Elise Contraires
Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon
En délégation au CRPP pour 2020-2021

“Dynamic wetting in complex interfaces “

The dynamic behavior of droplets impacting a surface or submitted to vibrations has been the subject of numerous studies for 15 years. Thereby, surface topography was developed to guide the displacement of droplet or control spreading and/or bouncing. During this talk, I present two aspects of the role of topography on the dynamic behavior of droplets in contact with rough substrates.
In a first part, I present the dynamic behavior of sessile droplets on textured polymer surfaces. The onset of sliding, the frequency and damping of the natural modes of vibrated droplets were measured through original imaging techniques. Surfaces were textured with several methods and the topography is used to modify the type of contact between the droplet and the surface and thus the real contact area. Our measurements focus on the impact of this contact area on the sliding of droplet and its phase shift. We then try to interpret the results as a function of the bulk and interfacial friction between the droplet and the surface.
In a second part, I focus on earthen building materials and study the impact of water droplets with the aim of reducing the wear of raindrops on such walls. Earthen materials are very heterogeneous, sensitive to water and with a complex and large roughness. This roughness affects the spreading of impacting droplet, as well as the penetration depth of water. We first measure with fast imaging the bouncing and spreading of droplet. We identify two types of impact, oscillations or damping, and show that the higher the roughness, the smallest the spreading is. Additional experiments using X-ray microtomography are made to analyze the penetration depth as well as the diameter. We could confirm the role of the surface topography on the conditions of water imbibition in the earthen substrate. In the long run, we could propose more efficient formulations for ecological building materials.