  Bubbles rising in viscoelastic liquids may exhibit a jump discontinuity of
the rise velocity as a critical bubble volume is exceeded. The phenomenon has
been extensively investigated in the literature, both by means of experiments
as well as via numerical simulations. The occurrence of the velocity jump has
been associated with a change of the bubble shape under the formation of a
pointed tip at the rear end and to the appearance of a so-called negative wake
with the liquid velocity behind the bubble, pointing in the opposite direction
to that in viscous Newtonian fluids. We revisit this topic, starting with a
review of the state of knowledge on the interrelations between the mentioned
characteristic features. In search for a convincing explanation of the jump
phenomenon, we performed detailed numerical simulations of the transient rise
of single bubbles in 3D, allowing for a local polymer molecular conformation
tensor analysis. The latter shows that polymer molecules traveling along the
upper bubble hemisphere are stretched in the circumferential direction, due to
the flow kinematics. Then, depending on the relaxation time scale of the
polymer, the stored elastic energy is either unloaded essentially above or
below the bubble's equator. In the former case, this slows down the bubble,
while the bubble gets accelerated otherwise. In this latter case, the relative
velocity of the polymer molecules against the bubble is increased, giving rise
to a self-amplification of the effect and thus causing the bubble rise velocity
to jump to a higher level. Detailed experimental velocity measurements in the
liquid field around the bubble confirmed the conclusion that the ratio of the
time scale of the Lagrangian transport of polymer molecules along the bubble
contour to the relaxation time scale of the polymer molecules determines the
sub- or supercritical state of the bubble motion.

Fluid Dynamics;Numerical Analysis;Numerical Analysis;Computational Physics