Effect of Superhydrophobic Composite Coatings on Drag Reduction in Laminar Flow

In this work, the drag reduction (DR) performance of superhydrophobic composite coatings was investigated. The DR behavior for both shortterm and long-term use was studied using a parallel-plate rheometer, and the morphology of the composite interface was studied by fluorescence staining. For short-term performance, the DR increased with higher silica NP content in the composition up to 40% by weight. The composite interface played a governing role in the DR of superhydrophobic coatings except for a very high particle loading sample. The surface topography effect on this composite interface was also studied. The increase in autocorrelation length was found to be related to the increased distance between wetted asperities and the decreased solid fraction. Furthermore, it was found that the reduced peak height also increased with autocorrelation length, providing a larger distance for meniscus sag and explaining the reason for increased DR with a larger autocorrelation length. Investigation of the long-term DR showed that sustainable DR can be provided by superhydrophobic coatings. In the case of the higher silica-loaded samples, the air plastron remained throughout the experiments and DR performance was retained. This has important implications for superhydrophobic coatings in DR for nautical applications.