A filament stretching device for measurement of extensional viscosity

Abstract

A filament stretching device for measuring the extensional viscosity of low-viscosity liquids is presented. The fluid sample is held between two disks which move apart at an increasing velocity so that the extension rate, based on the filament midpoint diameter, is constant. The device was used to measure the extensional stress growth coefficients of three ideal elastic solutions, including the model fluid M1 and a shear-thinning model fluid A1. The results indicate that all solutions containing high molecular weight polymer exhibit significant strain hardening as the fluid is extended. For the ideal elastic fluids, steady state in extensional stress was observed at strain above 4.5 and the steady Trouton ratio obtained for the fluids range from 2 to 5×103. For the fluid M1 the extensional viscosities obtained are higher than the apparent extensional viscosity obtained by other methods. This is the first time that the steady extensional viscosity has been measured for polymer solutions. The results obtained enable one to evaluate the numerous constitutive equations that have been proposed for polymer solutions.

Sub-cell approximations for viscoelastic flows—filament stretching

“…The aspect ratio of the filament was 0.54, and a fluid with moderate to high extensional viscosity was considered. Their results corroborated well with those extracted from the rheometer of Tirtaatmaja and Sridhar [20]. The finite element scheme reported in the present work has linear sub-cell form, similar to algorithms of [9,16].…”

“…Such a setting may be encountered in rheometry, for example, whilst measuring the elongational viscosity of a fluid [17,20]. Resistance of a material to stretching deformation can be characterised by its uniaxial extensional viscosity.…”

“…The filament-stretching rheometer, introduced by Tritaatmadja and Sridhar [20] has proved to be a useful experimental device. Both experimental and numerical studies have largely contributed to the understanding of extensional rheometry.…”

Interfacial hoop stress and instability of viscoelastic free surface flows

“…9 In a filament stretching device, fluid is placed in the gap between two coaxial circular disks. 10,11 The disks are then rapidly separated, and the fluid forms a nominally axisymmetric bridge that is very nearly cylindrical, except near the ends, where the flow is constrained by the no-slip boundary conditions on the disks. Under some conditions, this bridge loses axisymmetry quite dramatically, as illustrated by Spiegelberg and McKinley 9 -near the endplates, the filament splits into multiple fibrils and at later times the individual fibrils undergo a further splitting.…”

Instabilities in stagnation point flows of polymer solutions

“…In extensional flows of polymer solutions, when the velocity gradient (or strain rate, ε ) exceeds one-half of the relaxation rate of the polymer ( 0.5 λ , where λ is the characteristic relaxation time), such that the Weissenberg number Wi 0.5 λε = > , macromolecules can be significantly deformed from their equilibrium Gaussian coiled conformation, 1-3 leading to orders-of-magnitude increases in the apparent extensional viscosity of the fluid, even for dilute solutions. [4][5][6] It is this strain hardening property that is exploited in numerous industrial and biological processes and applications, ranging from enhanced oil recovery, inkjet printing, turbulent drag reduction and fiber-spinning, to flows of mucin secretions and polysaccharide solutions in the circulatory systems of animals. The extensional viscosity is recognized as a fundamental material function that must be quantified in order to fully characterize the rheology of complex fluids.…”