Capillary focusing close to a topographic step: Shape and instability of confined liquid filaments

TL;DR

This paper investigates the shape, stability, and breakup mechanisms of confined liquid filaments in microfluidic step-emulsification, combining experiments and numerical simulations to understand the transition between droplet formation regimes at different capillary numbers.

Contribution

It provides a detailed experimental and numerical analysis of filament breakup regimes, highlighting the transition between step- and jet-regimes in microfluidic droplet generation.

Findings

Characterized filament shapes across a range of capillary numbers.

Validated numerical simulations with experimental data.

Identified the transition mechanism between droplet breakup regimes.

Abstract

Step-emulsification is a microfluidic technique for droplet generation which relies on the abrupt decrease of confinement of a liquid filament surrounded by a continuous phase. A striking feature of this geometry is the transition between two distinct droplet breakup regimes, the "step-regime" and "jet-regime", at a critical capillary number. In the step-regime, small and monodisperse droplets break off from the filament directly at a topographic step, while in the jet-regime a jet protrudes into the larger channel region and large plug-like droplets are produced. We characterize the breakup behavior as a function of the filament geometry and the capillary number and present experimental results on the shape and evolution of the filament for a wide range of capillary numbers in the jet-regime. We compare the experimental results with numerical simulations. Assumptions based on the smallness of the depth of the microfluidic channel allow to reduce the governing equations to the Hele-Shaw problem with surface tension. The full nonlinear equations are then solved numerically using a volume-of-fluid based algorithm. The computational framework also captures the transition between both regimes, offering a deeper understanding of the underlying breakup mechanism.