Elucidating the mechanism of step-emulsification

TL;DR

This paper uses detailed simulations to uncover how pressure gradients and jet dynamics influence droplet formation in step-emulsification devices, proposing a new criterion for the transition between dripping and jetting.

Contribution

It introduces a novel understanding of droplet formation mechanisms and a new criterion for the dripping-jetting transition based on local Capillary and Weber numbers.

Findings

Adverse pressure gradient causes back-flow in the device.

Jet striction leads to droplet rupture.

Increasing dispersed phase flow stabilizes the jet, delaying rupture.

Abstract

Three-dimensional, time-dependent direct simulations of step emulsification micro-devices highlight two essential mechanisms for droplet formation: first, the onset of an adverse pressure gradient driving a back-flow of the continuous phase from the external reservoir to the micro-channel. Second, the striction of the flowing jet which leads to its subsequent rupture. It is also shown that such a rupture is delayed and eventually suppressed by increasing the flow speed of the dispersed phase within the channel, due to the stabilising effect of dynamic pressure. This suggests a new criterion for dripping-jetting transition, based on local values of the Capillary and Weber numbers.