# A Review of the Dynamics Progress of Bubble Collapse within Droplet and Droplet Splash
## 2 Typical Forms of Cavitation Bubble Collapse  
### Microjets  
Due to the influence of gravity on the droplet structure, researchers have conducted droplet cavitation experiments in spacecrafts [45; 57]. Figure 2 shows the microjets formed during the collapse process of a cavitation bubble within a droplet [45]. Under the condition of microgravity, it shows a nearly spherical drop structure, and the cavitation bubble was induced by electric spark. Figure 2a shows the maximum volume state of a cavitation bubble within a droplet at an eccentric position. Figure 2b shows the state of a bubble forming a microjet within a droplet with microjet velocity greater than 50 m/s. The jet on the right side of Figure 2c shows the formation of a double jets structure on the droplet surface caused by bubble collapse, with a jet velocity of 6 m/s. It should be noted that the structure on the left side of the droplet is a droplet splash structure. In addition, this research indicates that the process of jets formation during cavitation bubble collapse within droplets is consistent with the process of bubble collapse under the free surface. There are two kinds of micro-jets formed during the collapse of a droplet cavitation, the micro-jet formed by the cavitation inside the droplet and the micro-jet formed on the surface of the droplet caused by the shock wave induced by the bubble collapse.  
Figure 3 shows the jet structure during the collapse process of a cavitation bubble at a liquid surface [60]. During the process of bubble collapse, the bubble first collapses near the free surface, then the microjet passes through the center of the bubble, and finally evolves into a mushroom-shaped vapor cloud. At the same time, in the opposite direction to the bubble microjet, a pinnacle splash on the liquid surface forms. The vapor cloud mass is caused by the non-spherical evolution in the process of bubble collapse, and the vapor bubble moves away from the free surface. Additionally, the splash at the top is due to the increase of the pressure between the droplet and the bubble wall, resulting in the rupture of the free surface.  
Figure 3: Microjet structure during bubble collapse process [60].  
Figure 2: The Microjets formed during the collapse process of cavitation bubble within the droplet [45]. (**a**) Bubble with maximum radius. (**b**) Microjet. (**c**) Two liquid jets.  
Figure 4 shows the motion process of the bubble wall and the formation mechanism of the vortex ring structure [61]. Among them, Figure 4a shows that the bubble wall begins to approach the free surface at the initial moment. Figure 4b shows the reverse direction of the pinnacle jet and the microjet formed by the bubble collapse on the free surface. The reflected flow passes through the bubble, which also leads to the second collapse starting from the lower side of the bubble. Figure 4c shows the expansion of the bubble volume after a single collapse process. Figure 4d shows the vortex ring structure generated during the later stage of the bubble collapse, caused by the pressure gradient at the edge of the bubble wall. And, a high-pressure stagnant flow ring is formed.  
Based on this, it can be seen that due to the influence of a spherical free surface, the microjet process of the cavitation bubble within a droplet [46] is more complex than in the infinite fluid domain [62; 63]. There are not only microjet processes, the evolution of a collapsed vapor cloud [64], and secondary cavitation jets in a cavitation bubble within a droplet, but also splash jets on the surface of the droplet.