From flow focusing to vortex formation in crossing microchannels

The paper is concerned with the experimental and numerical investigations of the vortex formation and flow focusing inside a cross-shaped microchannel domain. The local hydrodynamics in the junction area, upstream of the focusing region, is analyzed with the aim to characterize the onset and the evolution of the vortical structures, in correlation with the operating parameters. The numerical simulations based on a finite-volume approach are validated by direct flow visualizations using epifluorescence and confocal microscopy. The main result of the study is a flow pattern map, providing comprehensive information on the flow dynamics inside the microchannel junction as a function of the input flow rates and the corresponding Reynolds numbers. The flow pattern map identifies the limits of the flow focusing regime and the critical values of the parameters at which the vortical structures are formed. Beyond the breakdown of the classical flow focusing scenario with one focused output stream, flow patterns with two and four output streams are identified.