Superhydrophobicity as a strategy against icing. Analysis of the water/surface dynamic interaction for icing mitigation

Abstract

The thesis illustrates the efforts made to proof how water repellent surfaces can help reducing ice accretion in icing conditions: tests performed in an icing wind tunnel showed that application of superhydrophobic coatings allow considerable savings in supplied heating power, necessary to keep an aerodynamic wing clean from ice, and reduce runback ice, which often forms in unprotected areas. The present thesis is the result of a three year research study, aimed at understanding the potential of superhydrophobicity for combating icing accretion on aircrafts. The project was performed in the framework of a collaboration between University of Bergamo, University of Alberta and Alenia Aermacchi, an Italian aeronautic company. A considerable part of the work has been devoted to understanding the dynamic interaction between the liquid and the solid surface through single drop impact experiments. Performing drop impact studies allowed explaining the role of different parameters, e.g. impact Weber number and surface wettability, during drop impact on a solid surface and identifying the mechanisms that promote water shedding from the surface. Water drop impact tests performed in isothermal conditions allowed to understand in which conditions surface wettability has an effect on drop dynamics, and also to evaluate characteristic times related to drop impact (e.g. spreading time and rebound time). Water drop impact in freezing conditions, i.e. on surfaces below 0°C, showed that surface temperature does not affect drop dynamics, unless frost is present on the surface.