The processes of surface wetting and film evaporation play a major role in any application using liquid fuels. Since the behavior of entire multi-liquid films is influenced by many simultaneously occurring physical processes, exact modeling is not yet possible. In order to reduce the complexity and to determine the basic effects in the spreading and evaporation of multi-component films, this study was carried out by placing single 5 μl droplets on a heated metal surface. Various alkanes, ethanol, and mixtures, as well as real gasoline, were studied at surface temperatures between 69°C and 140°C.
To describe the processes qualitatively and determine the time-dependent wetted surface area, the droplets were visualized using cameras. With the results, it was possible to determine the course of the wetted surface over time and to compare different liquids under varying surface temperatures. In addition, the temperature changes of the metal surface were determined by means of infrared (IR) thermography.
The experiments document the broad variety of multi-component fuel behavior. In contrast to the known spherically shaped droplets of one-component liquids, the droplets of multi-component fuels have a more complex propagation and evaporation behavior that cannot be predicted by conventional models. In order to better understand the physics, we elaborated on various subprocesses, which occur during the processes of propagation and evaporation. These subprocesses appear sequentially and partly simultaneously. It was found that the occurrence of individual subprocesses is dependent on the boundary conditions and the fuel composition.