Measurement of flow Characteristics of Viscoelastic Fluid by Mach-Zehnder Interferometry

Mach-Zehnder interferometry, as applied to the stress analysis of solid materials, was employed to investigate the flow of viscoelastic fluids. With this method, secondary principal values of a refractive index ellipsoid can be measured because a reference light is used. This is in contrast to the birefringence method, in which only the difference of principal values can be evaluated. A Poiseuille flow between two parallel plates was selected as a flow field. Schemes for measurement of direct and transverse stress optical coefficients, C₁and C₂, and absolute values of normal stresses were derived for each case where either poralized or nonpolarized light is led through the test section in the flow direction, the direction perpendicular to the shear plane, or the neutral direction. Such quantities can be evaluated if the phase difference can be measured as a function of shear stress and pressure on the wall and a stress optical coefficient C₁-C₂is known in advance. Preliminary experiments for polyethylene and polystyrene melts were carried out with the polarized light perpendicular to the shear plane. However measurements accurate enough to allow evaluation of the normal stresses etc. have not been attained in this study.