Shear Rate Imaging Using A Polarization Camera And A Birefringent Aqueous Cellulose Nanocrystal Suspension

Two aqueous cellulose nanocrystal suspensions are used to experimentally measure the shear and strain rates in a two-dimensional fluid flow. Cellulose nanocrystals are rod-like particles that align when subjected to shear, whereas at rest, they are randomly orientated by Brownian motion. The alignment causes birefringence, a phenomenon also known as flow-induced birefringence. The amount of birefringence is measured using a rotatable linear polarizer and a polarization camera. The linear polarizer is rotated to nine different positions. At each position, light from a light source becomes linear polarized before entering the birefringent fluid. Because of the birefringence the state of polarization is changed. This change is measured by the polarization camera. From the nine measurements the two-dimensional birefringence distribution is determined and from the amount of birefringence the strain rate is derived with the help of the data published by Lane et al. (2022a). We define Λ_0 as the angle between the (maximum) strain rate and the direction of flow. A shearing flow leads to Λ_0=45°, whereas Λ_0=0° and Λ_0=90° describe a flow down the centerline of a symmetrically converging and diverging channel, respectively. The measured strain rates are compared to simulations. The reference data in (Lane et al., 2022a) was taken at Λ_0=45°. For Λ_0≈45°, measurements and simulations correspond well to each other. If Λ_0≈0° is assumed, measured strain rates appear too high whereas they are too low for Λ_0≈90°. These results may be of interest for two reasons. First, shear rate imaging gave satisfying results in areas where Λ_0≈45°. Therefore, we propose the study of shear rates in a two-dimensional shearing flow by means of the presented method. Second, the results indicate that the flow state affects the degree of particle alignment and thus birefringence. This finding is in line with the statements made by Wayland (1960, 1964). The study of particle alignment and its influencing factors is of interest, and the experimental approach presented here is thought to be practicable for such studies.