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Microfluidic step-emulsification in axisymmetric geometry

Abstract

Biphasic step-emulsification (Li et al. Lab Chip, 2015, 15, 1023) is a promising microfluidic technique for high-throughput production of μm and sub-μm highly monodisperse droplets. The step-emulsifier comprises of a shallow (Hele-Shaw) microchannel operating with two co-flowing immiscible liquids and an abrupt expansion (i.e., step) to a deep and wide reservoir. Under certain conditions the confined stream of the disperse phase, engulfed by the co-flowing continuous phase, breaks into small highly monodisperse droplets at the step. Theoretical investigation of the corresponding hydrodynamics is complicated due to the complex geometry of the planar device, calling for numerical approaches. However, direct numerical simulations of the three dimensional surface-tension-dominated biphasic flows in confined geometries are computationally expensive. In the present paper we study a model problem of axisymmetric step-emulsification. This setup comprises of a stable core-annular biphasic flow in a cylindrical capillary tube connected co-axially to a reservoir tube of larger diameter through a sudden expansion mimicking the edge of the planar step-emulsifier. We demonstrate that the axisymmetric setup exhibits similar regimes of droplet generation as the planar device. Detailed parametric study of the underlying hydrodynamics is feasible via inexpensive (two dimensional) simulations owing to the axial symmetry. The phase diagram quantifying the different regimes of droplet generation in terms of governing dimensionless parameters is presented. We show that in a qualitative agreement with experiments in planar devices, the size of the droplets generated in step-emulsification regime is independent of the capillary number and almost insensitive to the viscosity ratio. These findings confirm that the step-emulsification regime is solely controlled by the surface tension. The numerical predictions are in excellent agreement with in-house experiments with axisymmetric step-emulsifier.

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Publication details

The article was received on 19 Jul 2017, accepted on 12 Sep 2017 and first published on 12 Sep 2017


Article type: Paper
DOI: 10.1039/C7LC00755H
Citation: Lab Chip, 2017, Accepted Manuscript
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    Microfluidic step-emulsification in axisymmetric geometry

    I. Chakraborty, J. Ricouvier , P. Yazhgur, P. Tabeling and A. Leshansky, Lab Chip, 2017, Accepted Manuscript , DOI: 10.1039/C7LC00755H

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