The phenomenon of droplet impact on a confined thin liquid film is encountered in a variety of industrial applications. Despite exhaustive research, the central rebound jet (CRJ) and its pinch-off are still far from being understood owing to their strong randomness and uncertainty in secondary pinch-off droplet numbers. This study investigated experimentally the CRJ and its pinch-off formed by the normal impact of a single droplet on a confined thin liquid film. The dynamic evolution of CRJ formation and its pinch-off are discussed for three typical Weber numbers ( We). Its morphology was analyzed by focusing on the effects of We and film thickness, and a qualitative comparison on CRJ height was made with the previous results. The critical thresholds of CRJ pinch-offs are characterized, and a novel concise prediction method was developed. The results show that the increase in dome diameter is caused not only by CRJ rising, but also by its fallback. Pinch-off heights of the CRJ usually lead to a critical threshold of We (or K), decreasing with increase in film thickness. The CRJ maximum height increases with increase in Fr and shows a power function. An active region of liquid film thickness taking a Gaussian normal distribution were found for CRJ formation and its pinch-off. The film thickness has significant influence on CRJ height in the active region, but little outside this region. A novel concise equation for predicting CRJ pinch-off and its droplet numbers was further obtained by an inverse multiple power-law function.

The phenomenon of droplet impact on heated wall along with its evaporation is encountered in a variety of scenarios in industrial production. The present work aims to experimentally study the evaporation heat transfer of single hydrous ethanol droplet impact on a heated wall at saturation temperature under lower velocity. Its dynamic evolution after the impact, along with the stable adhesion morphology, was studied at different wall temperatures. The characteristics of its heat transfer and evaporation were analyzed by focusing on the effects of wall temperature, ethanol concentration, and adhesion morphology. The results show that the wall temperature has little effect on the spreading time, but significant on the spreading area and retraction. The "pining effect" of the hydrous ethanol weakens with the ethanol concentration, until it completely disappears. The film thickness along with its contact angle in adhesion evaporation shows a linear decrease accompanied by oscillations at the later stage. Evaporation heat transfer rate gradually increases with the wall temperature and ethanol concentration, and shows a multiple power function with them. The saturation temperature in evaporation gradually increases with the wall temperature, while the ethanol concentration has a minor effect. The average heat flux in evaporation can reach the magnitude of 10E5, and increases with the wall temperature and ethanol concentration.