Secondary atomization of water-in-oil emulsion drops impinging on a heated surface in the film boiling regime

The present study experimentally examines the secondary atomization arising from the impingement of a water-in-oil emulsion drop onto a hot sapphire glass surface. The main aim of this study is to characterize secondary droplets emerged from the rim and lamella disruption. Drop impacts of n-dodecane and emulsions with a water volume content of 1.98%, 4.95%, 9.90% and 19.80% have been observed using a high-speed video system. The impact velocity and the target initial temperature have been varied. It is shown that the maximum spreading diameter of the impinging drop, the typical dimensionless diameter of the secondary drops, as well as the number of secondary drops correlate well with the Weber number for a pure liquid. Corresponding scaling relations D_max∼D₀We^1/2, D₃₂∼D₀We^−1/2 and N∼We^3/2 in the limit We≫1 are derived by considering the dynamics of spreading and breakup. The size of the secondary drops is reduced significantly for high emulsion concentrations. This effect is explained by the stabilizing role of the water drops in the emulsion, whose surface tension is much higher than the surface tension of the bulk liquid.