  Polydisperse fibre networks are the basis of many natural and man-made
architectures, ranging from high-performance bio-based materials to components
of living cells and tissues. The formation and persistence of such networks are
given by fibre properties such as length and stiffness as well as the number
density and fibre-fibre interactions. Studies of fibre network behavior, such
as connectivity or rigidity thresholds, typically assume fixed fibre length and
isotropic fibre orientation distributions, specifically for nanoscale fibres
where the methods providing time-resolved measurements are limited. Using
birefringence measurements in a microfluidic flow-focusing channel combined
with a flow-stop procedure, we here propose a methodology allowing
investigations of length dependent rotational dynamics of nanoscale
polydisperse fibre suspensions, including effects of non-isotropic orientation
distributions. Transition from rotational mobility to rigidity at entanglement
thresholds is specifically addressed for a number of nanocellulose suspensions,
which are used as model nanofibre systems. The results show that the proposed
method allows characterization of the subtle interplay between Brownian
diffusion and nanoparticle alignment on network dynamics.
