COIL-STRETCH TRANSITION OF MACROMOLECULES IN LAMINAR FLOW AROUND A SMALL CYLINDER

The analysis of mass transfer from a small cylindrical wire in a dilute polymer solution by using an electrochemical technique is reported here. Steady state measurements showed a large decrease of the mass transfer rate with respect to the newtonian behaviour, starting from a critical flow rate value. This phenomenon was ascribed to the fast stretching of the polymers coils which occurs when the elongational gradient in the upstream vicinity of the wire, equals the reciprocal of the polymer relaxation time. Fluctuations concurrently arise at the transition and result from an hydrodynamic instability induced by the viscosity increase during elongation. From a power spectrum density analysis, it was demonstrated that these instabilities actually take place near the stagnation point upstream.