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

Michael Hein, Shahriar Afkhami, Ralf Seemann, Lou Kondic

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


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 us 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.

Original languageEnglish (US)
Pages (from-to)911-917
Number of pages7
JournalMicrofluidics and Nanofluidics
Issue number5-6
StatePublished - May 1 2015

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Materials Chemistry


  • Capillary focusing
  • Drops and bubbles
  • Hele-Shaw flow
  • Step-emulsification
  • Volume-of-fluid


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