Elsevier

Applied Surface Science

Volume 469, 1 March 2019, Pages 864-869
Applied Surface Science

Full Length Article
Stability of frozen water droplets on highly hydrophobic porous surfaces: Temperature effects

https://doi.org/10.1016/j.apsusc.2018.11.027Get rights and content

Highlights

  • The freezing process of water droplets under low pressure (2 kPa) on highly hydrophobic surfaces was reported.

  • Three different mechanisms, influenced by the substrate’s temperature, control the frozen droplet disappearance.

  • Thermal instabilities and convective cells are observed on heated substrates.

  • A “Cassie ice state” can be reached on hydrophobic surfaces.

Abstract

We report on an experimental study of water droplets freezing on highly hydrophobic surfaces decorated by a random distribution of nanometric pores. We particularly analyze the influence of the substrate temperature ranging between 23 °C and 80 °C on the freezing phenomenon and the temporal stability of the resulting frozen droplets. The cooling and the subsequent spontaneous freeze of droplets were obtained by quickly reducing the pressure inside the cell from 101 kPa to 2 kPa. In all experiments the humidity rate was maintained very low (4%), allowing us to minimize its influence on the freezing process. The experimental results show that: (i) the freezing process is initiated by homogeneous nucleation near the gas-liquid interface, and is delayed by a factor between 2 and 3 on the substrates at the highest temperature. (ii) three different mechanisms, influenced by the substrate’s temperature, lead to the disappearance of the frozen droplets. Fragmentation and sublimation of frozen droplets are observed for all substrates’ temperatures. A melting of the ice shell is observed for droplets on substrate at 80 °C. This temperature appears as a minimal value from which we can expect to better control the water freezing and the frost formation on heated hydrophobic surfaces.

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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