  It is presently believed that flows of viscoelastic polymer solutions in
geometries such as a straight pipe or channel are linearly stable. Here we
present experimental evidence that such flows can be nonlinearly unstable and
can exhibit a subcritical bifurcation. Velocimetry measurements are performed
in a long, straight micro-channel; flow disturbances are introduced at the
entrance of the channel system by placing a variable number of obstacles. Above
a critical flow rate and a critical size of the perturbation, a sudden onset of
large velocity fluctuations indicates presence of a nonlinear subcritical
instability. Together with the previous observations of hydrodynamic
instabilities in curved geometries, our results suggest that any flow of
polymer solutions becomes unstable at sufficiently high flow rates.
