Hydrodynamics and crystallization of NaCl aqueous solution droplet impact on heated surface

The characteristics of NaCl aqueous solution droplet impact on heated surfaces were captured by a high-speed camera for temperatures from 80 to 480 degrees C and impact velocities from 1.17 to 4.50 m/s. The NaCl crystal morphologies after the solution vaporized were used to help understand the crystallization and flow. Below the saturation temperature, the droplet experiences impact, spreading, oscillation, evaporation and evaporation coupled with crystallization. The salts gradually accumulate at the precursor line which moves toward the droplet center to finally form regular crystals from the NaCl aqueous solution. The results show that the impact velocity directly affects the crystal shell formation, while the mass concentration mainly affects the crystal distribution. Compared with pure water, the NaCl aqueous solution droplets show different boiling regimes with surface temperature since the salt crystal deposition increases the boiling and the Leidenfrost temperature. Six typical boiling regimes were identified to classify the boiling characteristics based on the surface temperature and impact velocity. A central jet was observed for surface temperatures of 280 similar to 440 degrees C for an NaCl mass concentration of 0.2 due to the blocking effect from the violent boiling during the early spreading stage. The curves separating the central jetting regime and the other regimes are not linear.