Impact force of ring bouncing on superhydrophobic surface with a bead

Abstract

The impingement of drops on superhydrophobic surfaces with macrotextures would result in a reduced contact time, which is crucial in applications, such as anti-icing and anti-frost. We experimentally investigate the impact force and deformation of a water drop falling on superhydrophobic surfaces decorated with a water-repellent bead by employing a high-sensitivity force transducer and high-speed cameras operated in synchrony. The drop falling on the water-repellent bead bounces off in the form of a liquid ring when the Weber number We > 37, and this leads to a shorter contact time compared with an impingement on a flat surface. Four regimes, referred to as the air cavity, column jet, intact ring, and splashing, are identified based on the characteristics of drop deformation and peak forces. Two predominant peaks are identified in the force curves in most cases. The first peak originates from the momentum change of the inertial impact; the second peak in the first two regimes originates from the momentum change of the Worthington jet, while that in the last two regimes originates from the momentum change of the bouncing ring. Furthermore, a third peak appears when the oscillation of the lifting drop reattaches the substrate at a moderate Weber number. We investigate the drop dynamics in different regimes and theoretically explain the We dependence of peak force related to bouncing in the intact-ring regime. The discoveries obtained in this study will advance our understanding of the dynamics of drop impingement on superhydrophobic surfaces with macrotextures for reducing the contact time.