  Fluid and solute transfer in X-junctions between straight channels is shown
to depend critically on the junction angle 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 90\degree 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 con rm these results and show that, below the threshold
angle 33.8\degree recirculation cells are present in the center part of the
junction and separate the two injected flows of the two solutions. Reducing
further leads to the appearance of new recirculation cells with lower flow
velocities.
