The horizontal stretching of test liquid A1

Abstract

Tests are described in which a polyisobutylene-decalin solution (fluid A1) is stretched horizontally by the method previously used for fluid M1 (D. R. Oliver, J. Non-Newtonian Fluid Mech., 35 (1990) 251). In one type of testing, photographs were taken of the stretching jets and from these the stress, strain and strain rate were found for a series of points moving along the jet. In another type of testing, the speed of the rotating cylinder which pulls the jet was gradually increased and the same quantities were determined for the jet taken as a whole. The extensional viscosity of fluid A1 is of the order of 450 Pa s, corresponding to a Trouton ratio of 450 if the shear viscosity is taken as 1.0 Pa s. However, there are marked variations in extensional viscosity (100–3000 Pa s) depending on both extensional strain rate (1.0–50 s −1) and other test conditions. Furthermore, the fluid is highly shear thinning in character, the shear viscosity falling from 5.0 Pa s at a shear rate of 1.0 s −1 to 0.13 Pa s at a shear rate of 10 3 s −1. Thus the Trouton ratio has less meaning than for a fluid of constant viscosity, though it is undoubtedly high. Fluid A1 may also be treated, in extension, as a weak rubber-like solid and evidence is presented of a velocity at which this behaviour commences. the extensional modulus is typically 90 Pa, compared with 10 3 Pa for fluid M1 and 10 6 Pa for a rubber band. The results are discussed and the advantages and disadvantages of the present system pointed out; extensional viscosities below 100 Pa s are difficult to measure in this way. The main advantages are those of experimental simplicity and the production of data which may be examined in several different ways.