Real-time laser-based measurement of interface temperature during droplet impingement on a cold surface

This work presents a laser-based thermoreflectance technique to measure the real-time change in temperature of a liquid-solid interface when a heated liquid droplet impinges on a transparent substrate. Temperature variation at the interface results in refractive index changes in both the liquid and substrate, which, in turn, cause a reflectivity change at the interface. A 5 mW HeNe laser and a silicon photodiode are used to monitor the real-time reflectivity of the interface. The measurement is performed with two liquids, water and glycerol, impinging onto one surface of a prism made of F2 glass, with initial liquid temperatures of 0, 25, and 45 °C above room temperature. A temporal resolution of 8.8 ms and spatial resolution of 180 μm have been achieved in this work. The measurement uncertainty is ∼3.5-6.3 °C for water and 0.5 °C for glycerol. Higher temporal and spatial resolution can be readily obtained with minor modifications to the experimental apparatus. Measurement of liquid solidification and evaporation on a substrate may also be suitable for this technique, as the phase change causes an abrupt variation in reflectivity at the interface.