Heterogeneous flow kinematics of cellulose nanofibril suspensions under shear.

The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The in situ optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.

Flow-induced structures versus flow instabilities.

The Taylor-Couette flow of a dilute micellar system known to generate shear-induced structures is investigated through simultaneous rheometry and ultrasonic imaging. We show that flow instabilities must be taken into account since both Reynolds and Weissenberg numbers may be large. Before nucleation of shear-induced structures, the flow can be inertially unstable, but once shear-induced structures are nucleated, the kinematics of the flow become chaotic, in a pattern reminiscent of the elastically dominated turbulence known in dilute polymer solutions. We outline a general framework for the interplay between flow instabilities and flow-induced structures.

"The hydrogen atom of fluid dynamics"--introduction to the Taylor-Couette flow for soft matter scientists.

The flow between concentric cylinders is routinely used in soft matter studies. In many cases, the purpose of the setup is rheometric: the idea is to relate macroscopic changes in material properties to microscopic changes in the structure of the material. The correspondence between the modifications of the microscopic structure and the macroscopic flow often relies on viscometric assumptions, which require the flow to be at least laminar. Flow instabilities are usually neglected because the viscosities of the materials are high and the geometries are small, such that the creeping flow approximation can be used. Nonetheless, the phenomenology of viscoelastic instabilities that emerged in the last twenty years warns us that flows can become turbulent without inertia, in particular flows between concentric cylinders. Given the strong similarities between inertial hydrodynamic instabilities and viscoelastic instabilities, a general knowledge of the former is advised for any researcher working on complex fluids. In this tutorial review, we focus on the inertial instability of isothermal and incompressible Newtonian fluids flowing between concentric cylinders. We highlight important aspects that can guide the study and control of instabilities in complex fluids in general.