  We study the flow stability and spatio-temporal spectral dynamics of
cellulose nanocrystal (CNC) suspensions in a custom Taylor-Couette flow cell
using the intrinsic shear induced birefringence and liquid crystalline
properties of CNC suspensions for flow visualizations for the first time. The
analysis is performed at constant ramped speed inputs of the independently
rotating cylinders for several cases ranging from only inner or outer rotating
cylinders to three counter-rotation cases. All CNC suspensions have measurable
elastic and shear thinning, both increasing with CNC concentration. We show
that the flow patterns recorded are essentially Newtonian-like, with
non-Newtonian effects ranging from a decrease in wavenumbers to altering the
critical parameters for the onset of instability modes. Outer cylinder rotation
flow cases are stable for all concentrations whereas inner cylinder rotation
flow cases transition to axisymmetric and azimuthally periodic secondary flows.
However, unstable counter-rotation cases become unstable to asymmetric spiral
modes. With increasing CNC concentration a counter-rotation case was found
where azimuthally periodic wavy patterns transition to asymmetric spiral modes.
In contrast to polymeric solutions of similar low to moderate elasticity and
shear thinning, the shear-thinning region of CNC suspensions is expected to
lead to the breakdown of the chiral nematic phase, whose elastic constants
constitute the dominant structural elasticity mechanism. Thus, we interpret the
Taylor-Couette stability of the CNC suspensions as dominated by their
shear-thinning character due to the expected loss of elasticity in nonlinear
flow conditions.
