  The hydrodynamic interaction of two deformable vesicles in shear flow induces
a net displacement, in most cases an increase of their distance in the
transverse direction. The statistical average of these interactions leads to
shear-induced diffusion in the suspension, both at the level of individual
particles which experience a random walk made of successive interactions, and
at the level of suspension where a non-linear down-gradient diffusion takes
place, an important ingredient in the structuring of suspension flows. We make
an experimental and computational study of the interaction of a pair of lipid
vesicles in shear flow by varying physical parameters, and investigate the
decay of the net lateral displacement with the distance between the streamlines
on which the vesicles are initially located. This decay and its dependency upon
vesicle properties can be accounted for by a simple model based on the well
established law for the lateral drift of a vesicle in the vicinity of a wall.
In the semi-dilute regime, a determination of self-diffusion coefficients is
presented.
