Singular jets during droplet impact on superhydrophobic surfaces

TL;DR

This study investigates new jetting phenomena during droplet impacts on superhydrophobic surfaces, identifying a novel type of singular jets caused by horizontal inertia, and analyzes the impact dynamics across different regimes.

Contribution

It introduces the discovery of horizontal inertia-induced singular jets and provides a comprehensive analysis of impact regimes and transition conditions.

Findings

Identification of HI singular jets caused by horizontal inertia

Development of a regime map for singular jet formation

Scaling laws for impact regime transitions

Abstract

Hypothesis: The impact of droplets is prevalent in numerous applications, and jetting during droplet impact is a critical process controlling the dispersal and transport of liquid. New jetting dynamics are expected in different conditions of droplet impact on super-hydrophobic surfaces, such as new jetting phenomena, mechanisms, and regimes. Experiments: In this experimental study of droplet impact on super-hydrophobic surfaces, the Weber number and the Ohnesorge number are varied in a wide range, and the impact process is analyzed theoretically. Findings: We identify a new type of singular jets, i.e., singular jets induced by horizontal inertia (HI singular jets), besides the previously studied singular jets induced by capillary deformation (CD singular jets). For CD singular jets, the formation of the cavity is due to the propagation of capillary waves on the droplet surface; while for HI singular jets, the cavity formation is due to the large horizontal inertia of the toroidal edge during the retraction of the droplet after the maximum spreading. Key steps of the impact process are analyzed quantitatively, including the spreading of the droplet, the formation and the collapse of the spire, the formation and retraction of the cavity, and finally the formation of singular jets. A regime map for the formation of singular jets is obtained, and scaling relationships for the transition conditions between different regimes are analyzed.