Hydrodynamic instability of rod-like lyotropic liquid crystalline suspensions in Taylor-Couette flow: a case study

Liquid crystalline suspensions are prone to form distinctive flow patterns in flow geometries. Inspired by colorful patterns observed in PP flow for cellulose nanocrystals (CNC) aqueous CNC suspensions in recent work on the one hand, and the complexity of aqueous CNC suspensions in light of their viscoelasticity and shear-thinning behavior, on the other hand, we explored the flow instability patterns of CNC suspensions using direct birefringence patterns in Taylor-Couette (TC) flow, where CNC suspensions between two concentric cylinders are subjected to shear by the relative rotation of the cylinders. The setup is a custom-made TC visualization cell implemented on an Anton Paar MCR702e Space rheometer in a separate motor-transducer configuration. We employ cross-polarized optical imaging in our experiments, paving the way to observe flow transitions directly. Our results shed light on the combined effect of shear-thinning and viscoelastic properties of the CNC suspensions in determining the order of flow transitions. The classical primary and secondary transitions, i.e., the Laminar Couette Flow (LCF) to Taylor Vortex Flow (TVF) and Taylor Vortex Flow to Wavy Vortex Flow (WVF), respectively, are observed at a different range of elasticities irrespective of the shear-thinning level (increasing CNC concentration). Our observations point to the absence of flow instability intrinsic to viscoelastic effects, reminiscent of classical flow stabilities observed in Newtonian fluids and in agreement with the order of flow transitions in highly shear-thinning fluids with negligible to low elasticities. From the kinetics viewpoint of flow transitions, the joined impact of shear-thinning and high elasticity leads to the destabilization of the flow, shifting the critical Reynolds numbers to lower values. More interestingly, the counter-rotating of the cylinders of the TC flow system pointed to identification flow patterns with noticeable variations of conventional TVF and WVF.