Experimental study of liquid drop impact on granular medium: Drop spreading/splashing and particle ejection

The impact of a liquid drop on a granular medium is a common phenomenon in nature and engineering. The possible splashing droplets and ejected particles could pose a risk of pathogen transmission if the water source or granular medium is contaminated. This work studies the liquid drop impact on the granular medium using high-speed photography and considers the effects of liquid properties, drop impact characteristics, and granular medium properties. Four flow regimes, including direct penetration, prompt splashing, spreading, and corona splashing, are observed and a regime map is created to identify their thresholds. The spreading regime can eject a large number of particles, and the corona splashing regime can produce splashing droplets in addition to the ejected particles. For the splashing droplets, their median diameters and velocities are in the ranges 0.11 to 0.21 and 0.15 to 0.37 of the diameter and velocity of the impact drop, and their median splashing angles range from 14° to 27°. Two particle ejection mechanisms are observed, falling squeeze and forward collision, driven by the collapsing and forward spreading of the liquid lamella, respectively. The particles ejected by the latter mechanism have larger ejection velocities, angles and distances from the impact center, which can facilitate their long-range transmission. In addition, the process of spreading and retracting of the lamella formed by the drop impact is also studied, and it is found that the maximum spreading diameter of the lamella is proportional to the crater diameter. These results improve the understanding of the phenomenon after the drop impact on the granular medium and the characteristics of the splashing droplets and ejected particles, contributing to the prediction and risk assessment of contaminated particle transmission.