  We investigated the flow profile of a polymer solution in a thinning
capillary bridge. Fluorescent tracer particles with a diameter of 3$\mu$m were
used to visualize the flow. The cylindrical shape of the filament introduced
strong optical abberations that could be corrected for, and we were able to
characterize the flow in filaments with a thickness ranging from 150 to 30
$\mu$m. In the first regime when the filament was still sufficiently large, we
used a PIV algorithm to deduce the flow field. At later stages when the number
of particles in the observation plane decreased a PTV algorithm was used. The
main two results of our measurements are as follows. First, the flow profile at
the formation of the cylindrical filament is highly inhomogeneous and there is
only flow in the outer parts of the filament. Second, we find that in most
parts of the regime, where the temporal radius of the thinning filament can be
fitted with an exponential law the flow indeed is purely extensional.
