Shear-Induced Orientation Phenomena in Suspensions of Cellulose Microcrystals, Revealed by Small Angle X-ray Scattering

The alignment of cellulose microcrystals in a 6.9% (w/w) colloidal aqueous suspension under shear was characterized using small angle synchrotron radiation scattering. Prior to shearing, the microcrystals appeared randomly oriented and went through a two-step alignment process with increasing shear rate. At shear rates lower than 5 s-1, the X-ray scattering patterns indicated little or no preferential alignment in the shear direction; however, in the plane perpendicular to the shear direction, the microcrystals were preferentially oriented in the vertical direction. At shear rates of 5 s-1 and higher, the microcrystals were aligned horizontally along the shear direction. These orientation phenomena were found to be dependent on shear rate and completely reversible. They are likely due to the presence of planar domains of randomly oriented microcrystals which align at low shear rates and are broken up at higher shear rates enabling alignment of the individual microcyrstals.