A New Perspective for the Characterization of Crown Rim Kinematics

Abstract

Droplet impact on wall-films are typical examples where ‘splashing’, the fragmentation of the crown rim during its expansion, takes place. The triggering instability mechanisms are directly linked to crown rim kinematics. This experimental study analyses which geometrical parameter is physically the most suited for studying crown rim kinematics during expansion. The problems associated with the classical geometrical parameters rim radius $R_{R}$ and height $H_{R}$ (often considered separately) are presented. First, the radial and axial rim expansions have different durations which prevents the definition of rim expansion in a unified way. Second, considering separately $H_{R}$ and $R_{R}$ leads to an incomplete picture of the impact process in terms of momentum transfer to the rim since the crown aspect ratio varies strongly with the impact conditions. We show that considering the crown rim displacement instead solves these problems: a single peak during the impact process enables a clear definition of rim expansion (duration and magnitude). Furthermore, the temporal evolution of the rim displacement during expansion could systematically be fitted by a quadratic curve with high accuracy, which indicates a constant deceleration process. Thus, considering the rim displacement reveals important features of the crown rim kinematics.