Why Dry Spun Fibres have Serrated Cross Section?

In dry spinning polymer solution is extruded into a dry spinning cell. It is a jacketed vertical box, where heated air (inert gas) is circulated to evaporate the solvent. The evaporation of the solvent vapor from the core to the surface is quicker than the transfer of the solvent from the core to the surface. So, the surface of the skin will be relatively dry. This results in the skin drying up quickly, and the core remaining relatively viscous. Because of this, the cross-section collapses, and non-circular cross-section of the fibre results from a circular spinnerette cross section.

Cross sectional and Longitudinal View