Atomization behavior of burning stable emulsion droplets

Abstract

We investigate the atomization behavior of burning stable water-in-oil microemulsion droplets. The microemulsion compositions are prepared by altering the dispersed phase volume fraction (ϕ) and base oils (dodecane, xylene, and surrogate (90% dodecane and 10% xylene) by weight). The combustion of base oil follows classical droplet combustion, while two-stage combustion, i.e., classical droplet combustion and atomization-dominated combustion, is observed for emulsion droplets. We reveal that vapor bubble formation occurs primarily through heterogeneous nucleation, and the mechanisms leading to bubble formation can be accurately predicted and controlled for distinct, highly stable emulsions. During the initial phase of combustion, water vapor bubbles nucleate, while the nucleation of oil vapor is more prevalent in the later phase of combustion due to the depletion of water content. In both regimes, heterogeneous nucleation ensues due to the agglomeration of surfactant inside the burning droplet. The lower volatility and viscous nature of the oil inhibit the growth rate of the nucleated oil-vapor bubble within the oil–surfactant mixtures. On the contrary, the water vapor bubble exhibits a higher growth rate within the emulsion droplet, attributed to its higher volatility and low viscosity. We show that qualitatively, the global bubble dynamics associated with the emulsion droplets remain substantially consistent regardless of ϕ and the type of base oil. However, it is observed that the onset of nucleation, bubble growth and collapse cycles, and the size of secondary droplets display a strong dependence on the ϕ and the type of base oil. The normalized diameter at the onset of nucleation increases as ϕ increases, exhibiting a logarithmic trend. The optimal dispersed phase volume fraction for effective atomization is found to be ϕ=0.2, regardless of the type of microemulsion.