  We present a parametric study of the unsteady phenomena associated with the
flow of elongated gas bubbles travelling through liquid-filled square
capillaries under high Weber number conditions. These conditions consistently
induce the formation of a re-entrant jet at the back of the bubble that
commonly gives way to a deep liquid cavity. Subsequent steps include pinch-off
events in the cavity to generate one or multiple encapsulated drops which may
coalesce, in conjunction with the bursting of the bubble-liquid interface by
either the cavity or the drops. Some of these interfacial instabilities have
previously been reported experimentally (Olbricht 1996) and numerically
(Izbassarov & Muradoglu 2016) for liquid-liquid flow in microchannels. We carry
out three-dimensional direct numerical simulations based on a hybrid
interface-tracking/level-set method capable of accounting for the presence and
dynamic exchange of surfactants between the liquid bulk phase and the
liquid-gas interface. Our results indicate that the delicate interplay amongst
inertia, capillarity, viscosity, surfactant adsorption/desorption kinetics, and
Marangoni stresses has a dramatic influence over the non-axisymmetric
morphological structures of the encapsulated drops-elongated bubble. This
strong coupling also influences the pinch-off time, penetration depth of the
cavity, and number, size, and velocity of the encapsulated drops across the
bubble. The observed phenomena are summarised in three main morphological
regimes based on surfactant-related parameters and dimensionless groups. A
discussion of the flow regime maps is also provided.

Fluid Dynamics