On the threshold of drop fragmentation under impulsive acceleration

Secondary fragmentation of an impulsively accelerated drop depends on fluid properties and velocity of the ambient flow. The critical Weber number (Wecr) , the minimum Weber number at which a drop undergoes non-vibrational breakup, depends on the density ratio (ρ ) , the drop (Ohd) and the ambient (Ohₒ) Ohnesorge numbers. The current study uses volume-of-fluid based interface-tracking multiphase flow simulations to quantify the effect of different non-dimensional groups on the threshold at which secondary fragmentation occurs. For Ohd 0.1 , a decrease in Ohd was found to significantly influence the breakup morphology, plume formation and Wecr . The balance between the pressure difference between the poles and the periphery, and the shear stresses on the upstream surface, was found to be controlled by ρ and Ohₒ . These forces induce flow inside the initially spherical drop, resulting in deformation into pancakes and eventually the breakup morphology of a forward/backward bag. The evolution pathways of the drop morphology based on their non-dimensional groups have been charted. With inclusion of the data from the expanded parameter space, the traditional Wecr-Ohd diagram used to illustrate the dependence of the critical Weber number on Ohd was found to be inadequate in predicting the minimum initial We required to undergo fragmentation. A new non-dimensional parameter Cbreakup is derived based on the competition between the forces driving the drop deformation and the forces resisting the drop deformation. Tested using available experimental data and current simulations, Cbreakup is found to be a robust predictor for the threshold of drop fragmentation.