Axisymmetric numerical simulations of drop formation in viscoelastic jets*

Droplet formation of non-Newtonian fluids is of central importance to numerous industrial applications; these include spray-drying, atomisation, and paints, involving large interfacial deformations and complex spatio-temporal dynamics. We perform axisymmetric simulations of an impulsively-started viscoelastic jet exiting a nozzle and entering a stagnant gas phase using the open-source code Basilisk. This code allows for efficient computations through an adaptively-refined volume-of-fluid technique to capture the interface, and the log-conformation transformation, which provides a stable and accurate solution of the viscoelastic constitutive equation. For the first time, the entire jetting and breakup process of a viscoelastic fluid is simulated, including the flow through the nozzle which results in an initial radial stress distribution that affects the subsequent breakup dynamics. The velocity field and the shear stresses in the nozzle, and the early stages of the jet evolution, are validated against analytical solutions and linear stability predictions, respectively. We explore the effect of shear flow inside the nozzle on the thinning dynamics of the viscoelastic jet via analysis of the spatio-temporal evolution of the polymeric stresses. We also investigate systematically the dependence of the filament thinning rate and its breakup length on the axial momentum of the jet and the fluid relaxation time. Finally, we demonstrate the capacity of Basilisk to resolve the elasto-capillary regime of the breakup process, using mesh adaptivity, as a function of the finite extensibility of the polymeric chains.