Flow of elongated bubbles in microchannels under high We conditions: cavity formation, drop encapsulation, and bubble rupture*

We study the dynamics of surfactant-laden elongated bubbles surrounded by liquid flowing through a rectangular microchannel under high Weber number conditions. Systems exhibiting these characteristics are found in both high-end applications of microfluidic technologies (e.g., oil recovery) and real-life environments (e.g., pulmonary airways), wherein the balance amongst inertia, viscosity, and capillarity is altered by the lower surface tension and Marangoni effects introduced by the surfactants. An increase in We or capillary number is known to gradually disturb the curvature of the bubble back until a concave re-entrant liquid cavity develops and grows towards the bubble nose. We expand on these observations by presenting a systematic characterisation of the multiple interfacial singularities that occur under these conditions, which include pinch-off and coalescence events in the liquid cavity to produce encapsulated liquid drops within the bubble and bursting of the bubble by either the liquid cavity itself or the encapsulated drops. We perform direct numerical simulations based on a hybrid interface tracking/level-set method with the account of transport and exchange of surfactants at the interface and the bulk of the liquid phase for a range of dimensionless numbers related to the flow and the surfactant adsorption/desorption kinetics. Our results suggest that the Damköhler number has a dramatic effect on the unsteady topology of the encapsulated drops-bubble compound and other characterising parameters, such as the depth of the re-entrant cavity and the drop size. The numerous phenomena involved in the system include: i) cavity formation without drop encapsulation events; ii) drop encapsulations with subsequent stabilisation of the bubble back; and iii) drop encapsulations that result in highly distorted bubble-liquid interfaces.