Elastic vs. inertial instability in a polymer solution flow

The interrelation between elastic and inertial effects in destabilizing the flow of a polymer solution is studied experimentally. To achieve this goal, solution elasticity is varied by three orders of magnitude and a diagram of the flow states in a Couette-Taylor system is obtained. The regions of purely elastic and purely inertial flow instabilities and a crossover region between them are characterized. The main feature of the elastic instability, constant Deborah number at the instability threshold, is verified. An analogy between inertial and elastic flow transitions and dynamics is found and the concept of viscoelastic similarity is introduced.