2. Isothermal Newtonian model

This stationary model, which is the simplest, is, for example, a first approximation to solution spinning. Only one filament is considered here. An important point is that the extrusion part, i.e. the reorganization of the velocity profile and the transition from a Poiseuille profile in the spinneret to an almost homogeneous velocity profile in a section of the filament during elongation, is neglected. This will also be the case for most other models. This approximation is legitimate as soon as stretching is carried out over a length greater than the diameter of the capillary, since this reorganization of the velocity profile generally takes place over a length of the order of half a diameter. The limits of this approximation will be seen later. As this flow is essentially elongational, we first consider the stretching of a liquid specimen: this is Trouton's experiment