Morphological transitions of a Newtonian bubble rising in a viscoelastic liquid

A Newtonian bubble rising in a viscoelastic liquid can either elongate and split or retain a quasi-spherical (QS) shape, but a unifying mechanistic explanation for these distinct morphologies has been lacking. We show that the key factor is the location of hoop stress relaxation relative to the bubble interface. Localization of stress relaxation near the trailing interface sustains axial stretching and capillary pinch-off, resulting in elongation and either continuous or early-stage splitting. In contrast, migration of the relaxation zone away from the bubble interface suppresses tail growth and promotes recovery toward a QS shape. Phase-field computations based on the Giesekus model across ranges of Ca, Wi, and polymeric viscosity ratio (1−β) show that the competition between viscoelastic and capillary stresses delineates four morphological regimes and establishes a predictive connection between polymer relaxation dynamics and macroscopic bubble shape in viscoelastic multiphase flows.