The influence of pre-shearing on the elongational behaviour of dilute polymer and surfactant solutions

Abstract

Experimental results are presented which provide information about the elongational behaviour of pre-sheared dilute polymer and surfactant solutions. The viscosity behaviour of these pre-sheared solutions is explained on the basis of molecular data and by flow visualization. A rotational viscometer was modified in such a way that the sheared test fluid left the gap of the viscometer through an orifice in the outer cylinder. The flow-rate at a constant pressure drop through the orifice is related to the elongational viscosity of the fluid. The elongational viscosity of dilute polymer solutions increases when a critical shear gradient is exceeded. The critical shear gradient was found to be independent of the polymer concentration. It could be lowered by increasing the molecular weight of the polymer. Visualization of the orifice flows were done in a rectangular channel. A sudden increase of the apparent elongational viscosity, which is found for poly(isobutene) solutions when a certain Deborah number is exceeded, and an increase of the elongational viscosity, which for poly(acrylamide) solutions is independent of the pre-shearing deformation, can both be related to entrance vortices and instabilities of the entrance flow in the orifice. This was confirmed by flow visualization. The surfactant solutions which were used exhibit a sudden increase of the shear viscosity due to the formation of a shear-induced state (SIS) when a critical shear rate is exceeded. In the SIS their elongational viscosity is also increased. However, the Trouton ratio remains at a value of three which is characteristic for Newtonian fluids.