  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.
