Non-Contact And Non-Steady Rheo-Optical Measurement Of Hydrodynamic Stress Fields Inside A Cylindrical Tube

For measuring non-contact and non-steady rheo-optical measurement of hydrodynamic stress fields, we utilize a high-speed polarization camera that captures an unsteady phase-retardation field. It is known that the retardation has a proportional relationship to secondary principal stress difference. A solution of sodium iodide including crystal polymers (CNCs, cellulose nano crystals) was used as the working fluid. We have successfully measured the retardation induced by flow birefringence of CNCs without compromising the Newtonian fluid properties of the solution. In this paper, we propose a visualization technique for the retardation field of fluid in a cylindrical glass tube that incorporates index matching and discuss its comparison with the spatial intensity distribution of theoretical hydrodynamic stress field. Experimental results show that the retardation increases with an increase in flow rate. In addition, by utilizing the orientation angle data measured with the high-speed polarization camera, the vector field of principal stress difference is visualized, which is suggested to be useful for confirming the effects of direction of stresses. To investigate whether the measured retardation filed agrees with the theoretical secondary principal stress difference, the spatial intensity distribution of the two was compared. The overall trend agrees well while the retardation at the center of the tube is different, possibly owing to the effect of structure-induced retardation due to self-assembly of CNCs. The structure-induced retardation data were obtained by a combined measurement of a rotational rheometer and the high-speed polarization camera. As a result, the theoretical secondary principal stress difference distribution with considering structure-induced retardation showed better agreement with the retardation field than the case without considering it.