Impact dynamics of compound drops of fluids with density contrast

The dynamics of compound drops impacting on a flat substrate is numerically investigated using a ternary-fluid diffuse-interface method, with the aim of assessing the effect of a density difference between the inner and outer droplets (denoted by lambda) on the evolution of the interfaces. With the help of numerical simulations, we find that, at the intermediate stage of drop impact, the inner droplet exhibits a self-similar deformation at lambda = 1 and relatively high Weber number, and experiences more or less a uniform acceleration for various.. In particular, the acceleration magnitude at lambda not equal 1 can be correlated with the acceleration at lambda = 1 and the Atwood number. When the inner droplet is denser than the outer one, a lamella occurs at the spreading front of the inner droplet. We present a scaling analysis of the thickness of the lamella, and the resultant theoretical prediction is in good agreement with numerical results. At the maximal spreading of the compound drop, a bulging structure is formed around the symmetry axis due to the presence of the inner droplet, thereby effectively reducing the liquid supply to the spreading front and leading to a decrease of maximal spreading ratio beta(max) as compared with a pure drop. We proposed a corrected Weber number We(lambda)* by taking account of the combined effects of lambda, volume fraction of the inner droplet, Weber number and morphology of the compound drop. Integrating We(lambda)* with the universal model of beta(max) for impacting pure drops, we successfully build up a new model for predicting the maximal spreading ratio of impacting compound drops with various lambda.