Orientation Distributions of Cellulose Nanofibrils and Nanocrystals in Confined Flow

The pursuit of sustainable and environmentally friendly materials has been driving a tremendous interest in biobased alternatives in the last decade. Nanocellulose has been widely seen as a prime contender due to its impressive properties as well as being abundant and biodegradable. Recently, it has been demonstrated how nanocellulosic materials can be hydrodynamically aligned in flows and assembled continuously into materials with tunable macroscopic properties. However, the aligning mechanisms of the highly entangled system of elongated nanoparticles in different flow situations still remain largely unknown. Here, we investigate the orientation distributions of cellulose nanofibrils and nanocrystals (CNF and CNC) in a straight quadratic channel at various flow rates using small-angle X-ray scattering (SAXS), where CNF and CNC are aligned by strong shear flow close to the walls. In dilute systems, CNC behave as Brownian ellipsoids, while at semi-dilute concentrations there seems to be a limit to how high alignment of CNF and CNC can be achieved in a shear dominated flow even though particle interactions clearly aid in aligning the system at low flow rates. Furthermore, we show how some essential parameters in the orientational distribution can be obtained with polarized optical microscopy.