Ice Adhesion on Superhydrophobic Coatings in an Icing Wind Tunnel

Abstract

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.