Central rebound jet of a droplet normal impact on a confined thin liquid film

Abstract

The phenomenon of the impact of a droplet 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 the uncertainty in secondary pinch-off droplet numbers. This study experimentally investigated 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 along with its pinch-off is discussed for three typical Weber numbers ( We). Its morphology (base diameter and height) was analyzed by focusing on the effects of We and film thickness on the formation mechanism for droplets, and a qualitative comparison of CRJ height with the previous results was made. The critical thresholds of the CRJ pinch-offs are characterized, and a novel concise prediction method was developed. The results show that the increase in the dome diameter is caused not only by the CRJ growth but also by its fallback. Its maximum value is positively correlated with the increase in We and film thickness. The pinch-off height of the CRJ column is characterized by the critical threshold of We (or K), decreasing with the increase in the film thickness. The maximum height of the CRJ increases with the increase in the Froude number ( Fr) and shows a power function. An active region of the liquid film thickness taking a Gaussian normal distribution was found for CRJ formation and its pinch-off. The film thickness has a significant influence on the 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 a multiple inverse power-law function of We with Ohnesorge number ( Oh), Re/ Fr, and viscosity effects.