(601e) Non-Monotonic Jet Dynamics As Droplet Impacts on Liquid Films

Water drops impact on liquid surfaces play a critical role in applications such as inkjet printing, pesticide delivery, and fuel injections, making their study essential for advancing process optimization in these fields. In our work, water droplets of varying diameters impact a water pool at varying speeds, creating a transient cavity and an upward jet. The complex interplay between kinetic energy and surface tension - quantified by the droplet Weber number - is modulated by the pool height and governs the overall dynamics of the process.

Detailed high-speed imaging of our experiments uncovers a distinct progression in jet dynamics as the pool height is varied: the jet speed rises sharply with increasing pool height, peaks, and then gradually declines before reaching a stable plateau. This non-monotonic behaviour is mirrored by the number of secondary droplets produced. Detailed analysis involving droplet diameter, pool height, and impact speed will be used to understand the underlying dynamics and explain the observed trends, highlighting the role of cavity formation, cavity-substrate interaction and Rayleigh–Plateau instability in jet fragmentation.

Ultimately, our findings contribute to a deeper understanding of multiphase flow dynamics and offer practical insights for streamlining processes that depend on controlled secondary droplet formation. This work not only bridges experimental observations with theoretical modelling but also provides a robust framework for predicting and managing droplet ejection in real-world applications, paving the way for more efficient and precise process designs.