An experimental note on the deformation and breakup of viscoelastic droplets rising in non-Newtonian fluids

Abstract

The deformation of viscoelastic droplets rising in non-Newtonian fluids is examined experimentally. Similarly to the case of bubbles and Newtonian droplets traveling in viscoelastic fluids, a critical volume exists but there is no jump in velocity nor a drastic change in droplet geometry. Nevertheless, a tail appears as well as a negative wake. For large-enough volumes, the thickness of the tail is much larger than that for the Newtonian droplets; this is also true for the magnitude of the negative wake and for the extension of the tail. The head of the droplet is subjected to a bi-axial elongation which deforms the spherical part of the droplet converting it into an elongated shape. This elongated shape blends into a tail and the droplet is converted into a viscous elongated teardrop. The tail is then subjected to a uniaxial extensional flow under the action of elongational stresses along the length of the tail. This extensional flow is counteracted by the shear stresses acting on the interface between the tail and the outer surrounding fluid. This interaction will determine the tail breakup. The influence of the elastic properties of the surrounding fluid and the extensional viscosity of the droplet will be responsible for the length and thickness of the tail as well as for its breakup mechanism.