Impact and Shedding of Microdroplets on Hydrophilic and Superhydrophobic Surfaces

Book excerpt: The impact and shedding phenomena of water microdroplets on substrates with various wettabilitties are studied in this work. The analysis is aimed at illustrating the differences in behavior between micro, sub-millimeter and millimeter-sized droplets. This involved the evaluation of different parameters such as droplet maximum spreading, contact time, restitution coefficient as well as the critical air velocity for droplet shedding. The work focuses on the results obtained using a hydrophilic aluminum surface, which is the standard material used in aeronautics, and a superhydrophobic surface. After a comparative study on droplet size and surface wettability, the surface roughness effect on the impact of droplets is reported for both substrates. In addition, the adhesion of a sessile droplet on the two substrates is related to its corresponding shedding velocity. The analysis is considered a step forward in studying the behavior of cloud-sized (less than 100 æm) droplets especially on superhydrophobic surfaces. The first step of the current investigation was to design a dedicated test rig to work experimentally with microdroplets. The setup is developed to allow the microdroplet generator, camera, lighting and the designed shedding nozzle to work together without interfering with droplets imaging. Since the impact, deformation/bouncing, and shedding of the microdroplets occur in a matter of microseconds, high speed imaging is implemented. In addition, the MATLAB image processing toolbox is used to quantify the required parameters from the camera raw images by tracking their boundaries. The impact results show that the maximum spreading and recoiling of cloud-sized droplets on superhydrophobic surfaces are reduced when compared to sub-millimeter and millimeter sized droplets. This is depicted to the fact that the roughness of the superhydrophobic surface is in the same order of magnitude as the microdroplet size. Furthermore, the shedding tests illustrate that the smaller the droplet size, the higher the free stream incipient velocity needed for its shedding. The results also demonstrate the ease to remove impinged droplets from the superhydrophobic substrate when compared to the hydrophilic substrate, even at sub-zero temperatures.