Gelation dynamics of charged colloidal rods: critical behaviour and time-connectivity superposition principle

TL;DR

This study investigates the gelation dynamics of charged rod-like colloids, specifically cellulose nanocrystals, revealing critical behavior, a superposition principle, and asymmetries in gelation mechanisms through rheological analysis.

Contribution

It introduces a detailed rheological analysis of CNC gelation, demonstrating a time-connectivity superposition principle and critical dynamics unique to anisotropic charged colloids.

Findings

Identification of a well-defined gel point with critical behavior.

Master curves demonstrating time-connectivity superposition.

Asymmetry and non-universality in critical exponents.

Abstract

Charged colloidal particles can self-assemble into gel networks upon screening of electrostatic repulsion by added salt. While gelation of spherical colloids has been extensively studied, much less is known about the gelation dynamics of anisotropic colloids. Here, we focus on cellulose nanocrystals (CNCs) as prototypical rigid, highly charged rod-like colloids. In aqueous solution with salt, CNCs display a rich phase diagram ranging from gel at low solid content to glassy phases at higher concentrations. Building on our previous work [Morlet-Decarnin et al., ACS Macro Lett., 2023, 12, 1733], we present an extensive study of the mechanical recovery dynamics of CNC suspensions following a strong shear. Time-resolved mechanical spectroscopy reveals a liquid-to-solid transition characterized by a well-defined critical gel point. The evolving viscoelastic spectra can be rescaled onto master curves, demonstrating a time-connectivity superposition principle and critical dynamics on both sides of the gel point. By varying the CNC weight fraction and salt concentration, we identify a boundary between gel and attractive glass states marked by clear changes in rheological observables, including the elastic and viscous moduli at the gel point and their high-frequency power-law exponents. Analysis of dynamic critical exponents and hyperscaling reveals pronounced asymmetry between pre-gel and post-gel dynamics and non-universal values of the dynamic exponent. These findings highlight gelation mechanisms specific to highly charged rod-like colloids and call for complementary microstructural characterization and theoretical modeling.