Divergent streamlines and free vortices in Newtonian fluid flows in microfluidic flow focusing devices

Monica Oliveira, F.T. Pinho, M.A. Alves

Research output: Contribution to journalArticle

17 Citations (Scopus)
107 Downloads (Pure)


The appearance of divergent streamlines and subsequent formation of free vortices in Newtonian fluid flows through microfluidic flow-focusing geometries is discussed in this work. The micro-geometries are shaped like a cross-slot but comprise three entrances and one exit. The divergent flow and subsequent symmetric vortical structures arising near the centreline of the main inlet channel are promoted even under creeping flow conditions, and are observed experimentally and predicted numerically above a critical value of the ratio of inlet velocities (VR). As VR is further increased these free vortices continue to grow until a maximum size is reached due to geometrical constraints. The numerical calculations are in good agreement with the experimental observations and we probe numerically the effects of the geometric parameters and of inertia on the flow patterns. In particular, we observe that the appearance of the central recirculations depends non-monotonically on the relative width of the entrance branches and we show that inertia enhances the appearance of the free vortices. On the contrary, the presence of the walls in three-dimensional geometries has a stabilizing effect for low Reynolds numbers, delaying the onset of these secondary flows to higher VR. The linearity of the governing equations for creeping flow of Newtonian fluids was invoked to determine the flow field for any VR as a linear combination of the results of three other independent solutions in the same geometry.
Original languageEnglish
Pages (from-to)171-191
Number of pages20
JournalJournal of Fluid Mechanics
Early online date28 Sep 2012
Publication statusPublished - 25 Nov 2012


  • low Reynolds number flows
  • microfluidics
  • divergent streamlines
  • vortex dynamics
  • free vortices
  • Newtonian fluid flows
  • microfluidic
  • flow-focusing devices

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