Multiscale transitional flow in anisotropic nanoparticle suspensions revealed by time-resolved X-ray scatter microscopy

Complex fluids transition from laminar to transitional flow above a critical control parameter, in a similar manner to their Newtonian counterparts. In a continuum mechanics sense, fluid elements follow the ensuing complex trajectories, which gives rise to secondary flows in terms of macroscopic vortices. However, it is not known whether their trajectories exhibit the same spatiotemporal behaviour as the macroscopic flow field if idealized fluid elements are replaced with actual anisotropic nanoparticles. Here, to address this fundamental question, we develop a multiscale method that combines small-angle X-ray scatter microscopy with polarized light imaging, thereby bridging seven orders of magnitude in length scales. We demonstrate proof of principle for Taylor–Couette flow—a classical transitional flow problem—of platelet-like graphene oxide nanoparticle and rod-like cellulose nanocrystal suspensions. The analysis shows markedly different multiscale dynamics underlying flow stability: whereas the platelet-like particles follow the macroscopic dynamics of the secondary flows, as expected for high rotational Péclet numbers, the rod-like particles exhibit characteristic high-frequency orientational motion at moderate rotational Péclet numbers that correlates with the turnover frequency of the vortex instabilities.