Stokes flow paths separation and recirculation cells in X-junctions of varying angle between straight channels.

摘要: Fluid and solute transfer in X-junctions between straight channels is shown to depend critically on the junction angle α in the Stokes flow regime. Experimentally, water and a water-dye solution are injected at equal flow rates in two facing channels of the junction: Planar Laser Induced Fluorescence (PLIF) measurements show that the largest part of each injected fluid “bounces back” preferentially into the outlet channel at the lowest angle to the injection; this is opposite to the inertial case and requires a high curvature of the corresponding streamlines. The proportion of this fluid in the other channel decreases from 50% at α= 90◦ to zero at a threshold angle. These counterintuitive features reflect the minimization of energy dissipation for Stokes flows. Finite elements numerical simulations of a 2D Stokes flow of equivalent geometry confirm these results and show that, below the threshold angle αc= 33.8◦, recirculation cells are present in the center part of the junction and separate the two injected flows of the two solutions. Reducing further α leads to the appearance of new recirculation cells with lower flow velocities.Flow control in microchannels has become an important area of research in microprocess engineering1. The behavior of the fluids at junctions in microfluidic circuits is particularly critical for applications such as mixing, chemical reactions or heat exchange: this is for instance the case when two different fluids are injected separately into a junction of flow channels. While, for T-junctions, the flow of the two fluids at the outlet depends weakly on the junction angle at low Reynolds numbers2, 3, there are very diverse patterns in X-junctions …