  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.
