Direct observation of hydrogel network formation during sonication of cellulose dispersions

Nanocellulose dispersions and hydrogels represent a promising class of sustainable soft materials with dynamic responsiveness to external stimuli. This study employs visualization, statistical analysis, and particle image velocimetry to investigate the dynamic behavior of cationic (CCNF) and TEMPO-oxidized (TOCNF) cellulose nanofibril dispersions under sonication, focusing on hydrogel network formation at varying concentrations (0.5-2.0 wt% for CCNF and 1.0-2.0 wt% for TOCNF). CCNF dispersions rapidly formed physically crosslinked layers even at low concentrations due to attractive interactions via quaternary ammonium groups, resulting in persistent hydrogel networks. In contrast, TOCNF required concentrations over 1.3 wt% to overcome electrostatic repulsion between carboxylate groups and formed weaker, transient gels. Image sequence analysis revealed that crosslinked layer thickness and lifetime increased with concentration for both nanocellulose types, with 2.0 wt% samples exhibiting robust, resilient hydrogel structures throughout sonication. While CCNF showed resistance to ultrasonic disruption, TOCNF networks degraded rapidly due to weaker intermolecular interactions. Results highlight how nanofibril surface chemistry and concentration govern the interplay between ultrasound-induced network formation and its destabilization. The study provides mechanistic insights into sonication-driven gelation and establishes a methodological framework for designing nanocellulose-based materials with tailored real-time structural responsiveness, bridging nanostructural dynamics and macroscopic behavior for advanced soft material applications.