Super-Hydrophobic Coatings as a Part of the Aircraft Ice Protection System

This paper reviews the current knowledge on super-hydrophobic coatings (SHC). Using an ideal super-hydrophobic surface patterned with identical cylindrical flathead posts forming a square network with constant periodicity, models are proposed to explain SHC, wear and ice adherence on SHC. The models demonstrate that SHC based on Cassie-Baxter state improve the bead mobility compared to SHC based on Wenzel state and more suitable for aircraft application. Their erosion resistance can be improved by increasing the post height and the hydrophobic material thickness. Their ice adhesion reduction factor (IARF) is better but SHC based on Cassie-Baxter state have a limitation to reduce ice adherence dependence on the surface pattern and IARF of the hydrophobic material. The bead mobility is calculated from advancing and receding water contact angles (WCA). The airflow test is recommended to measure the advancing and receding WCA because the test is more representative of phenomena observed for aircraft flight in icing clouds. Experiments show that no ridge forms when a wing anti-icing system operating in running wet mode is used in combination with SHC. The reason is not well understood but water shedding is assumed to be the main driver. If the water film breaks into beads, the beads roll on the surface due to the bead sphericity resting on SHC and the nonuniform velocity profile over the wing surface. A possible explanation for the water shedding is that the rolling creates an aerodynamic force that lifts the bead off the wing.