Qualitative analysis of the capillary flow stability of spurting materials by using transmitted light intensity measurements

The stability in the capillary flow of spurting materials was analyzed by following the temporal variation of their average birefringence. Two different fluids were analyzed, a high-density polyethylene melt, and an aqueous micellar solution of Cetylpyridinium Chloride and Sodium Salicylate. Birefringence changes were detected through measurements of the transmitted light intensity and video images of the flow channel. Transmitted light intensity measurements were more sensitive and provided better information about the flow stability than pressure ones. An unstable flow region was present in the micellar solution before the onset of spurt. Also, there were periodic changes in the optical properties of both fluids, in the spurt region and at higher shear rates, which indicate that the flow is also unstable in the high shear rate branch, in contrast to the generalized assumption of the stable flow in such regime. Several frequency components of the transmitted light intensity were observed, coincident with different spurts, in the unstable flow regimes for both fluids. Finally, an outstanding decrease in the transmitted light intensity was observed in both fluids under flow conditions where slip was present.