Relaxation time of rod-like particle alignment under shear or extensional flow: the effect of particle polydispersity

Understanding how rod-like particles align and relax under the flow is crucial for improving the quality of compound materials and their manufacturing processes. It is predicted that monodisperse rod-like particles have a single time scale for relaxation, while polydisperse rod-like particles have been reported to exhibit multiple time scales. However, the relationship between these multiple time scales and flow conditions remains unclear. We studied the relaxation time of polydisperse cellulose nanocrystals (CNC) using a glass microchannel designed to generate simple shear and extensional flows. The birefringence induced by CNC alignment was measured with a high-speed polarization camera. After reaching a steady state, the flow was stopped, and the relaxation time was measured. The relaxation time decreased non-linearly with increasing strain rate in the low strain rate region and became constant in the high strain rate region. The CNC polydispersity qualitatively explained these results. Further experiments using samples with monodispersity and different polydispersity are needed for a better understanding. This research provides fundamental knowledge on the alignment control of rod-like particles.