Fragmentation of liquids into droplets via secondary flows of Newtonian and viscoelastic fluids.

Fragmentation of liquids into a population of smaller droplets is of great importance in many natural and industrial processes. In this talk we explore a new method of fragmentation that is based on secondary flows around a spinning top. We first use dye injection and visualize the flow around the spinning top as it rotates in a transparent tank filled with either Newtonian or viscoelastic liquids. In the Newtonian case, inertially-driven secondary flows generate toroidal vortices that roll around the geometry in an ever-spinning spiral. For viscoelastic liquids, presence of long macromolecules leads to normal stresses that defy inertial effects. Thus, the flow separates into viscoelastic and inertial zones, giving rise to a unique butterfly pattern that revolves around the spinning top. We use the kinematics of these secondary flows for atomization of viscous immiscible oils. As we introduce a volume of the immiscible oil into the tank, the oil blob follows the secondary flows and stretches into a continuous manifold that resembles the geometry of a toroidal sheet. The gentle nature of the secondary flow stretches the thinning oil layer continuously until it eventually breaks into a set of small droplets. We study the size distribution of these droplets under different operational conditions and provide a theoretical framework that explains our measurements.