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| Dynamics of impacting droplets |
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 Boiling morphology at the liquid-solid interface
Besides studying the thermal behaviour of the impinging droplets together with the morphological and quantitative description of the secondary atomization, we are interested in characterizing the boiling morphology at liquid-solid interface. To accomplish with this, droplet impact is recorded by two high-speed cameras disposed to occur side and bottom views of the lamella. The optical access to the interfacial region, from the bottom view images is assured by using transparent targets, heated by Joule effect, using a transparent film of Indium Oxide (In2O3). This procedure allows observing the boiling mechanisms within a wide range of surface temperatures. Here, we are using the contact temperature Tc=(el Tl+eWTW)/ (el +eW) - where e is the thermal effusivity and the subscripts l and W refer to the liquid and to the surface, respectively - for comparison of results. The Figure illustrates some morphological features observed at the impact of an HFE7100 droplet within the bubble and film boiling regimes. It is evident that liquids such as HFE7100, with low surface tension and latent heat of evaporation easily form cellular structures which degenerate into dry out regions.
Bubble boiling
Film boiling
Figure 1. Details of the temporal evolution of the boiling mechanism of HFE7100 droplets (D0=2.0mm, U0=1.3ms-1) impacting onto heated transparent substrates within different surface temperatures a) Tc=86ºC, b) Tc =115ºC.
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