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

TL;DR

This study uses 3D simulations to explore how surfactants affect the complex behaviors of bubbles and encapsulated drops in high Weber number flows within capillaries, revealing new insights into interfacial dynamics.

Contribution

It introduces a hybrid numerical method to analyze surfactant effects on bubble encapsulation and bursting in microchannels, highlighting the influence of surfactant kinetics and Marangoni stresses.

Findings

Surfactants significantly alter bubble and drop morphology.

The interplay of forces affects pinch-off timing and drop size.

Flow regimes depend on surfactant parameters and dimensionless groups.

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 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.