Drop impact on superheated surfaces

TL;DR

This study experimentally investigates droplet impact behaviors on superheated surfaces, identifying regimes and universal scaling laws for droplet spreading, and measures vapor layer thickness to understand the underlying physics.

Contribution

It provides a comprehensive experimental analysis of impact regimes and establishes a universal scaling law for droplet spreading on superheated surfaces.

Findings

Impact regimes depend on surface temperature and droplet properties.

Universal scaling law $\gamma o ext{We}^{2/5}$ for droplet spreading.

Vapor layer thickness measured interferometrically.

Abstract

At impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surfaces (``contact boiling''), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film boiling''), or both forms the Leidenfrost layer and ejects tiny droplets upward (``spraying film boiling''). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading $\gamma$ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number $\We$ ($\gamma\sim\We^{2/5}$) -- regardless of surface temperature and of liquid properties -- which is much steeper than for the impact on non-heated (hydrophilic or hydrophobic) surfaces ($\gamma\sim\We^{1/4}$). We also intereferometrically measure the vapor thickness under the droplet.