Rheology of concentrated and highly concentrated enzymatic cellulose nanofibril hydrogels during lubricated compression

Processing cellulose nanofibril (CNF) hydrogels with a high concentration is a solution to reduce logistics costs and drying energy and to produce CNF-based materials with good dimensional stability. However, the rheology of concentrated and highly concentrated CNF hydrogels is poorly understood due to the difficulties to characterise them using standard shear rheometers. In this study, enzymatic CNF hydrogels in the concentrated and highly concentrated regimes (3-13.6 wt%) were subjected to lubricated compression at various strain rates. At low strains, compression curves exhibited a linear regime. At higher strains and low strain rates, a heterogeneous and marked hardening of stress levels was observed and accompanied with a two-phase flow with significant fluid segregation and network consolidation. At high strain rates, a homogeneous and incompressible one-phase plateau-like regime progressively established. In this regime, a yield stress was measured and compared with literature data, showing a good agreement with them.