the mechanical device, which can seriously compromise the energy efficiency. It is thus not surprising that significant effort has been expended to develop surfaces that substantially reduce the ice adhesion or delay its formation [8], [9], [10], [11], [12]. Several approaches which prevent ice formation or reduce ice adhesion and accumulation have been proposed over the last decades. Some of these studies conclude that the ice adhesion reduces with increasing hydrophobicity of the surface, rendering the superhydrophobic surface a more interesting candidate for anti-icing applications. These surfaces, characterized by high water repellency and low friction at liquid-solid interface, are expected to minimize the ice or frost creation and their adhesion to the surface. A substantial amount of research has focused on the potential icephobic properties of such surfaces.
Thus, in the last few years, several authors [2], [13], [14] have shown that a delayed ice accretion occurs on superhydrophobic surfaces. Others authors have correlated the contact angle hysteresis with the ice adhesion [15], [16]. This statement and the role played by surface roughness on the icephobic behavior have been however critically questioned [17], [18], [19], [20], [21]. For example, Varanasi et al. [17] reported that the frost formation inside the texture of superhydrophobic surfaces could compromise their effectiveness in reducing the ice adhesion, which increases with surface roughness. Jung et al. [19] have shown that hydrophilic surfaces with nanometer (nm)-scale roughness and higher wettability present longer freezing delays compared with typical superhydrophobic surfaces with larger hierarchical roughness and very low wettability. In fact, the efficiency of anti-icing material can also be limited by the presence of frost on the surface whose formation is highly influenced by the environmental humidity [18], [21], [22]. On the other hand, more recently, the durability of the icephobic properties of superhydrophobic surfaces has been questioned [23], [24], [25], [26] and some controversial conclusions appear.
These numerous investigations evidence clearly the influence of many parameters (e.g., hydrophobicity, surface roughness, environmental humidity and fragility of surface structures…) on the ice formation on a solid surface and its adhesion or stability. The role played separately by each one of them in the ice formation remains still unclear.
We report here an experimental study of water droplet freezing on highly hydrophobic surfaces. We particularly investigate the influence of the substrate temperature, ranging between 23 °C and 80 °C in the freezing process and its temporal stability. In all experiments the humidity rate was maintained very low (4%), allowing us to minimize its influence in the freezing process. Independently of the substrate temperature, the droplets freeze when the environing pressure inside the vacuum chamber is abruptly reduced. The freezing process is delayed at higher temperatures due to the increase of the heat transfer rate from liquid-solid contact interface. We observe that the thin ice layer formed melts in few seconds. We discuss the morphology adopted by the frozen droplet and its temporal evolution as a function of the substrate’s temperature.
Section snippets
Experimental section
55 μm-thick anodisc aluminum oxide membranes purchased from Sigma-Aldrich were used as substrate. The surfaces are patterned with a random distribution of nanometric pores characterized by a mean diameter of 200 nm and a mean center-to-center distance of 316 nm (see Fig. 1). These geometrical characteristics lead to a solid fraction in contact with liquid of ∼0.69.
To provide stable highly hydrophobic surfaces these membranes were grafted with fluoroalkyl-silane (C16F17H19O3Si, named FAS-17)
Wetting properties
The advancing and receding contact angles measured on processed surfaces, at room temperature, were θA = 143°±3° and θR = 118°±4°. These values result from a statistical study having taken into account measurements carried out on at least twenty samples. In our experiments, we used water drops having a static contact angle θ of ∼135°. The relative high contact angle hysteresis (Δθ = 25°) suggests that a partial pore’s imbibition should occur. Thus, the water droplets should be found in an
Conclusion
We have experimentally investigated the freezing process of water droplets and its temporal stability on highly hydrophobic surfaces. In particular, we have highlighted the influence of the substrate’s temperature. From our experiments, it clearly appears that independently of the substrate temperature, the droplets freeze when the environing pressure inside the vacuum chamber is abruptly reduced to 2 kPa. The freezing process is delayed at higher temperatures due to the increase of the heat
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