Search

Liquid/liquid second-dispersion signifies the process of breaking dispersed droplets, for the purpose of further reducing the droplet size. This study introduces a systemic experimental study on the second-dispersion process in different constricted microchannels and presents the flowing behaviors of liquid plugs passing through them. Both breakup, non-breakup and their transition of plugs were observed at the operating conditions of Ca ranging from 0.0002 to 0.025 and Re ranging from 0.74 to 27.3 in the orifice. Main factors influencing the plug breakup process, such as surfactant concentration, plug length, flow rate, viscosity and channel geometry, were carefully examined. We found the liquid/liquid second-dispersion process, driven by the capillary pressure criterion, is both a statically confined and dynamical confined process. To realize second-dispersion, the hydraulic radius ratio of downstream microchannel to constrictive orifice should be larger than 1.89 and the operating condition should be lower than the critical capillary numbers (~10-3) of continuous phase and dispersed phase. We find these critical capillary numbers reduce as the rising of hydraulic radius of the orifice but changed little with the enlargement of downstream microchannel. The orifice geometry also dominates the daughter droplets, whose radii are at least two times of the orifice hydraulic radius. [ABSTRACT FROM AUTHOR]

Microchannel emulsification requires large amounts of surfactant to prevent coalescence and improve emulsions lifetime. However, most numerical studies have considered surfactant-free mixtures as models for droplet formation in microchannels, without taking into account the distribution of surfactant on the droplet surface. In this paper, we investigate the effects of nonuniform surfactant coverage on the microfluidic flow pattern using an extended lattice-Boltzmann model. This numerical study, supported by micro-particle image velocimetry experiments, reveals the likelihood of uneven distribution of surfactant during the droplet formation and the appearance of a stagnant cap. The Marangoni effect affects the droplet breakup by increasing the shear rate. According to our results, surfactant-free and surfactant-rich droplet formation processes are qualitatively different, such that both the capillary number and the Damköhler number should be considered when modeling the droplet generation in microfluidic devices. The limitations of traditional volume and pressure estimation methods for determining the dynamic interfacial tension are also discussed on the basis of the simulation results. [ABSTRACT FROM AUTHOR]