Evolutions of liquid droplets impacting on hot porous substrates: spreading, imbibition, and evaporation

The evolutions of droplets impacting on hot porous substrates are investigated, focusing on spreading, imbibition, and evaporation. The Level-Set model is employed to track droplet morphology, and the effects of droplet material, substrate temperature, droplet size, impact velocity, and substrate porosity are quantified. The results indicate that under evaporation conditions, the spreading factor significantly decreases compared to non-evaporative scenarios. The penetration depth is observed to increase initially to a maximum before declining, with droplet size and impact velocity positively correlating with spreading. Additionally, increases in substrate temperature, droplet size, and impact velocity are found to enhance evaporation, thereby significantly reducing the penetration depth—except in cases with varying porosity. On heated porous surfaces, the capillary pinning effect is weakened, leading to earlier evaporation at the droplet rim and a shorter wetting line. Notably, a polynomial relationship between evaporation mass and time proves to be more suitable for ethanol droplets and cases with higher substrate temperatures.