No abstract

Recent studies have shown the potential of water-repellent surfaces such as superhydrophobic surfaces in delaying ice accretion and reducing ice adhesion. However, conflicting trends in superhydrophobic ice adhesion strength were reported by previous studies. Hence, this investigation was performed to study the ice adhesion strength of hydrophobic and superhydrophobic coatings under realistic atmospheric icing conditions, i.e., supercooled spray of 20 μm mean volume diameter (MVD) droplets in a freezing (-20 °C), thermally homogeneous environment. The ice was released in a tensile direction by underside air pressure in a Mode-1 ice fracture condition. Results showed a strong effect of water repellency (increased contact and receding angles) on ice adhesion strength for hydrophobic surfaces. However, the extreme water repellency of nanocomposite superhydrophobic surfaces did not provide further adhesion strength reductions. Rather, ice adhesion strength for superhydrophobic surfaces depended primarily on the surface topology spatial parameter of autocorrelation length (Sal), whereby surface features in close proximities associated with a higher capillary pressure were better able to resist droplet penetration. Effects from other surface height parameters (e.g., arithmetic mean roughness, kurtosis, and skewness) were secondary.

Superhydrophobic coatings have shown promise in reducing both ice accretion and accumulation on a surface. However, recent studies revealed conflicting reports of ice adhesion strength on superhydrophobic surfaces. Therefore, a comprehensive experiment was conducted to measure the ice adhesion strength of a variety of hydrophobic and superhydrophobic coatings by subjecting test substrates to a super-cooled spray consisting of 20 µm droplets in a walk-in cold chamber, and at an air temperature of -20°C. The accreted ice was then removed by pressurized air in a tensile direction for a mode-1 fracture. The relationships between surface wettability, roughness parameters and ice adhesion were then studied in detail. Results showed that for hydrophobic surfaces, a high contact angle and receding contact angle resulted in a lower ice adhesion strength. However, ice adhesion strength for superhydrophobic surfaces correlated weakly with receding contact angle. It was discovered that low surface autocorrelation lengths for superhydrophobic surfaces would result in low ice adhesion strength. This is due to the fact that closely spaced surface features create a high capillary pressure between the surface asperities to resist the penetration of impacting super-cooled droplets to result in ice formation at a Cassie wetting state. However, if the surface asperities are infiltrated with water droplets, the ice adhesion strength can be affected by secondary surface roughness parameters such as arithmetic mean roughness, kurtosis and skewness. Nomenclature 2τ= cohesive fracture energy ω = adhesive fracture energy υ = Poisson's ratio for ice A = area c = "defect" diameter h = ice accretion thickness E = Young's modulus of ice z = surface feature amplitude P c = critical ice fracture pressure S a = arithmetic surface roughness S sk = surface skewness S ku = surface kurtosis 2 S al = surface autocorrelation length S q = surface root mean square roughness t x = autocorrelation function in the x-direction t y = autocorrelation function in the y-direction