Use of Coupled Birefringence and LDV Studies of Flow Through a Planar Contractionto Test Constitutive Equations for Concentrated Polymer Solutions

LIDIA MARÍA QUINZANI; ROBERT ARMSTRONG; ROBERT BROWN

JOURNAL OF RHEOLOGY

JOURNAL RHEOLOGY AMER INST PHYSICS

Lugar: Nueva York; Año: 1995 vol. 39 p. 1201 - 1228

0148-6055

Laser Doppler velocimetry and flow-induced birefringence are usedto measure the rate of deformation and the principal components of therefractive index tensor in a 5% polyisobutylene (PIB) solution in tetradecane(C14) flowing along the centerplane of an abrupt 3.97:1 planar contraction. Thestress optical law is used to interpret the birefringence data in terms of thenormal stress difference, which is used to calculate a transient elongationalviscosity defined along the centerplane. These measurements are compareddirectly to predictions of six multimode, differential constitutive models(Oldroyd-B, White-Metzner, Aciemo et al., Giesekus, Bird-DeAguiar, and Phan-Thien-Tanner)that are fit to steady and small amplitude oscillatory shear flow data for the PIB/Cl4solution. The fluid exhibits slight elongational thickening followed byapparent extensional thinning at higher elongation rates. We believe that this?thinning? behavior is due to the decreased residence time of the polymermolecules in the high-strain-rate region as the flow rate (and maximumelongation rate) is increasedT. he nonlinear constitutive equations, except forthe White-Metzner model, are virtually indistinguishable in their descriptionof the dynamical response of the fluid in this experiment; however, the Phan-Thien-Tannermodel gives the best quantitative fit to the data. These results point to theneed for experiments in which the fluid flowing along the centerline issubjected to a greater total elongational strain.