Hierarchical organization governs nonlinear mechanical reversibility in $ι$-carrageenan gels

Abstract

Carrageenan gels are thermoreversible polysaccharide networks whose mechanical properties emerge from ion-mediated helix association, yet how their molecular organization controls nonlinear deformation remains poorly understood. Here, we investigate the temperature-dependent rheology of $ι$-carrageenan gels formed in KCl solutions using linear and nonlinear oscillatory rheology combined with normal-force measurements. $ι$-carrageenan forms homogeneous and mechanically reversible gels whose elastic modulus increases continuously with quench depth. Deep quenches generate pronounced strain stiffening before yielding, associated with the development of internal stresses revealed by negative normal forces. Remarkably, large deformations preserve the small-strain elastic modulus while progressively suppressing strain stiffening, demonstrating a partial mechanical reversibility of the network. We interpret these observations using a hierarchical network picture in which a persistent intermolecular network controls linear elasticity, while a more fragile mesoscale organization enables cooperative alignment and stress amplification under deformation. Our results highlight that nonlinear mechanics of thermoreversible polysaccharide gels are governed not only by molecular connectivity, but also by the reversible formation and eventually destruction of mechanically adaptive hierarchical structures.