Time-Dependent Mechanics of Cellulose Nanofibril Gels Under Shear and Compression

Background Gels made of cellulose nanofibrils (CNF) are biodegradable and renewable with numerous applications. Prior efforts to quantify rheological properties have considered the material primarily as a fluid and have been hindered by strain localization and sample size dependence. Objective We identify important considerations in designing experiments to quantify the time-dependent mechanics of CNF gels and devise experimental methods to quantify the viscoelastic properties of CNF gels on time scales on the order of hours. Methods Shearing experiments were designed using a custom-built loading device to apply uniform shear strain while simultaneously imaging the sample for digital image correlation analysis, enabling quantification of the shear relaxation modulus. In parallel, triaxial experiments were performed by compressing thin CNF gels, enabling quantification of the creep compliance in response to changes in volume. Results The shear data resulted in a relaxation modulus between 0.4 and 0.8 kPa at short times and a viscoelastic relaxation time on the order of tens of minutes. The triaxial compression tests revealed two regimes of strain response, which occurred above and below the osmotic pressure of the gels. Below the osmotic pressure, the creep compliance decreased with increasing compressive stress, whereas above the osmotic pressure, the creep compliance was relatively insensitive to the applied stress. Conclusions Our study presents a summary of challenges in experiments on CNF gels and designs experimental methods that overcome limitations of prior work. The data quantifying relaxation modulus and creep compliance in response to changes in shape and volume complements data reported in prior literature. These methods and findings can be used in future studies to assess how composition and processing affects the viscoelastic properties of CNF gels.