Dynamics of thermal and wetting footprint of a volatile droplet during Leidenfrost transition

TL;DR

This study visualizes the thermal and wetting footprints of volatile droplets during the Leidenfrost transition, revealing partial wetting behavior and dynamics using infrared thermography and internal reflection imaging.

Contribution

It provides new insights into the wetting and thermal behavior of volatile droplets during the Leidenfrost transition using advanced optical techniques.

Findings

Thermal footprint shows a hot central zone with cooler surroundings.

Wetting imaging confirms partial wetting during transition.

Droplet dynamics are stable under specific impact conditions.

Abstract

The levitation of a volatile droplet on a highly superheated surface is known as the Leidenfrost effect. Wetting state during transition from full wetting of a surface by a droplet at room temperature to Leidenfrost bouncing, i.e., zero-wetting at high superheating, is not fully understood. Here, visualizations of droplet thermal and wetting footprint in the Leidenfrost transition state are presented using two optical techniques: mid-infrared thermography and wetting sensitive total internal reflection imaging under carefully selected experimental conditions, impact Weber number < 10 and droplet diameter < capillary length, using an indium-tin-oxide coated sapphire heater. The experimental regime was designed to create relatively stable droplet dynamics, where the effects of oscillatory and capillary instabilities were minimized. The thermography for ethanol droplet in Leidenfrost transition state (superheat range of 82K-97K) revealed thermal footprint with a central hot zone surrounded by a cooler periphery, indicative of a partial wetting state during Leidenfrost transition. High-speed total internal reflection imaging also confirmed the partial wetting footprint such that there are wetting areas around a central non-wetting zone. Result presented here using ethanol as a test fluid shed light on the geometry and dynamics of a volatile droplet footprint in Leidenfrost transition state.