Determine TIR imaging signature of the evaporative liquid microlayer

Determine how an evaporative liquid microlayer beneath a growing interfacial vapor bubble is represented in total internal reflection (TIR) imaging, including its optical signature and detectability, during magic carpet breakup of a water droplet impacting a heated sapphire substrate in a depressurized environment (chamber pressure approximately 2.5 kPa).

Background

The paper synchronizes high-speed surface temperature measurement using a thin-film Fe–Ni thermocouple array with TIR imaging to investigate bubble growth and microdroplet formation during drop impact onto a heated surface under reduced pressure. While microdroplets at the receding contact line suggest the presence of an evaporative microlayer, the microlayer was not directly resolved in TIR.

Because TIR relies on evanescent-wave reflection sensitive to nanometric separations, the optical appearance of a very thin, parallel liquid–vapor interface layer is nontrivial. Clarifying the TIR signature of the microlayer is necessary to enable direct observation and quantification of microlayer dynamics in these experiments.