Macromolecular dynamics in extensional flows: 1. Birefringence and viscometry

Instabilities in stagnation point flows of polymer solutions

“…Most commonly, still images appear to show two birefringent strands widely separated near x = 0 and converging downstream of the stagnation point. This seems on first sight to be analogous to the birefringent pipe structures reported in axisymmetric opposed jet flows of polymer solutions, 41,43 however video imaging (in this instance captured at approximately 80 fps, with 5 ms exposure) shows clearly that the strands are not steady in space or time but fluctuate rapidly. A space-time diagram derived from analysis of the retardation intensity measured between the dashed white lines in Fig.…”

“…8(b)) and assembling them into a contour plot as a function of the Weissenberg number. A similar approach was taken by Carrington et al 43 for presenting the FIB structures they observed over a range of Wi in the opposed jets apparatus. In Fig.…”

“…[41][42][43] However, there are two important points to be aware of with regard to these early works: 1) the experimental studies were performed under conditions of very high 31,[35][36][37] In this study we examine the critical onset conditions for viscoelastic flow instabilities in polymer solutions using the ideal planar extensional flow field generated within the OSCER device. We employ a wide range of complex fluids formulated from different polymers of various molecular weights and concentrations in aqueous solution to provide fluids with a very wide range of viscoelastic properties, as characterized by the elasticity number, viscosity ratio and extensibility parameter.…”

Extensional flow oscillatory rheometry

“…Along the axial region of the opposed jets, a fluid element will be accelerated from the central zone towards the jet entrance. The flow field created is simple uniaxial extension to a first approximation, high strain rates can be realised and the pressure drop across the jets provides a measure of the effective extensional viscosity of the fluid (Carrington et al [16,17]). Those streamlines which pass close enough to the stagnation point provide fluid elements with sufficient residence time to accumulate very high strain (Crowley et al [18]).…”

Shear and extensional rheology of solutions of mixtures of poly(ethylene oxide) and anionic surfactants in ionic environments

“…Fluid inertia in the region between the jets, as well as the shear flow at the entrance to the capillaries, makes the flow depart from the ideality of uniaxial extension. However, the ratio P /ε can be interpreted as an apparent extensional viscosity [36]. It is relevant to represent the data in terms of a relative extensional viscosity ratio, defined by η e η e,s = P P s , (11) where η e /η e,s is the ratio between the apparent elongational viscosities of the solution and the solvent (water), P is the pressure drop measured for the solution and P s is the pressure drop for water at the same strain rate.…”