ID 16

On the scaling of crown rim diameter during droplet impact on thin wall-films

 

Abstract:

Liquid droplet impact on thin wall-films is encountered in a variety of scenarios. Some common examples include droplets interacting with wetted plant leaves and fuel droplets impacting on lubricating wall-films in internal combustion engines. Even though there are numerous studies on liquid droplet interaction with wall-films of the same liquid (‘one-component’ interaction) very few studies have considered liquid droplet impacting on wall-films of a different liquid (‘two-component’ interaction). With this in mind, an experimental study on the impact of different combinations of silicon oil, hexadecane, and hyspin droplets on silicon oil, hexadecane, and hyspin wall-films is reported here. The impact velocities are selected to cover the droplet impact regimes of deposition, transition, and splashing. The focus of the present study is to understand the scaling of crown rim radius (Rc,u) at the end of crown expansion stage (when crown height is maximum). This scaling involves droplet and wall-film properties, droplet impact velocity, and wall-film thickness. Comparison with a similar scaling for the global maximum of Rc,u is performed to highlight the effect of crown dynamics after the expansion stage. The effect of wall-film on the scaling of maximum crown rim radius is highlighted by comparing with the semi-empirical correlations for maximum droplet spread during impact on dry solid surfaces. Finally, comparison with predictions from theoretical models for ‘one-component’ droplet–wall-film interaction is carried out to highlight the gap in our current understanding on the effects of ‘two-component’ interaction, wall-film thickness, droplet and wall-film viscosities, and crown morphology.