A recent paper from GE Global Research and MIT mechanical engineering researchers casts doubt on the effectiveness of superhydrophobic surfaces ability to block ice formation on aircraft, wind turbines, communications towers and other applications where frost frequently appears.
The authors of the paper, which is published in Applied Physics Letters, note that tests done during ice formation studies on how supercooled water acts on superhydrophobic surfaces appear to have been based on spraying or pouring the water. While this provides some important information, they it is incomplete and may mask more serious dangers, viz., it doesn’t take into account the process of frost formation (i.e., ice formation without going through a liquid phase).
The bad news is that, based on their microscopy experiments looking at frost nucleation, growth and adhesion, they believe that the icephobic properties of superhydrophobic surfaces are questionable. In fact, they say that ice adhesion can actual increase “wherever frost can form indiscriminately on the surface.”
“In-flight ice accretion on aircraft surfaces is usually attributed to the freezing of supercooled water droplets suspended in clouds that come into contact with aircraft surfaces. However, recent studies show that icing clouds could be unexpectedly supersaturated resulting in heterogeneous ice nucleation. Hence, frost formation could also be an important in-flight ice accretion mechanism on aircraft surfaces. Therefore, it is important to consider frost formation while designing icephobic surfaces and extreme caution must be exercised in the use of superhydrophobic surfaces for icephobic surface treatments on ground and in-flight applications.”
The good news is that they say the insights from their studies suggestion new designs for better anti-icing surfaces.
“[A]pproaches that can spatially control nucleation (e.g., promote nucleation on top portions of the texture to form [Cassie-Baxter state] ice) . . . could reduce ice adhesion and improve the robustness of textured surfaces for icephobicity.”
The reference to Cassier-Baxter state is explained in the diagram below. A water droplet on a solid surface and surrounded by a gas forms a characteristic contact angle θ. If the surface is rough, and the liquid is in close contact with the droplets, the droplet is in what is known as the “Wenzel” state, which promotes ice formation. If the liquid rests on the tops of the asperities, it is in the “Cassie-Baxter” state, which discourages ice formation.