Thermographic analysis of interfacial heat transfer mechanisms on droplet/wall interactions with high temporal and spatial resolution

This paper explores the use of time resolved infrared (IR) thermography combined with high-speed imaging to describe the liquid-surface interfacial heat transfer phenomena occurring at droplet/wall interactions. A first set of experiments is used to infer on the validation of custom made calibration and post-processing methods to evaluate the potential to obtain accurate data, useful to describe the observed phenomena. In this context, the technique showed good potential to capture very well particular details on droplet dynamics and heat transfer, allowing to identify air bubble trapping at the impact region, as well as the temperature variations at the formation of the rim. The combined analysis of droplet dynamics (e.g. the spreading factor) with the radial temperature profiles, heat flux and cooling effectiveness allowed establishing qualitative and quantitative trends on the effect of various parameters on the heat transfer occurring at droplet/wall interactions. Particularly, details on the temperature distribution within the lamella, reflected on the surface temperature fields, which are related to the spreading dynamics, are explored and discussed. Furthermore, extreme wetting scenarios, such as superhydrophobicity are studied in detail. The analysis performed show that they limit the heat transfer between the droplet and the surface. However, the thermal analysis coupled with the spreading dynamics reveals that a major reason for this is not just related to the reduced contact time of the droplet (due to rebound) or air entrapment, but is also associated to the reduced wetted area caused by low wettability, which is not obvious from the analysis of the spreading diameter alone, but becomes evident when this is coupled with the evaluation of the temperature fields on the heated surface.