Experimental investigation of boiling regime transitions during ethanol droplets impact using synchronized infrared thermography and high-speed visualization

Droplet impact on heated solid surfaces governs heat transfer in spray cooling and thermal management, yet the coupled interfacial dynamics and transient heat transfer remain challenging to probe experimentally. This work introduces a novel experimental platform, featuring a transparent ITO-coated sapphire substrate that enables simultaneous high-speed infrared thermography (2000 Hz, and 25 µm/pixel) of the liquid-solid interface, synchronized with side- and bottom-view visualization. This unique tri-modal approach (side-view, bottom-view, and IR thermography) captures causal links between wall superheat, bubble genesis, wetting behavior, and transient cooling during ethanol droplet impact over a wide temperature range (120 −240°C) and Weber numbers (103−194). We identify three distinct regimes: i)At low wall temperatures (≤ 150 °C), sustained bubbly boiling with vigorous nucleation yields the highest heat transfer, where the thermal contact area exceeds geometric spreading by up to 50%. ii) At intermediate temperature (∼ 180 °C), a remarkable fingering boiling pattern emerges in the transition boiling, characterized by spatially non-uniform heat flux and reduced liquid -solid contact; increasing Weber number compresses the vapor layer, suppressing fingering. iii)At higher wall temperatures (≥ 210 °C), Leidenfrost-like bouncing occurs, with bottom-view confirmation of a continuous vapor layer at 240 °C. The regime transitions are quantified through cooling effectiveness, which peaks at 0.56 at 150 °C, and drops to 0.06 in film boiling. Comparison with prior studies suggest that Jakob number can be used as a universal predictor for boiling transitions upon droplet impact, independent of fluid properties. These findings provide a comprehensive experimental framework for distinguishing boiling regimes during droplet impact, which can inform the design of droplet and spray-based thermal management systems by identifying the optimal superheat for nucleate boiling and the conditions that trigger inefficient transition and film boiling.