On the crown rim expansion kinematics during droplet impact on wall-films

•Experimental study of drop impact on thin wall-films with different silicone oils.•Crown rim displacement used as length scale to define rim expansion.•Scaling of rim displacement, velocity and acceleration derived from parabolic fits.•Discussion on the role of impact parameters and their interplay on rim kinematics.•Comparison of experimental kinematic parameters with models reported in literature. During droplet impact on thin wall-films, a liquid lamella, the so-called ‘crown’, expands radially and vertically away from the impact point. This phenomenon is often associated with the growth of fingers at the crown rim, leading to the fascinating crown-type splashing. The present study reports an experimental investigation on droplet impact on wall-films, where the droplet and wall-film are made of different silicone oils. The focus is to characterize the crown rim kinematics in terms of relevant length, velocity, and acceleration scales during the expansion phase. The analysis showed that the crown rim displacement is a physically meaningful parameter to characterize the rim expansion. From this parameter, it is possible to clearly define the maximum expansion of the crown rim as well as the average velocity and the constant deceleration of crown rim expansion. The effects of droplet and wall-film properties (especially, their viscosities), droplet impact velocity, and wall-film thickness on these length, time, velocity, and acceleration scales associated with crown rim expansion are studied. Furthermore, the experimental data extracted from our vast database are systematically compared with the theoretical predictions from relevant models reported in literature to discuss their range of applicability.