Seeing Is Believing: Coupling Between Liquid Crystalline Ordering and Rheological Behaviour in Cellulose Nanocrystals Suspensions

Abstract

Nanomaterial engineering technologies have the potential to revolutionize industrial food systems, addressing issues related to health and sustainability. Some nanomaterials have unique physico-chemical properties that can be exploited for beneficial effects on food, leading to increased shelf life, enhanced flavour release, and increased absorption of nutrients and other bioactive components. In food production, processing steps like pipe flow, mixing and evaporation can clearly affect - and be affected by - mesoscopic arrangement of components. A nanomaterial characterization challenge includes the consideration of physico-chemical changes as a nanoscale material moves from formulation through incorporation into final products. These complexities could include the effective nanomaterial size, physical state, and behaviour under stress, which is quantified in terms of rheological performance. Here, we investigate the interplay between ordering and rheological behaviour in suspensions of Cellulose Nanocrystals (CNC) extracted with acid hydrolysis from plant biomass waste. In particular, through a microscopy shear cell we gain access to CNC textures and local degree of alignment upon oscillatory deformations, and compare them with linear and nonlinear rheological response. We find that, within the liquid crystalline and gel phases of CNC, some of the pre-shear protocols typically used in rheological measurements - involving mechanical yielding imposed via large amplitude oscillations or continuous shear - are associated to perturbation and persistent alignment of CNC domains along the shear direction, which in turn weaken the linear mechanical CNC response. Complete relaxation of shear-induced ordering and full recovery of linear moduli only occur over hundreds of seconds. We thus show that the combination of optical and rheological characterization can provide a valuable tool for the assessment of non- linear behaviour in complex fluids like CNC suspensions.