  We investigate the margination of microparticles/platelets in blood flow
through complex geometries typical for in vivo vessel networks: a vessel
confluence and a bifurcation. Using 3D Lattice-Boltzmann simulations, we
confirm that behind the confluence of two vessels a cell-free layer devoid of
red blood cells develops in the channel center. Despite its small size of
roughly one micrometer, this central cell-free layer persists for up to 100
$\mu$m after the confluence. Most importantly, we show from simulations that
this layer also contains a significant amount of microparticles/platelets and
validate this result by in vivo microscopy in mouce venules. At bifurcations,
however, a similar effect does not appear and margination is largely unaffected
by the geometry. This anti-margination towards the vessel center after a
confluence may explain in vivo observations by Woldhuis et al. [Am. J. Physiol.
262, H1217 (1992)] where platelet concentrations near the vessel wall are seen
to be much higher on the arteriolar side (containing bifurcations) than on the
venular side (containing confluences) of the vascular system.
