Microfluidic step-emulsification in axisymmetric geometry

Abstract

Biphasic step-emulsification (Z. 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 consists 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 consists of a stable core-annular biphasic flow in a cylindrical capillary tube connected co-axially to a reservoir tube of a 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 to the planar device. A 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 qualitative agreement with experiments in planar devices, the size of the droplets generated in the 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 surface tension. The numerical predictions are in excellent agreement with in-house experiments with the axisymmetric step-emulsifier.

Mechanism of bubble formation in step‐emulsification devices

“…The formation of droplet in step‐emulsification device can be divided into dripping and balloon mechanisms. It is found that the formation of droplets under dripping mechanism is solely controlled by surface tension . The formation of droplets is hardly affected by the capillary number and the viscosity ratio of both phases.…”

“…These microfluidic devices usually include T‐junction, cross‐focusing, and coaxial microchannel . In addition to the conventional microdevices, the emerging emulsification device includes triangular cavity, step‐emulsification device, and so on. These devices are usually based on step‐emulsification principle, which are beneficial for the generation of monodispersed droplets or bubbles, as the volume of bubbles or droplets in such devices is mainly determined by the geometric parameters of the equipment and the physical properties of the fluid …”

Flow-assisted droplet assembly in a 3D microfluidic channel

“…The production of these droplets is omitted, as the step-emulsifier is typically much smaller than the size of the channel, allowing for separation of the two processes. 25,35 We note that, although the droplets are already in a cluster initially, their relative rearrangement is still important as it determines the cluster morphology in the final state. The latter results primarily from the droplet-flow interaction and has direct consequence on the photonic properties of the droplet lattice, as we will discuss in details below.…”

“…Here, the focus is on the clustering and droplet interactions within a short distance from their initial release rather than the production of the droplets, which has been studied previously. 25 Our aim is to elucidate the physical picture of the hydrodynamic interactions, potentially improving the design of more efficient microfluidic systems.…”

Creating monodispersed droplets with electrowetting-on-dielectric step emulsification

“…Methods of producing monodisperse droplets 1 includes T-junction, 2 co-flow, 3,4 flow-focusing, 5,6 and step emulsification. [7][8][9] Because step emulsification is based on the principle of Laplace pressure, it is insensitive to the flow rate and yields a constant and high monodispersity.…”