  The buoyancy-driven dynamics of a pair of gas bubbles released in line is
investigated numerically, focusing on highly inertial conditions under which
isolated bubbles follow non-straight paths. In an early stage, the second
bubble always drifts out of the wake of the leading one. Then, depending on the
ratios of the buoyancy, viscous and capillary forces which define the Galilei
($Ga$) and Bond ($Bo$) numbers of the system, five distinct regimes specific to
such conditions are identified, in which the two bubbles may rise independently
or continue to interact and possibly collide in the end. In the former case,
they usually perform large-amplitude planar zigzags within the same plane or
within two distinct planes, depending on the oblateness of the leading bubble.
However, for large enough $Ga$ and low enough $Bo$, they follow nearly vertical
paths with small-amplitude erratic horizontal deviations. Increasing $Bo$ makes
the wake-induced attraction toward the leading bubble stronger, forcing the two
bubbles to realign vertically one or more times along their ascent. During such
sequences, wake vortices may hit the trailing bubble, deflecting its path and,
depending on the case, promoting or hindering further possibilities of
interaction. In some regimes, varying the initial distance separating the two
bubbles modifies their lateral separation beyond the initial stage. Similarly,
minute initial angular deviations favour the selection of a single vertical
plane of rise common to both bubbles. These changes may dramatically affect the
fate of the tandem as, depending on $Bo$, they promote or prevent future
vertical realignments.
