Shear rate imaging using a polarization camera and birefringent aqueous cellulose nanocrystal suspensions

Abstract

A fluid that is composed of anisotropic particles may show birefringence when under the effect of shear. This phenomenon is known as shear induced birefringence and it is of interest to researchers for two reasons. First, to study the rheological behaviour as birefringence is linked to particle alignment. Second, to study and visualize fluid flows. The most common way to measure birefringence is to send polarized light through the birefringent material and to measure the change in polarization. This thesis considers five aspects of flow birefringence measurements which were also the subject of five corresponding publications. First, we give guidance for researchers calibrating a polarization-sensitive camera. For this, we define an error that quantifies the quality of the polarization measurements and discuss multiple factors influencing the measurement quality. We show that sufficiently large f-numbers no longer influence measurement quality and argue that lens design and focal length have little influence. Second, we describe a two-dimensional birefringence measurement technique that is based on a rotatable linear polarizer and a polarization camera. By measuring the first three Stokes parameters, the technique determines the relative position of the refractive index axes and the relative phase difference. The measurement range of the optical retardation is half the wavelength of the illumination. Third, we present an experimental procedure to measure the optical response of a birefringent fluid to shear. The experimental set-up is based on a Taylor-Couette flow where transparent end plates at the top and bottom enable optical access. Using a polarization camera, the absolute difference between the two main refractive indices and the relative position of the refractive index axes with respect to the flow direction (commonly referred to as extinction angle) are determined. Fourth, we report birefringence measurements in aqueous cellulose nanocrystal suspensions. Suspensions with concentrations between 0.7 and 1.3% per weight are sheared with shear rates up to 31 1/s and show extinction angles of 23-40° and birefringence in the order of 1e-5. Fifth, the birefringence response of aqueous cellulose nanocrystal suspensions is used to measure shear and strain rates in a two-dimensional fluid flow. The results show that the study of shear rates in a two-dimensional shearing flow by means of flow birefringence is feasible and therefore encourage the use of aqueous CNC suspensions for birefringent flow studies. However, the results indicate that the angle between principal strain rate and direction of flow affects particle alignment and thus birefringence.