The die swell effect is the swelling of an extruded polymer solution.
The image compares water (left) and polymer solution (right) jets, which exhibits a swelling near the pipe mouth. The swelling is caused by the relaxation of the polymer chains elongated by the shear flow in the pipe. Liquids are colored in blue for the illustration with methylene blue. The polymer solution is composed of polyethylene oxide of a high molecular weight.