Drop encapsulation and bubble bursting in surfactant-laden flows in capillary channels
Abstract
We present a parametric study of the unsteady phenomena associated with the flow of elongated gas bubbles
travelling through liquid-filled square capillaries under highWeber number conditions. These conditions induce
the formation of an indentation at the back of the bubble that commonly gives way to a deep re-entrant liquid
jet penetrating the bubble. Subsequent steps include pinch-off events in the penetrating liquid to generate one or
multiple encapsulated drops which may coalesce, in conjunction with the bursting of the bubble-liquid interface
by either the liquid jet or the drops. Some of these interfacial instabilities have previously been reported experimentally
[65] and numerically [43] 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 liquid, 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.
Domains
Engineering Sciences [physics]
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