Studying Ice Nucleation and Freezing Front Propagation with Infrared and Laser Speckle Imaging

Abstract

Clear ice formation on aircraft is a large economic and safety problem. The phenomenon, also known as ‘freezing rain’, usually occurs when subcooled rain droplets impact the aircraft while it descends from high altitude to prepare for landing. It can quickly increase the aerodynamic drag by up to 80% and reduce the lift force by 50%. Inefficient heating methods or toxic or corrosive chemicals are used to keep flying safe. In this thesis, infrared imaging (IRI) and laser speckle imaging (LSI) are used to observe freezing of water on surfaces with different material properties. A freezing front can be seen to travel through a molecular liquid layer of water (MLL) that covers all surfaces. If this freezing front has passed, impacting droplets freeze much sooner and at unusually high temperatures, which could explain the formation of clear ice on aircraft surfaces.

In the past decade, the interest in anti-icing research has spiked. However, no systematic study has been performed to find out the origin of clear ice. Using the new insight of a freezing molecular liquid layer of water, this thesis is able to explain the origin of clear ice and runback icing. It also suggests that the ongoing conflict in literature on the anti-icing performance of superhydrophobic surfaces might be caused by the presence of the molecular layer of water.

With a systematic study, the surface temperature, contact angle and relative humidity were found to be able to delay freezing of the water layer and the contact angle was able to decrease the velocity of the freezing front. Using this insight, coating architectures and scalable production methods were used to produce hydrophilic-hydrophobic patterns for passive anti-icing coatings.