  Knowledge about the flow field of the plasma around the red blood cells in
capillary flow is important for a physical understanding of blood flow and the
transport of micro- and nanoparticles and molecules in the flowing plasma. We
conduct an experimental study on the flow field around red blood cells in
capillary flow that are complemented by simulations of vortical flow between
red blood cells. Red blood cells were injected in a 10x12 micrometer
rectangular microchannel at a low hematocrit and the flow field around a single
or two cells have been characterized thanks to a highspeed camera and by
tracking 250 nm nanoparticles in flow behaving as tracers. While the flow field
around a steady croissant shape is found to be relatively similar to that of a
rigid sphere, the flow field around a slipper shape exhibits a small vortex at
the rear of the red blood cell. Even more pronounced are vortex-like structures
observed in the central region between two neighboring croissants. Conclusions:
The rotation frequency of the vortices is to a good approximation, inversely
proportional to the distance between the cells. Our experimental data are
confirmed and complemented by numerical simulations.
