High-speed shadow imagery to characterize the size and velocity of the secondary droplets produced by drop impacts onto a heated surface

This study presents the developmental stages of an optical technique that aims at measuring simultaneously the droplet size and velocity. This technique is based on shadowgraphy associated with high-speed imaging. It is used to investigate the impacts of droplets onto a wall heated above the Leidenfrost temperature. Image analysis involves a detection of the droplet outlines for measuring their size and a tracking of the droplet trajectories to determine their velocities. This paper describes the different steps of the image processing. Solutions are also proposed to address the problems inherent to blurred, deformed, and overlapping droplets as well as the size-dependant effect of the depth of field. The technique is finally applied to several cases of rebound and splashing. Results show the many advantages resulting from the combination of size and velocity measurements.