Flow instability in polymer solutions and melts

The existing experimental data concerning the problem of flow instability in polymer solutions and melts is considered and critically discussed. The instability is understood as both regular distortions of the jet surface shape and turbulence of the flow as such. The visual manifestations and physical mechanisms determining the development of flow instability are analyzed and classified. The following principal forms of instability are distinguished: small-scale regular surface defects, periodic oscillations with the scale of the jet diameter, the slip—stick periodic transition phenomenon, self-oscillations of the stream, jet spurt, and large-scale distortions passing into stream discontinuities. In all cases, the instability of the jet is due to rubber elasticity of polymer fluids, a property which causes storage of elastic energy during deformation with its subsequent release in the form of stream distortions. Therefore, the general criterion for the onset of instability is a certain critical value of the Weissenberg number. The key factors determining the loss of the flow stability are concentration of stresses at the channel outlet, transition from laminar flow to slip along a solid wall (adhesive ruptures) under certain critical conditions, and mechanical fracture (cohesive ruptures) of a material. In the appearance of hysteresis oscillations, bulk elasticity and compressibility of the melt also play a certain role. Alternative mechanisms proposed in the literature are also discussed. Examples illustrating the possibility of suppressing jet distortions are given; this suppression is important for many industrial applications in polymer processing.