Mitigation of biological adhesion to aircraft leading edge surfaces

Contamination caused by insects on aircraft leading edge surfaces can result in premature transition of the boundary layer, leading to an increase in skin friction drag and fuel consumption. Consequently, the use of novel low surface energy coatings to mitigate insect residue adhesion was investigated. In order to determine the effect of surface characteristics on insect residue adhesion a range of surfaces, from superhydrophobic to hydrophilic, were evaluated. Surfaces were characterized in terms of their microstructure and hydrophobic properties by profilometry and dynamic contact angle measurements, respectively. Insect impact tests were conducted using two different test facilities, an insect delivery device and a laboratory scale wind tunnel, both capable of producing single and multiple insect impacts at speeds of ca. 100 m/s. Topography of insect residues was investigated using Scanning Electron Microscopy (SEM) and Confocal Laser Scanning Microscopy (CLSM). A screening and evaluation technique, utilizing a modified wet abrasion scrub tester, was developed to determine the effectiveness of candidate coatings against insect residue adhesion. The test method provided a direct quantification of insect residue adhesion to a surface. The use of an analogue as a substitute to live insect testing was evaluated, with Schneider’s Insect Medium shown to be successful in mimicking the adhesive properties of Drosophila melanogaster haemolymph. The exposure to environmental factors (e.g. relative humidity, exposure to water) on the adhesive properties of insect haemolymph was found to be negligible. The dynamics of an insect impact event and its influence on the wetting and adhesion mechanisms of insect residue to a surface were studied. It was found that the effect of angle of impact and impact speed significantly influenced the insect residue patterns. Exposure to a constant airflow during the insect impact event imparts a shear force, resulting in an increase in the residue area and a decrease in the height measurements. The dominant factor influencing the rupture velocity (i.e. the lowest speed needed to fracture the exoskeleton) was found to be the orientation of the insect body relative to the surface upon impact. Tests with different insect species were conducted to investigate the effect of insect size and type on the effectiveness of the coatings and evaluation procedure. The influence of substrate temperature on insect impact dynamics and adhesion was evaluated and shown to have a minimal effect. Insect residue areas were theoretically predicted using high speed liquid droplet theory and compared to experimentally obtained results. The surface topography and chemistry, in particular the sliding angle of a coating, was found to have a significant influence on the adhesion of insect residue. Only superhydrophobic coatings with a specific microstructure and low sliding angle showed a significant reduction in insect residue adhesion.