As the droplet impacts a surface heated above the dynamic Leidenfrost temperature, it levitates on a self-generated vapor cushion, leading to rebound without direct contact. The structure and dynamics of this vapor layer critically influence both droplet deformation and thermal transport. The work investigates the evolution of the vapor layer beneath viscoplastic droplets and identifies a distinct interfacial oscillation at the droplet base during the rebound phase. This oscillation, marked by repeated dimple formation and collapse, emerges from the interplay of transient vapor generation, surface tension, and non-Newtonian rheology. Numerical simulations based on the volume-of-fluid approach, coupled with microscale evaporation modeling, are conducted to quantify the effects of the Weber number (We), the Jakob number (Ja), the Ohnesorge number (Oh), and Bingham capillary number (B). Results show that inertial and thermal effects amplify the oscillation, while viscous forces suppress it. The oscillatory behavior of the vaporization rate of the liquid droplet correlates closely with droplet interface motion, offering insights into vapor–liquid coupling in complex fluid systems. It is observed that the interfacial oscillations are governed by a strong coupling between vapor film thickness, vapor pressure, and interface curvature, with pressure peaks at minimum film thickness acting as a pneumatic restoring mechanism, which drives interface deformation and motion reversal. Furthermore, a geometry-based correlation for local evaporation rate was developed for viscoplastic droplets, accounting for variations in We, Oh, and Ja. It is found that, despite non-Newtonian behavior, evaporation remains strongly governed by droplet geometry.
Kumar et al. (2026) studied this question.