ISBN 978-3-8439-4638-4

978-3-8439-4638-4, Reihe Thermodynamik

Ronan Bernard
Macro and Micro Dynamics of Droplet Impact onto Wall-films made of different Liquids

232 Seiten, Dissertation Universität Stuttgart (2020), Softcover, A5

Zusammenfassung / Abstract

Droplet impacts onto wetted surfaces (wall-films) are ubiquitous but no less fascinating. The impact process leads to the formation of a complex liquid structure named crown. Although droplet impact processes are often considered as inertio-capillary systems, important viscous effects can be observed on the crown dynamics. These viscous effects are still unclear, particularly when droplet and wall-film are made of different liquids. They are mostly due to shear intensified by the vicinity of a solid wall, characteristic of droplet impact onto thin wall-films. Hence, a combined analysis of the macroscopic and microscopic crown dynamics is undertaken experimentally to better understand viscous effects during the impact. A setup based on high-speed shadowgraphy is developed with a macroscopic side view and a microscopic bottom view. At the macroscopic level, the sensitivity of the splashing limit and the crown expansion to the impact parameters influencing shear is investigated. At the microscopic level, a time- and spatially-resolved velocity field in the crown base flow is obtained by micro-Particle Image Velocimetry (micro-PIV) with large depth-of-field to tackle the challenges associated with the unsteady, short, and multi-scale impact process. The measured velocities exhibit a stronger deceleration than inviscid theoretical predictions from literature. This stronger deceleration can be recovered by considering the effect of shear caused by the simultaneous wall-film decay and boundary layer growth, corroborating the macroscopic effects of wall-film thickness and viscosity on the crown dynamics. Besides, the influence of miscibility and wettability on splashing, crown expansion and crown kinetics is analyzed. The role of interfacial tension in storing capillary energy and in acting as a retraction force is quantified. Last, the outlook presents the promising ability to study important features of multiple droplet impacts from the knowledge of single crown dynamics.