Calibration of stress-optic coefficients for visualizations of fluid stress fields

We apply the photoelastic method to fluids, which allows direct visualizations of stress. The photoelastic method is a stress measurement technique based on changes in optical anisotropy due to internal stress and has been developed in the field of solids. The orientation of non-spherical particles dispersed in a fluid in the direction of stress induces a phenomenon known as flow birefringence. This phenomenon can be associated with the stress (strain) inside the fluid by the 'stress-optic law'. To estimate the fluid stress field from flow birefringence, stress-optic coefficients are needed, which represent the sensitivity of flow birefringence to fluid stress. The stress-optic coefficient can be determined by simultaneously measuring the flow birefringence with respect to the shear (stress) applied to the fluid (rheo-optical measurement). The aim of this study is to calibrate the stress-optic coefficient by rheo-optical measurements using a rheometer, which is a dynamic viscoelastic measuring device. In this presentation, the results of the calibration of stress-optic coefficients of rod-shaped cellulose nanocrystal suspensions will be presented.