Experimental investigations on the interactions between water/deicing fluids and icephobic coatings pertinent to aircraft anti-/de-icing

Abstract

Aircraft icing is widely recognized as a significant hazard to flight safety in cold climates. The detrimental effects of ice accretion over aircraft wings on the aerodynamic performance of aircrafts can cause lift loss, drag increase and stall margin reduction as well as additional rolling moment during the flight. Passive anti-icing approaches using hydro-/ice-phobic surface coatings are suggested to be promising strategies for aircraft icing mitigation. While a number of hydro-/ice-phobic coatings, have been demonstrated to be effective to delay/suppress ice accretion over airfoil/wing surfaces, “rain erosion” resistance performance of the icephobic coatings (i.e., the ability to prevent the material loss of coatings caused by the continuous high-speed impact of water droplets) has not been fully explored. In the present study, a comprehensive experimental campaign was conducted to evaluate the degradation characteristics of typical hydro-/ice-phobic coatings, i.e., Lotus-leaf inspired superhydrophobic surfaces (SHS), undergoing continuous impingement of water droplets at relatively high speeds (i.e., up to ~ 100 m/s). While the surface wettability and the ice adhesion strength on the hydro-/ice-phobic coatings were quantified as a function of the duration of the rain erosion experiments, surface topology changes on the eroded coatings were also measured by using an Atomic Force Microscope (AFM) system. The degradation characteristics of the surface wettability and ice adhesion strength on the eroded test surfaces are correlated with the AFM measurements to elucidate the underlying physics for a better understanding about the “rain erosion effects” on icephobic coatings. The detrimental effects of aircraft deicing fluids on the anti-icing performance of icephobic coatings was also explored through a comprehensive experimental campaign. The contact angle and ice adhesion stress over icephobic coatings was evaluated before and after being treated by aircraft deicing fluids to quantify the degradation of the anti-icing performance of icephobic coatings. The variations of surface topology and surface chemistry were evaluated by scanning electron microscope (SEM) and Fourier-transport infrared spectroscopy (FTIR), respectively, to understand the mechanism of the degradation in the anti-icing performance of icephobic coatings after being treated by aircraft deicing fluids. By leveraging advanced flow diagnostic techniques such as Digital Image Projection (DIP) and high speed imaging, the transient flow-off process of the Newtonian and Non-Newtonian deicing fluids over a flat surface subject to airflows with different wind speeds were also are investigated systematically to simulate the runback of deicing fluids over the aircraft wing during aircraft take-off. High speed imaging was used to qualitatively evaluate the thickness variation and view the wave structures over the deicing fluid during the experiment. DIP was used to quantify the temporal evolution in the volume of aircraft deicing fluids and the corresponding wave characteristics. The importance of the surface wave in the flow-off and mass transport of aircraft deicing fluids subject to airflows was also fully discussed and analyzed based on experimental results. DIP was combined with Stereo Particle Image Velocimetry (SPIV) to investigate the aerodynamic force and adhesion force exerted on wind-driven droplets to provide solid experimental results for predicting the runback of droplets subjects to airflows. The three-dimensional configuration of droplets was retrieved by DIP so that the adhesion force could be obtained. Then, while the velocity fields downstream of wind-driven droplets were measured via SPIV, the aerodynamic force over wind-driven droplets was obtained through momentum conservation law. The new findings derived from the present studies will be very helpful to gain further insights into the underlying physics about the complex interactions between water/deicing fluids and hydro-/ice-phobic coatings for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft operations in cold weathers.