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 analog of four-roll mill, where a plane linear flow is realized. We found that the dynamics of a single vesicle is significantly altered by the presence of another vesicle at separation distances up to about 3.2 \div 3.7 times of effective radius of the vesicles. This is supported by direct measurements of a single vesicle back-reaction on the velocity field. Thus, the experiment provides the lower bound for the interaction scale of vesicles and so the corresponding upper bound for the volume fraction \phi=0.08 \div 0.13 of non-interacting vesicle suspensions.