Characteristic spatial scale of vesicle pair interactions in a plane linear flow

We report the experimental studies on interaction of two vesicles trapped in a microfluidic four-roll mill, where a plane linear flow is realized. We found that the dynamics of a vesicle in tank-treading motion is significantly altered by the presence of another vesicle at separation distances up to 3.2-3.7 times of the vesicle effective radius. This result is supported by measurement of a single vesicle back-reaction on the velocity field. Thus the experiment provides the upper bound for the volume fraction phi = 0.08-0.13 of noninteracting vesicle suspensions.