# A Review of the Dynamics Progress of Bubble Collapse within Droplet and Droplet Splash
###### Abstract
When the surface shear stress is high enough, the water droplets will be torn from the surface wave [38; 39; 40; 41; 42; 43]. Figure 1: Introduction figure of the fields involved in cavitation bubbles within droplets. (**a**) fuel atomization [44]; (**b**) inkjet printing [9]; (**c**) underwater explosion [14]; (**d**) micropump research [18]; (**e**) acoustic droplet vaporization technology [26]; (**f**) needle-free injection [36]; (**g**) marine aerosol research [42]. In many fields, cavitation bubbles within droplets have broad application prospects. The essence of cavitation bubbles within droplets is that the process of bubble collapse leads to a severe multiphase flow phenomenon, accompanied by rich fluid phenomena. The main features include the formation of microjets [45; 46; 47], bubble migration [48; 49], and the formation of microbubbles [45; 50] during the collapse process of the cavitation bubble. During the splash process of the droplet, splashes and jets of different modes [51; 52; 53] and directions [54; 55] are formed. A shock wave is generated during the collapse of the bubble within a droplet and causes a change in the pressure field in the droplet. When the pressure wave propagates to the free surface of the droplet, it will cause the surface of the droplet to oscillate and excite the droplet to splash. During the non-spherical collapse of the bubble under the free surface, a high-speed jet will be generated while moving away from the free surface. The current research on the dynamics of bubbles within droplets is still in the early stage. Many unsolved problems limit its wide applications. For example, the theoretical model suitable for the dynamics of cavitation bubble collapse within droplets still suffers many limitations. Based on these, the topic of cavitation bubbles within droplets is being given increasing attention by researchers [56]. The research progress of cavitation bubbles within droplets could be briefly introduced as follows. [PERSON] et al. [52] used laser-induced suspended droplets and captured the process of bubble collapse within droplets through high-speed photography. The different situations were observed on the surfaces of droplets, including atomization, film formation, and droplet fragmentation. The propagation process of the shock wave inside the droplet was researched. [PERSON] and [PERSON] [45; 50; 57] found that the collapse of a cavitation bubble within a droplet can generate two droplet jets. In addition, the surface of the droplet will reflect the shock wave generated by the collapse of the bubble, thereby inducing the generation of a large number of microbubbles. Subsequently, the evolution process of the shock wave of bubble within a droplet was verified through numerical simulation. [PERSON] et al. [7] established a mathematical model focusing on the effect of viscosity. [PERSON] et al. [58] conducted numerical simulation (Open FOAM) research on the splash processes of suspended droplets. It showed that the radial acceleration of the droplet splash was caused by the oscillation of an internal bubble. [PERSON] et al. [59] investigated the bubble initial position within the droplet and its effects on the form of the droplet splash. This paper mainly introduces the dynamic processes of bubble collapse and induced droplet splash and summarizes the important research achievements in recent years.