Slow dynamics and time-composition superposition in gels of cellulose nanocrystals

TL;DR

This study reveals that cellulose nanocrystal gels exhibit a universal aging behavior where their elastic properties can be rescaled onto a single master curve across various salt concentrations, demonstrating a form of time-composition superposition.

Contribution

It introduces the discovery of a universal superposition principle in the aging dynamics of CNC gels, linking salt concentration to the temporal evolution of their elastic modulus.

Findings

Elastic modulus follows a sigmoidal master curve spanning 13 orders of magnitude in time.

Time-shift factors decay steeply with increasing salt concentration, saturating at high salt levels.

The superposition principle applies across different salt types and CNC contents, indicating universality.

Abstract

Cellulose nanocrystals (CNCs) are rodlike biosourced colloidal particles used as key building blocks in a growing number of materials with innovative mechanical or optical properties. While CNCs form stable suspensions at low volume fractions in pure water, they aggregate in the presence of salt and form colloidal gels with time-dependent properties. Here, we study the impact of salt concentration on the slow aging dynamics of CNC gels following the cessation of a high-shear flow that fully fluidizes the sample. We show that the higher the salt content, the faster the recovery of elasticity upon flow cessation. Most remarkably, the elastic modulus $G'$ obeys a time-composition superposition principle: the temporal evolution of $G'$ can be rescaled onto a universal sigmoidal master curve spanning 13 orders of magnitude in time for a wide range of salt concentrations. Such a rescaling is obtained through a time-shift factor that follows a steep power-law decay with increasing salt concentration, until it saturates at large salt content. These findings are robust to changes in the type of salt and in the CNC content. We further show that both linear and nonlinear rheological properties of CNC gels of various compositions, including, e.g., the frequency-dependence of viscoelastic spectra and the yield strain, can be rescaled based on the sample age along the general master curve. Our results provide strong evidence for universality in the aging dynamics of CNC gels, and call for microstructural investigations during recovery as well as theoretical modelling of time-composition superposition in rodlike colloids.