Yong Han Yeong

Superhydrophobic Nanocomposite Surface Topography and Ice Adhesion

A method to reduce the surface roughness of a spray-casted polyurethane/silica/fluoroacrylic superhydrophobic nanocomposite coating was demonstrated. By changing the main slurry carrier fluid, fluoropolymer medium, surface pretreatment, and spray parameters, we achieved arithmetic surface roughness values of 8.7, 2.7, and 1.6 μm on three test surfaces. The three surfaces displayed superhydrophobic performance with modest variations in skewness and kurtosis. The arithmetic roughness level of 1.6 μm is the smoothest superhydrophobic surface yet produced with these spray-based techniques. These three nanocomposite surfaces, along with a polished aluminum surface, were impacted with a supercooled water spray in icing conditions, and after ice accretion occurred, each was subjected to a pressurized tensile test to measure ice-adhesion. All three superhydrophobic surfaces showed lower ice adhesion than that of the polished aluminum surface. Interestingly, the intermediate roughness surface yielded the best performance, which suggests that high kurtosis and shorter autocorrelation lengths improve performance. The most ice-phobic nanocomposite showed a 60% reduction in ice-adhesion strength when compared to polished aluminum.

Drop Impact and Rebound Dynamics on an Inclined Superhydrophobic Surface

Due to its potential in water-repelling applications, the impact and rebound dynamics of a water drop impinging perpendicular to a horizontal superhydrophobic surface have undergone extensive study. However, drops tend to strike a surface at an angle in applications. In such cases, the physics governing the effects of oblique impact are not well studied or understood. Therefore, the objective of this study was to conduct an experiment to investigate the impact and rebound dynamics of a drop at various liquid viscosities, in an isothermal environment, and on a nanocomposite superhydrophobic surface at normal and oblique impact conditions (tilted at 15°, 30°, 45°, and 60°). This study considered drops falling from various heights to create normal impact Weber numbers ranging from 6 to 110. In addition, drop viscosity was varied by decreasing the temperature for water drops and by utilizing water-glycerol mixtures, which have similar surface tension to water but higher viscosities. Results revealed that oblique and normal drop impact behaved similarly (in terms of maximum drop spread as well as rebound dynamics) at low normal Weber numbers. However, at higher Weber numbers, normal and oblique impact results diverged in terms of maximum spread, which could be related to asymmetry and more complex outcomes. These asymmetry effects became more pronounced as the inclination angle increased, to the point where they dominated the drop impact and rebound characteristics when the surface was inclined at 60°. The drop rebound characteristics on inclined surfaces could be classified into eight different outcomes driven primarily by normal Weber number and drop Ohnesorge numbers. However, it was found that these outcomes were also a function of the receding contact angle, whereby reduced receding angles yielded tail-like structures. Nevertheless, the contact times of the drops with the coating were found to be generally independent of surface inclination.

Temperature and humidity effects on superhydrophobicity of nanocomposite coatings

This work investigates temperature and humidity effects on the superhydrophobicity of polyurethane/organoclay nanocomposites. Previous reports of superhydrophobic degradation at decreasing surface temperatures for both low and high humidity were generally conducted in open environments. However, the present setup allows a thermally homogeneous environment, i.e., the temperature of the nanocomposite, air and water droplet are equal with no spatial temperature gradients. In such conditions, results showed stable retention of superhydrophobicity for both low humidity (RH   80%) cool-down cycle, though superhydrophobicity degraded during the warm-up cycle, which was attributed to dew condensation.

Atmospheric Ice Adhesion on Water-Repellent Coatings: Wetting and Surface Topology Effects

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.

Microscopic Receding Contact Line Dynamics on Pillar and Irregular Superhydrophobic Surfaces

Receding angles have been shown to have great significance when designing a superhydrophobic surface for applications involving self-cleaning. Although apparent receding angles under dynamic conditions have been well studied, the microscopic receding contact line dynamics are not well understood. Therefore, experiments were performed to measure these dynamics on textured square pillar and irregular superhydrophobic surfaces at micron length scales and at micro-second temporal scales. Results revealed a consistent “slide-snap” motion of the microscopic receding line as compared to the “stick-slip” dynamics reported in previous studies. Interface angles between 40–60° were measured for the pre-snap receding lines on all pillar surfaces. Similar “slide-snap” dynamics were also observed on an irregular nanocomposite surface. However, the sharper features of the surface asperities resulted in a higher pre-snap receding line interface angle (~90°).

Nanocomposite coating superhydrophobicity recovery after prolonged high-impact simulated rain

Polyurethane/fluoroacrylic/organoclay superhydrophobic nanocomposites were exposed to high-flux, high-speed 1 mm drops traveling at 25 m s−1 (more than 105 droplet impacts at Weber numbers exceeding 104), which is equivalent to decades of rainfall impacting a moving surface. After dry-out from this simulated rain exposure, high contact angles and reasonable roll-off were generally recovered for samples oriented both normally and tilted to the impacting droplets, though some microscale osmotic swelling was noted. After refunctionalizing the vertically impacted surface with fluoroacrylic, an increase in performance was observed.

Water Droplet Impact Dynamics at Icing Conditions with and without Superhydrophobicity

Superhydrophobic coatings have shown promise in reducing both ice accretion and accumulation on a surface. However, recent studies of super-cooled water droplet impact dynamics on a freezing superhydrophobic surface have been limited to either low Weber numbers (Wep 90) the room temperature droplet was unable to sustain the inertial forces and therefore broke into smaller droplets upon surface impact while a more viscous super-cooled water droplet was able rebound without disintegration. A regime map depicting the various super-cooled droplet rebounding outcomes on the freezing superhydrophobic surface was also plotted as functions of the Weber and Ohnesorge number. We observed that viscosity effects would start to affect the droplet rebound dynamics at Weber numbers of approximately 90 or greater.

Ice Adhesion Strength on Hydrophobic and Superhydrophobic Coatings

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.

Ice Adhesion on Superhydrophobic Coatings in an Icing Wind Tunnel

Researchers have recently focused on superhydrophobic coatings as an ice-mitigation tool. These surfaces have a high degree of water-repellency and were shown in previous low-speed droplet studies to reduce surface ice adhesion strength. However, there has been little research regarding testing in aerospace icing conditions, that is, high-speed super-cooled droplet impact (>50 m/s) on a cold substrate in an environment where the air temperature is below freezing. A detailed set of experiments was conducted in an icing wind tunnel to measure the ice adhesion strength of various superhydrophobic coatings by subjecting the surfaces to a super-cooled icing cloud consisting of 20 μm droplets at a constant liquid water content (LWC) of 0.4 g/m3. Test conditions included air speeds of 50 and 70 m/s and in glaze (−5°C) and rime ice regimes (−15°C). The accreted ice was then removed by pressurized nitrogen in the tensile direction in an ice adhesion test. The pressure required for ice removal and the fraction of ice remaining were combined into an overall adhesion parameter (AP). Results showed that the present superhydrophobic coatings generally resulted in increased ice APs relative to the baseline titanium surface. The strongest indicator of ice adhesion performance for these coatings was found to be the surface roughness lateral auto-correlation length. Only superhydrophobic coatings with length-scales less than 40 μm reduced the ice AP. When compared to previous results, it can be seen that increased droplet impact speeds tended to increase the ice adhesion strength on the superhydrophobic coatings. This was because of the increased droplet impact Bernoulli and hammer pressures which exceeded the resistive capillary pressure of the surface features induced by large surface lateral auto-correlation lengths.