Secondary Atomization of a Droplet in Diverse Interaction Settings

Abstract

Secondary atomisation refers to the process by which liquid droplets, which are already in a dispersed state, are further atomised down into smaller droplets. This process occurs after primary atomisation, which is the initial breakup of a bulk liquid into droplets. Understanding and controlling secondary atomisation is crucial for optimising various industrial and natural applications. In this study, we investigate the secondary atomisation of a single droplet in three different interaction settings: shock waves, vortices, and porous surfaces (such as facemasks). In shock-droplet interaction (first setting), the multiscale phenomenon is classified into two stages: wave dynamics (stage I) and droplet breakup dynamics (stage II). Stage I involves the formation of different wave structures after an incident shock impacts the droplet surface. Stage II involves induced airflow interaction with the droplet, leading to its deformation and breakup. Primarily, two modes of droplet breakup, i.e., shear-induced entrainment (SIE) and Rayleigh–Taylor piercing (RTP) (based on the modes of surface instabilities) are observed for the studied range of Weber numbers (We∼ 30 - 15000). A criterion for the convenient transition between two breakup modes is also discussed. For measurement of drop sizes during a shock-drop interaction process, the two-sensor depth from defocus (DFD) technique is further developed to achieve higher spatial and temporal resolution, to improve estimates of spatial size distribution and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. Furthermore, the secondary atomisation of liquid metal droplets has been investigated using Galinstan as a test fluid. The study explores crucial questions, such as the applicability of atomisation results obtained from conventional fluids like DI water to liquid metal atomisation. The study sheds light on how surface oxidation of liquid metal plays a significant role in regulating atomisation dynamics and the shape of fragmented droplets. In vortex-droplet interaction (second setting), we elucidate the mechanism of co-axial interaction of a droplet with a vortex ring of different circulation strengths (Γ = 45 - 161 cm^2 s^(-1)). We focus on both the droplet and the vortex dynamics, which evolve in a spatial and temporal fashion during different stages of the interaction, as in a two-way coupled system. In the droplet dynamics, different regimes of interaction are identified, including deformation (regime-I), stretching and engulfment (regime-II), and droplet breakup (regime-III). In vortex dynamics, we compare the interaction’s effect on different characteristics of the vortex rings. Vortex-droplet interaction leads to a reduction in these parameters. In droplet porous-surface (facemask) interaction (third setting), we show that high-momentum, large-sized (>250 μm) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent. The penetrated droplets can atomise into numerous much smaller (<100 μm) droplets, which could remain airborne for a significant time. The possibility of secondary atomisation of high-momentum cough droplets by hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the single/double-layer mask material must be considered in determining mask efficacy. The results of droplet atomisation are compared in terms of droplet penetration, size distribution, and volume transmission. Theoretical models for the criteria of droplet penetration, breakup time, and droplet size prediction agree well with the experimental data. To conclude the discussion, we investigate an interaction test case at a low Weber number value. In this scenario, we examine a periodic interaction between a vortex ring and a droplet, where surface tension force is dominant over inertial force (low Weber number), and secondary atomisation does not occur. This type of interaction has the potential to modify the droplet’s evaporation and crystallisation characteristics. Our findings reveal that the droplets’ evaporation characteristics depend on the strength of the vortex, while the crystallisation dynamics remain independent of it.