Micro-rheology of nanocellulose suspensions with smoothed particle hydrodynamics simulations

Nano-cellulose fibres in suspension/gel states are known to have complex structures, depending on e.g. concentration and ionic strength. These complex structures pose both opportunity and challenges: creating novel functional structures is challenged by difficult rheology and processability of NFC suspensions. To better assess the chemical and rheological impacts on the formation of nano-cellulose structures, we propose a novel particle-based method based on smoothed dissipative particle hydrodynamics. This numerical approach treats both fluid and solid phases in a unified way. Both fluid and solid phases are described as a set of particles exchanging momentum and/or interacting through Derjaguin-Landau-Verwey-Overbeek (DLVO) potentials. Additionally, nano-cellulose fibres are represented as strings of solid particles connected through extensional/bending springs. Brownian motion is also accounted for as a dissipative term. We used this model to simulate the micro rheology of NFC suspensions The simulations were carried out for the Couette geometry in a gap of the order of one micrometer. The goal was to determine the effect of various properties of cellulose nanofibre and its interaction strength on forming micro flocs and on local viscosity. The parametric effects on micro scale are expected to reflect in the observed macroscopic flow behavior as this behavior depends critically on the local aggregation dynamics. In the simulations with very large shear rates, floc formation takes place rapidly for all studied parameter combinations. Moreover, due to slippage, local shear rate becomes higher close to the walls than on average. Thus, the effective viscosity has higher value in the central region of the flow compared to the boundary regions. As to the varied parameters, fibril geometry (especially length) and suspension concentration appear to have much stronger effects on floc formation and viscosity than fibril interaction (double layer thickness) or bending stiffness.