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We present a parametric study of the unsteady phenomena associated with theflow of elongated gas bubbles travelling through liquid-filled squarecapillaries under high Weber number conditions. These conditions consistentlyinduce the formation of a re-entrant jet at the back of the bubble thatcommonly gives way to a deep liquid cavity. Subsequent steps include pinch-offevents in the cavity to generate one or multiple encapsulated drops which maycoalesce, in conjunction with the bursting of the bubble-liquid interface byeither the cavity or the drops. Some of these interfacial instabilities havepreviously been reported experimentally (Olbricht 1996) and numerically(Izbassarov & Muradoglu 2016) for liquid-liquid flow in microchannels. We carryout three-dimensional direct numerical simulations based on a hybridinterface-tracking/level-set method capable of accounting for the presence anddynamic exchange of surfactants between the liquid bulk phase and theliquid-gas interface. Our results indicate that the delicate interplay amongstinertia, capillarity, viscosity, surfactant adsorption/desorption kinetics, andMarangoni stresses has a dramatic influence over the non-axisymmetricmorphological structures of the encapsulated drops-elongated bubble. Thisstrong coupling also influences the pinch-off time, penetration depth of thecavity, and number, size, and velocity of the encapsulated drops across thebubble. The observed phenomena are summarised in three main morphologicalregimes based on surfactant-related parameters and dimensionless groups. Adiscussion of the flow regime maps is also provided.

Drop encapsulation and bubble bursting in surfactant-laden flows in capillary channels